Condensed Matter Resnick seminar
The fractional quantum Hall effect, first observed some four decades ago, is a system where Anyons - particles of fractional charges and fractional quantum statistics - flourish. In the last year, several important developments have brought the physics of Anyons back into the limelight. In particular, Anyons were shown to be quantum particles that can interfere as waves, and their traditional "alma mater", the fractional quantum Hall effect, has been shown to exist even without the application of any magnetic field.
I will review some of these developments, focusing on the theory behind them, and making minimal assumptions of prior knowledge.
Berry phases strongly affect the properties of crystalline materials, giving rise to modifications of the semiclassical equations of motion that govern wave-packet dynamics. In non-Hermitian systems, generalizations of the Berry connection using the bi-orthogonal formalism have been argued to characterize the topology of these systems. Since non-Hermitian Hamiltonians could be relevant for many types of open and lossy systems it is important to understand how these new quantities enter equations of motion for semi-classical wavepackets. Since generally for non-Hermitian systems the adiabatic theorem fails, this poses a challenge to the theory. I will discuss how to define observables and the conditions under which we can still apply the single band limit, and introduce the type of anomalous terms that may appear in the equation of motion and are present already in one-dimensional systems. I will also discuss the conditions for observing these anomalous contributions and potential extensions of the formalism to include complex electric fields, and magnetic fields.
Boundary-induced electronic phenomena (edge-physics) play a crucial role in explaining the fundamental mechanism driving conductivity in exotic material systems where conventional band theory predicts a nonmetallic phase, advancing applicative quantum materials research. Yet despite a decade of scientific advancement in this field, limited layered material systems have been demonstrated to exhibit edge states, and the harnessing of such states for technological device applications remains challenging. In this talk, I will show experimental evidence for the emergence of confined-edge current flow rising from the Commensurate Charge Density Wave (CCDW) phase of the Van der Waals material 1T-TaS2. Through the fabrication and conductance analysis of meso-scale 1T-TaS2 crossbar devices we demonstrate the ability to toggle between high and low resistance states via anisotropic write currents. By spatially mapping the current density via scanning SQUID microscopy, we reveal the current density path in the low-resistance conduction state resides dominantly along the device edges. Surprisingly, the edge flow is confined to a single side of the device which selects a preferable edge and can be manipulated. This single confined edge current flow, which can be explained by CDW domain-wall or CDW single domain formation, raises many question regarding its nature and possible applications of confined current manipulation.
Discrete and local responses of crystalline matter structures are pivotal elements facilitating the ongoing information revolution. A direct way to switch the properties and response of a given structure is to modify its crystalline symmetries by changing the relative atomic positions. Structural rearrangements, however, are challenging due to the solid interatomic bonds involved, which limits current technologies to alternating electronic orders without moving the atoms.
Interestingly, recent experiments show efficient control of atomic scale shifts in layered 2D crystals along their van der Waals (vdW) interfaces. The layers exhibit discrete sliding steps between meta-stable crystalline configurations in response to external electric fields or stress. These 2D vdW polytypes include periodic configurations that preserve substantial interlayer band hybridizations with distinct structural symmetries and diverse electronic/optical/magnetic properties. Their local switching occurs via mobile incommensurate partial dislocations lines, free to slide in a super-lubricant manner to replace one polytype with another.
The talk will outline the many possible polytypes in mono and binary compounds, their typical stacking energies, orbital inter-layer overlaps, and discrete symmetries. Following that, I will discuss the corresponding response of each polytype, including its internal charge redistribution, electric polarization, and underlying band structure. I will emphasize our recent reports of interfacial ferroelectricity [1], ladder-like cumulative polarization [2], doping-dependent polarization in elemental graphitic polytypes [3], and the microscopic dynamics of dislocation boundary lines between polytypes. Finally, we will discuss opportunities to extend this conceptual "slide-tronics" switching mechanism to efficient swapping between structural symmetries and orientations that should turn Sliding vdW Polytypes into a vast field of research.
- "Interfacial ferroelectricity by van-der-Waals sliding"
https://www.science.org/doi/10.1126/science.abe8177
- "Cumulative Polarization in Conductive Interfacial Ferroelectrics"
https://www.nature.com/articles/s41586-022-05341-5
- "Spontaneous Electric Polarization in Graphene Polytypes"
https://arxiv.org/abs/2305.10890
Manipulating quantum matter via external stimuli is at the frontier of quantum materials science. In this talk, I will discuss the potential of using coherent bosonic fields—such as photons, phonons, and polaritons—to manipulate material properties on demand. A judicious choice of the system configuration and the bosonic driving field can lead to exotic non-equilibrium phenomena, including topological phases, spontaneous symmetry breaking, and metal-to-insulator transitions. In pursuit of realizing effective driving sources, I will introduce our latest work on developing a hyperbolic phonon-polariton laser. This device can serve as a tool for inducing and probing novel non-equilibrium phases in solid-state materials, with potential applications in electronics, quantum sensing, and computation.
Cloud-based quantum computing platforms allow researchers to access and run quantum simulations remotely and have the potential to revolutionize our understanding of complex physical systems, including materials science, chemistry, and condensed matter physics. In this talk, we will discuss the basics of quantum simulations and how they can be implemented on quantum computers. We will then focus on the advantages and challenges of using cloud-based quantum computing platforms for quantum simulations. Specifically, we will discuss the benefits of cloud-based quantum computing platforms, including the ability to access state-of-the-art hardware and software without investing in expensive infrastructure. We will also address the challenges of working with quantum computers, including the need for specialized programming languages and the potential for errors in calculations. Finally, we will present some case studies of quantum simulations that have been successfully run on cloud-based quantum computing platforms, and discuss the implications of these simulations for the fields of materials science, chemistry, and condensed matter physics. Overall, this talk will provide an introduction to the exciting world of quantum simulations using cloud-based quantum computing platforms, and highlight some of the key challenges and opportunities in this field.
In transition metal dichalcogenides (TMDs) such as H-NbSe2 an H-TaS2 , superconducting properties are retained down to a single layer, making them useful platforms for studying thickness-dependent effects. Specifically, NbSe2 exhibits a reduction in its TC from 7.2 K in the bulk to approximately 3 K in the single-layer limit. In TaS2 , conversely, TC increases from 0.8 K in the bulk to approximately 3 K in the single layer limit. This contradicting behavior, which long puzzled researchers, could be related to a thickness-dependent suppression of superconductivity by the competing charge density wave (CDW) phase. I will present measurements of device-based high-resolution tunneling spectra in TaS2 , where we track the gap structure from the bulk all the way to a single layer. Our devices allow for simultaneous evaluation of the gap ∆, TC , and the upper critical field HC2 . Although TaS2 is considered as a dirty superconductor, we find that HC2 is proportional to ∆ 2 , a relation expected for clean superconductors. Even more curiously, we find that the same ratio between HC2 and ∆2 holds for other TMDs: NbSe2 of all thicknesses, and bulk TaSe2 , covering 4 orders of magnitude in HC2 and covering both clean and dirty limits.
The excitonic insulator phase is characterized by spontaneous formation of excitons, which condense and open a gap in the single-particle spectrum. Nevertheless, conclusively recognizing an excitonic insulator poses difficulties due to the concurrent occurrence of structural phase transitions in candidate materials. I will address the ongoing, decade-long debate surrounding the semimetal-to-insulator transition in Ta2NiSe5 and the contentious discussion about its physical origin.
Dipolar excitons in semiconductors and transition metal dichalcogenide (TMD) heterostructures have recently emerged as a promising experimental platform for realizing strongly correlated quantum many-body phenomena. In my talk, I will discuss multilayered structures that expose the anisotropic nature of dipolar interactions. Specifically, I will present a trilayer system that allows stabilizing a quantum quadrupolar state via tunneling events of holes between outer layers, as was recently observed experimentally. With increased exciton density, dipolar interactions destabilize the quadrupolar state and nucleate a staggered dipolar phase with broken layer symmetry. Furthermore, I will show that quadrilayer structures can support an interlayer molecular ("pair") superfluid phase and argue that it can be experimentally detected via a jump discontinuity in the exciton spectral shifts. Lastly, tuning exciton densities drives a quantum phase transition belonging to the (2+1)D XY universality class, marked by breaking interlayer molecules and softening of the associated binding energy. If time permits, I'll present recent unpublished results indicating a trimer condensate in antiparallel dipolar bilayers.
In this talk, I will discuss the role of antiferromagnetic interactions in two-dimensional crystal melting. I will show that, for strong enough magnetic interactions, fundamental lattice dislocations become prohibitive due to magnetic frustration. This gives rise to new types of hexatic and tetratic phases that are topologically ordered, in the sense of containing bound fundamental dislocations and free dislocation pairs. I will derive these phases through a field-theoretic tretment, and will demonstrate them numerically in an experimentally realizable system of hard spheres confined between parallel plates. I will discuss the possible survival of antiferromagnetic order in molten crystals.
Time crystals are quantum systems characterized by broken discrete time-translational symmetry, manifested by sub-harmonic response to periodic driving (simply put – the frequency of the response of the system is smaller than the frequency of driving). Originally suggested by Wilczek, time-crystals have become a hallmark example of non-trivial self-organization in may-body quantum systems, and has been measured extensively. However, the vast majority (if not all) of the measurements of time crystals are done via optical means, which present various limitations and challenges. It would be highly beneficial to be able to measure time-crystalline behavior in other experimental setups, but this presents a challenge, because many experimental setups are actually open to an environment, and common wisdom is that opening a quantum system to an environment is detrimental to the time-crystalline phase.
We thus ask two questions: (1) what are the conditions under which a time-crystal state can survive opening the driven system to an external environment? (2) which new nanoscale systems can exhibit time-crystal order?
To answer these questions, we use the theory of open quantum systems (in Lindblad form), and formulate a symmetry condition which we call “weak local dynamical symmetry”, which guarantees the stability of the time-crystal state even if an environment is present. We show that such conditions can be met in transport measurements of driven quantum dot arrays, where the current holds the signature of the time-crystal phase. Finally, we discuss the possibility of measuring time-crystalline behavior in molecular nano-magnet arrays, which display a unique, and to some extent yet unexplained, dynamical features.
[1] S. Sarkar and Y. Dubi, Signatures of discrete time-crystallinity in transport through an open Fermionic chain, Nature Communications Physics 5, 155 (2022).
[2] S. Sarkar and Y. Dubi, Emergence and Dynamical Stability of a Charge Time-Crystal in a Current-Carrying Quantum Dot Simulator, Nano Letters 22, 11, 4445 (2022)
[3] S. Sarkar and Y. Dubi, Time crystals in molecular nanomagnet arrays, Nano Letters (under review)
When systems containing spin-orbit interaction are subjected to a magnetic field, the Fermi surface becomes asymmetric, resulting in non-linear transport components (V ∼I 2 ). In this talk, I will show that by focusing on such non-linear transport effects, the role of spin-orbit interaction in the (111) LaTiO3/SrTiO3 interface can be understood. We observed that the non-linear resistance exhibits a dramatic increase at a critical in-plane magnetic field Hc. Furthermore, we find that this increase vanishes when a minuscule out-of-plane magnetic field is applied. By employing a minimal model that incorporates Rashba-type spin-orbit interaction, we demonstrate that these results can be explained by the intersection of the spin-split Fermi contours at the critical field Hc. The presence of an out-of-plane magnetic field opens a gap through the Zeeman interaction, which we show to substantially diminish the effect. We speculate that certain magnetoresistance effects previously observed in various interfaces could be understood within the framework of the Fermi contour intersection studied here.
Spin liquids are highly-entangled phases of quantum spins that host fractional excitations. The best-known examples of such states arise in the Kitaev honeycomb model, which is fine-tuned to exhibit extensively-many conserved quantities that permit an exact solution. I will introduce the model and show that including two 'real-world' effects -- vacancies and breaking of integrability -- yield qualitatively different physical properties. Additionally, I will explain how Kitaev spin liquids could be realized with subextensively many conserved quantities by using boundary states of one-dimensional SPTs instead of local spins as building blocks.
Superconducting quantum interference devices (SQUIDs) are commonly known to be very sensitive sensors of magnetic flux. The source for their sensitivity is the strong dependence of the critical current on magnetic field. Scanning nanoscale SQUIDs are of growing interest due to their highly sensitive imaging of magnetic and transport properties of low-dimensional systems [1]. The critical current of the nanoSQUID, however, is also temperature dependent, giving rise to the most sensitive cryogenic scanning nano-thermometer [2-3]. The combination of these two capabilities allow us to visualize and study intricate electronic flow patterns in quantum systems [4-5].
In the main part of this talk, I will discuss the direct observation of vortices in an electron fluid [6]. Vortices are the hallmarks of hydrodynamic flow. Recent studies indicate that strongly-interacting electrons in ultrapure conductors can display signatures of hydrodynamic behavior including negative nonlocal resistance, higher-than-ballistic conduction, Poiseuille flow in narrow channels, and a violation of the Wiedemann-Franz law. Here we provide the first visualization of whirlpools in an electron fluid. By utilizing our nanoSQUID on a tip (SOT) we image the current distribution in a circular chamber connected through a small aperture to an adjacent narrow current-carrying strip in the high-purity type-II Weyl semimetal WTe2. In this geometry, the Gurzhi momentum diffusion length and the size of the aperture determine the vortex stability phase diagram. We find that vortices are present only for small apertures, whereas the flow is laminar (non-vortical) for larger apertures. Moreover, near the vortical-to-laminar transition, we observe a single vortex in the chamber splitting into two vortices, a behavior that can occur only in the hydrodynamic regime but is not expected for ballistic transport. These findings suggest a novel mechanism of hydrodynamic flow in thin pure crystals: the spatial diffusion of electrons’ momenta is enabled by small-angle scattering at the planar surfaces, instead of the routinely invoked electron-electron scattering, which becomes extremely weak at low temperatures. This surface-induced para-hydrodynamics [7], which mimics many aspects of the conventional hydrodynamics, including vortices, opens new avenues for exploring and utilizing electron fluidics in high-mobility electron systems.
A new transport method of measuring superconducting coherence length x will be presented. This method is particularly useful for perpendicular coherence length xc in layered superconductors. The method requires dedicated theoretical development. The theory will be tested in simple cases and criticized in difficult cases. The method will be applied to a cuprate superconductor where xc is thought to be on atomic scale. The implication of our finding will be discussed.
We experimentally explore extensions to quantum mechanics, that have been proposed to resolve the long-standing problem of quantum measurement. Two on-going experiments will be discussed. The first one, which is based on the inverse Faraday effect in ferrimagnetic resonators, will explore an internal inconsistency originated from the collapse postulate. The second experiment, which is done using spins in diamond, addresses the question whether dynamical instabilities are possible in quantum systems having a Hilbert space of finite dimensionality.
The Hubbard model is to interacting quantum electronic systems as the Ising model is to classical statistical mechanics. Its apparent simplicity and widely believed relevance to the high-temperature superconductors have motivated an enormous amount of work over the past six decades. Still, much remains unknown. I will describe what is known with theoretical confidence about the phase diagram of the Hubbard model, both analytically and numerically. I will then present results from a determinant quantum Monte Carlo study of a related bilayer system that is free of the sign problem, and discuss the extent to which it may serve as a computational proxy to the Hubbard model.
We study a long topological Josephson junction with a ferromagnetic stripe between the superconductors. The low-energy theory exhibits a non-local in time and space interaction between chiral Majorana fermions, mediated by the magnonic excitations in the ferromagnet. The spontaneous breaking of a Z2-symmetry at the mean-field level leads to a tilting of the magnetization and the opening of a fermionic gap (Majorana mass). This is equivalent to the Peierls instability in the commensurate Fröhlich model. Within a Gaussian fluctuation analysis, we identify critical values for the temporal and spatial non-locality of the interaction, beyond which the symmetry breaking is stable at zero temperature – despite the effective one-dimensionality of the model. We conclude that non-locality, i.e., the stiffness of the magnetization in space and time, stabilizes the symmetry breaking. In the stabilized regime, we expect the current-phase relation to exhibit an experimen- tally accessible discontinuous jump.
At nonzero temperatures, as usual in the 1D Ising model, the long-range order is destroyed by solitonic excitations, which in our case carry each a Majorana zero mode.
In recent years it has become increasingly appreciated that electrons in solids possess quantum geometric structure that impact the electronic properties of the system. Typically, this takes the form of a Berry curvature for single electron states, and is physically manifested as an anomalous velocity. In this talk we discuss quantum geometric properties of collective modes. We show that generally such excitations possess their own type of quantum geometric measure, closely related to an electric dipole moment, which we call the quantum geometric dipole (QGD). We will focus on two examples of this, excitons and plasmons in two-dimensional systems, and discuss some physical implications of a non-vanishing QGD. While these realizations involve neutral excitations which may be described as two-body states, we will also present a many-body formulation of the QGD that is independent of specific wavefunction forms. As an example, we apply the formalism to collective excitations of fractional quantum Hall states, and show that these generically carry non-zero QGD’s.
Graphene, a two-dimensional honeycomb lattice of Carbon atoms, has many fascinating mechanical and electronic properties. In the presence of a strong perpendicular magnetic field, it forms nearly four-fold degenerate Landau levels (with two spins and two valleys). When an integer number of these four Landau levels are filled, the system becomes a quantum Hall ferromagnet in the spin/valley space. The ordering of the ferromagnet is determined by the interactions. In this talk I will present the history of experimental and theoretical work on this problem at charge neutrality. I will end with our contribution, which considers the most general possible interactions at low energies, and provide a set of possible phase diagrams for this problem.
We study a free fermion model where two sets of non-commuting non-projective measurements stabilize area-law entanglement scaling phases of distinct topological order. We show the presence of a topological phase transition that is of a different universality class than that observed in stroboscopic projective circuits. In the presence of unitary dynamics, the two topologically distinct phases are separated by a region with sub-volume scaling of the entanglement entropy. We find that this entanglement transition is well identified by a combination of the bipartite entanglement entropy and the topological entanglement entropy. We further show that the phase diagram is qualitatively captured by an analytically tractable non-Hermitian model obtained via post-selecting the measurement outcome. Finally we introduce a partial-post-selection continuous mapping, that uniquely associates topological indices of the non-Hermitian Hamiltonian to the distinct phases of the stochastic measurement-induced dynamics.
In this talk, I will present a proof of delocalization in spin chains symmetric under a combination of mirror and spin-flip symmetries and with a nondegenerate spectrum. The proof applies to two prominent examples: the Stark many-body localization system (Stark-MBL) and the symmetrized many-body localization system (symmetrized–MBL). I will also provide numerical evidence of delocalization at all energy densities in these models and show that the delocalization mechanism appears robust to weak symmetry breaking.
Twisted bilayer graphene systems have emerged as a new platform for intriguing many-body states of matter. The application of an external electric field drives the system from an insulating state to a topological state that does not conduct in bulk but does conduct on the sample edges and even to superconducting states. I will introduce and analyze a model that sheds light on the interplay between correlated insulating states, superconductivity, and flavor-symmetry breaking in magic-angle twisted bilayer graphene. Using a variational mean-field theory, we determine the normal-state phase diagram of our model as a function of the band filling. Our model elucidates how the intricate form of the interactions and the particle-hole asymmetry of the electronic spectrum determine the phase diagram. It also explains how subtle differences between devices may lead to experimentally observing different behaviors. A similar model with minor modifications is suitable for describing other systems, such as untwisted and twisted trilayer Graphene and transition metal dichalcogenides structures.
Anisotropic dipolar systems are ideal for the study of quantum phase transitions, as the weak magnetic dipolar interaction and the moderate anisotropy allow the induction of appreciable quantum fluctuations at applicable transverse magnetic fields. Furthermore, upon dilution, random interactions and effective random fields emerge, allowing the study of their interplay with quantum fluctuations in the ferromagnetic and spin glass phases. Here I will review some of the intriguing phenomena resulting from this interplay and from the uniqueness of the dipolar interaction, including the destruction of spin glass order at infinitesimal transverse field, a new type of Imry-Ma like disordering of the ferromagnetic phase, and recent results showing that an effective three body interaction is needed to account for the anomalous dependence of the critical ferromagnetic temperature on transverse field. Relevance to classical dipolar systems will be discussed.
TBD
Ferroelectricity superconductivity and ferromagnetism are orders which rarely coexist due to their different symmetry and orbital requirements. A possible avenue for combining these orders is by interface design, where orders formed at the constituent materials can overlap and interact through emergent conducting electrons. In my talk I will describe our struggle to combine these orders at a conducting oxide interface. We combined transport and scanning SQUID imaging of the current flow to study and understand ferroelectric-superconductor 1 and ferroelectric-magnetic 2 heterostructures. I will also show our recent preliminary results on a magnetic-superconductor new interface where we believe that a magnetoelectric coupling can be realized.
Ability for the on-demand generation and manipulation of non-classical states of light is a cornerstone of scalable quantum information processing. In my talk, I will describe how emerging field of waveguide quantum electrodynamics (WQED), studyin interaction of photons propagating in a waveguide with localized quantum emitters, paves new routes towards flexible control over the photon-photon correlations. Specifically, I will consider emergence of photonic entanglement in a waveguide-coupled array of two level systems with modulated resonance frequencies. It will be shown how modulation may impose symmetry constraints on multi-photon emission leading to the symmetry-protected quantum correlations. Furthermore, I will describe a protocol for deterministic generation of designed multi-photon entangled states in the modulated arrays of two-level systems.
Photocurrents in topological materials have been associated with topological properties of the electronic bands, such as the Berry connection. However, despite some experimental demonstrations, it has remained unclear what part the topology actually plays in the process. In this talk I will discuss our use of time- and angle-resolved photoemission spectroscopy (trARPES) to resolve photocurrents in the excited electronic states of a topological insulator. By analyzing the rise times of the population following the optical excitation, we gained a complete view of the occupied and unoccupied electronic states, and how they are coupled by the light. Our work provides a microscopic understanding of how to control photocurrents in materials with spin-orbit coupling and broken inversion symmetry, and paves the way to control of currents in topological states. I will also briefly describe trARPES experiments on a magnetic Weyl semimetal.
A combination of spin-orbit coupling and breaking of time reversal symmetry gives rise to plenty of physical phenomena of a fundamental interest and attractive for spintronic applications. In this seminar I will focus on asymmetric electron scattering in semiconductors and metals underlying anomalous Hall and topological Hall effects as well as so-called spin swapping phenomena. We have contributed to the field by demonstrating a crossover between qualitatively different regimes of skew scattering on magnetic centers and chiral spin textures leading to either chiral or topological Hall effect. I will also discuss the chiral Friedel oscillations of the spin density universally appearing in conducting systems with spin orbit coupling and broken time reversal symmetry
Light-matter interactions are playing an increasingly crucial role in the understanding and engineering of new states of matter with relevance to the fields of quantum optics, solid state physics, chemistry and materials science. Experiments have shown that significant modifications of material properties and transport can occur in a cavity in the regime of collective strong light-matter coupling even without external irradiation – “in the dark”. Theoretical modeling of these effects is often impaired by the large number of relevant degrees of freedom in experiments and the subtle nature of light-matter interactions in a cavity. In this talk we choose to focus on paradigmatic models for disordered spins coupled to the photon field of cavity and discuss how collective light matter interactions can dramatically alter the many-particle spin wavefunctions in the limit of vanishingly small photon numbers. Subtle, permanent changes in the wavefunctions result from the combined effects of vacuum hybridization and long-range cavity-mediated couplings between the spins. A surprising, general, result is the realization of “semilocalization”, an elusive effect usually associated to critical states of Anderson-like transitions. We discuss possible implications for energy transport mediated by collective light-matter dark states in molecular physics and quantum optical systems.
Weak measurement is an alternative to von Neumann’s dogma of the “collapse” of a wave function: it avoids the necessity of the latter’s complete destruction, while capable of extracting partial information from the system measured. Concomitantly, measurement is associated with a back-action of the detector on the system’s state. This back-action can be harnessed for the purpose of steering a quantum state into a pre-designated target state, and for quantum engineering of non-trivial states of matter. I will try to package this all together, elucidating the difference between a stupid passive protocol, and an active one, where decisions made by a smart observer may improve the performance of quantum steering.
Machine Learning (ML) is an increasingly valuable tool for the study of condensed matter physics. However, despite many successes, applications where ML conceptually drives theoretical research are scarce. Here we set ML the harder task of providing insight into a problem for which there is very little understanding. Dimer models, one of the oldest problems in statistical physics, were recently introduced to Ammann Beenker tilings. The model showed unexpected features such as long-ranged anisotropic non-monotonic dimer-dimer correlations. A deeper understanding of their origin remains elusive.
To elucidate these results we worked co-operatively with ML, using a combined analytical and numerical approach incorporating the recently developed Mutual Information Renormalization Group (RG) scheme. Remarkably, ML identifies that the statistics can be expressed in terms of emergent large-scale 'super-dimers'. The result reveals an emergent scale invariance, demonstrating proximity to a non-conformal RG fixed point understood as the same dimer problem re-appearing at a coarse-grained level. Our findings provide a rare example of successfully applying RG to a 2D quasicrystal model, and portray a new approach for discovering coarse-grained representations in complex systems.
Quantum-enhanced sensing techniques allow us to detect single spins, both nuclear and electron. This capability is promising not only because it is necessarily impressive from a technological point of view, but also because it enables us to “peek” under the hood of the ensemble average employed by the more “traditional” methods. Recent studies, making use of NMR-based dynamical decoupling schemes, have shown how to estimate both range and solid angle of the detected spin with respect to the sensor, or in other words its precise position. In the first part of the seminar, I will present a method extended and developed in my group, which makes use of simple radio pulses to perform nanoscale tomography of electron spins. The sectioning is implemented using a rotating magnetic field with varying strengths. For our lab, this is the first step towards nanoMRI, and allows us to pinpoint the location of spins in sample space. In the second part of the seminar, I will discuss one exciting direction we are pursuing to improve the sensitivity of our technique, with the aim of reaching the Heisenberg limit. This method makes use of what is currently known as adaptive Bayesian estimation. I will show our current state in this endeavor and explain how the use of such means is projected to improve the signal-to-noise ratio in quantum sensing setups.
Electron hydrodynamics, which is the viscous flow of current in mesoscopic high-mobility devices, has become a valuable tool in characterizing quantum materials, and can be used to (re)create a number of hydrodynamic phenomena, both old and new. However, the formation of a whirlpool - arguably the most striking hydrodynamic phenomenon - has so far been elusive.
I discuss recent results on ultrapure, thin WTe2 flakes in which it is for the first time possible to directly observe hydrodynamic vortices using spatially resolved current-imaging techniques. From a theory perspective, this observation is rather challenging: Both from the measured bulk mean free path and from ab-initio calculations, one would instead expect ballistic transport, and not hydrodynamic flow. Here, I show how this discrepancy can be resolved in a kinetic theory that incorporates weak surface scattering from the top and bottom surfaces, thereby leading to a novel para-hydrodynamic regime where ballistic and hydrodynamic mechanisms coexist. Surprisingly, our findings imply that para-hydrodynamic transport appears generically in ultraclean devices, meaning it is probably observable in a range of other currently studied materials.
One cannot overestimate the importance of entanglement as a fundamental aspect of quantum mechanics. Recently it has also gained a central role in the study of many body systems in condensed matter and high energy physics. After reviewing this, I will pose the main question of our recent studies: How are symmetries, which give rise to conservation laws, manifested by entanglement measures? Similarly to the system Hamiltonian, a subsystem's reduced density matrix is composed of blocks characterized by symmetry quantum numbers, or charge sectors. I will present a geometric method for extracting the contribution of individual charge sectors to a subsystem’s entanglement measures (entropies and negativities) within the replica approach, via threading of appropriate conjugate Aharonov-Bohm fluxes through a multi-sheeted Riemann surface.
Specializing to the case of 1+1D conformal field theory, I will describe a general exact result for the entanglement characteristics, in the ground state as well as following a quench. I will apply it to a variety of systems, ranging from free and interacting fermions to spin and parafermion chains, and verify it against exact asymptotic results and numerics. For example, I will show that the total ground-state entanglement entropy, which scales as the logarithm of the subsystem size, is composed of square-root of log contributions of individual subsystem charge sectors for interacting fermion chains, or even subsystem-size-independent contributions when total spin conservation is also accounted for. I will proceed to nonequilibrium current-carrying steady states, and show that they exhibit very unusual extensive entanglement between far-away segments with peculiar distribution between symmetry sectors.
I will conclude by describing how measurements of the contributions to the entanglement or negativity from separate charge sectors can be performed with ultracold bosons or fermions and similar systems, and how they could be calculated efficiently from tensor network states.
Topological quantum materials have been extensively studied in recent years. After introducing the band structure topology and related materials, I will talk about novel electric and optical phenomena emerging in quantum materials caused by topology. For example, Weyl semimetals can exhibit anomalous Hall effect (AHE) due to the monopole-like Berry curvature and also an exotic nonlinear AHE due to the Berry curvature dipole. Further, we found that magnetic quantum materials exhibit exotic optical responses, for instance, a giant photocurrent caused by the quantum geometry beyond Berry phase.
The Kondo insulator state (KIS) realized in the symmetric Anderson model at half filling is studied in the framework of a mean field approach. It is shown that a Kondo insulator state with doubled lattice period is stabilised, where the gapped electron liquid behaves like a gapless Majorana spin liquid. Local moments of the d-electrons form a static Z2-field in which the band electrons move. The gap in the quasi-particle excitation spectrum decreases with increasing external magnetic field and closes at its critical value. The behavior of the electron liquid is studied for an arbitrary dimensionality of the model. The proposed approach leads to a description of KIS without the need to resort to any artificial symmetry breaking.
In this talk, I will present a fundamentally new type of scanning probe microscope, the Quantum Twisting Microscope (QTM), capable of performing local quantum interference measurements at a twistable interface between two quantum materials. Its working principle is based on a unique tip made of an atomically-thin two-dimensional material. This tip allows electrons to coherently tunnel into a sample at many locations at once, with quantum interference between these tunneling events, making it a scanning electronic interferometer. With an extra twist degree of freedom, our microscope becomes a momentum-resolving local probe, providing powerful new ways to study the energy dispersions of interacting electrons. I will present various experiments performed with this microscope, demonstrating quantum interference at room temperature, probing the conductance of in-situ twisting interfaces, and imaging local energy dispersions of graphene and twisted bilayer graphene.
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Zoom link: https://us02web.zoom.us/j/88692035706
Although it is recognized that Anderson localization always takes place for a dimension d less or equal d = 2, while it is not possible for hopping V (r) decreasing with the distance slower or as r−d, the localization problem in the crossover regime for the dimension d = 2 and hopping V (r) / r−2 is not resolved yet. Following earlier suggestions we show that for the hopping determined by two-dimensional anisotropic dipole-dipole or RKKY interactions there exist two distinguishable phases at weak and strong disorder. The first phase is characterized by ergodic dynamics and superdiffusive transport, while the second phase is characterized by diffusive transport and delocalized eigenstates with fractal dimension less than 2. The crossover between phases is resolved analytically using the extension of scaling theory of localization and verified using an exact numerical diagonalization.
Floquet engineering - designing phases of matter “on-demand" using time-periodic drives has recently emerged as a powerful tool for inducing exotic non-equilibrium phenomena. This talk will focus on Floquet phases arising in steady states and quasi-steady states of periodically driven interacting electronic systems. In the first part of my talk, I will discuss a spontaneously broken symmetry phase in the steady-state of a laser-driven semiconductor coupled to reservoirs of phonons and photons. In the second part, I will consider slowly driven one-dimensional electronic systems and show that despite strong interactions between the electrons, these systems can form exponentially long quasi-steady states exhibiting universal transport.
Reference: https://www.nature.com/articles/s41467-021-25511-9
We examine the properties of a one-dimensional Fermi gas with attractive intrinsic (Hubbard) interactions in the presence of spin-orbit coupling and Zeeman fields. Such a system can be realized in the setting of ultracold atoms confined in a 1D optical lattice, and has been proposed to host exotic topological phases and edge modes. In absence of any external fields, this system shows a trivial Bardeen–Cooper–Schrieffer (BCS) phase. Introduction of Zeeman field takes the system to a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) phase, where the quasi-long range superconducting order co-exists with magnetic order in the system, as indicated by its pair momentum distribution. We explore the effect of spin-orbit coupling in this system. Next, we show that the addition of a smooth parabolic potential yields a phase with exponentially decaying pair binding and excitation energy gaps, which is expected to be associated with topological edge modes in the system. However, we show that this ground state degeneracy is susceptible to local impurities, and argue that the exponential splitting in the clean system is similar to a phase with only conventional order.
Chaos is an important characterization of classical dynamical systems. However, in recent years, a quantum Lyapunov exponent λL, and a butterfly velocity vB for ballistic spread of local perturbation, computed from the so-called out-of-time-order commutator (OTOC) have emerged as important measures for chaos and thermalization in interacting quantum many-body systems having some well-defined semiclassical limit. In the first part of the talk, I will describe curious interplay of chaos, quantum fluctuations, symmetry breaking and complex dynamics across dynamical transition in a quantum spin glass model. I will discuss the implications of the results in the classical limit of the model which describes dynamics in supercooled liquid in structural glasses. In the second part of the talk, I will talk about a surprising noise-induced many-body chaotic to non-chaotic transition in the classical Langevin dynamics of interacting integrable and non-integrable systems.
We know that the structure and symmetry of the crystal lattice are closely connected to magnetism. We try to adjust these factors in order to change the alignment of spins in conventional ways, for instance, by applying pressure or strain. However, these effects can only be achieved statically, and many materials may break under the load required to make significant changes in the magnetic order. In this talk, I will show how coherent collective vibrations of atoms, known as optical phonons, provide an intriguing alternative. The development of powerful terahertz sources in recent years has enabled resonant driving of optical phonons, yielding vibrational amplitudes so large that the dynamics are governed by nonlinear interactions between phonons and other degrees of freedom. I will lay out the steps that lead to the prediction of phono-magnetic effects, nonlinear phononic phenomena that can be regarded as vibrational analogs of opto-magnetic effects in nonlinear optics. In this process, I will show how optical phonons can be used not just to control spin waves coherently and induce spin polarization, but also to produce magnetic responses in otherwise entirely nonmagnetic materials.
Quantum fluids of matter with long range, anisotropic interactions display rich excotic collective phenomena, from Roton instabilities, quantum droplets, and supersolidity [1]. A prominent example is the dipole-dipole interaction, which has recently been addressed by a growing community, both from atomic physics as well as from condensed matter physics, with the latter being focused on dipolar quantum fluids of two-dimensional excitons, bound pairs of an electron and a hole in a semiconductor. Very recently, interacting dipolar polaritons, which dressed superposition states of dipolar excitons and confined photons have been reported. These strongly interacting dipolar exciton and polariton systems offer opportunities to explore new collective phenomena which are currently inaccessible with atomic dipolar gases, and the possibility to demonstrate new types of quantum devices on the level of two-particle interaction.
I will present several recent results in systems of dipolar excitons and polaritons. These include experimental evidence for the dynamical formation of a robust dark dense liquid phase of dipolar excitons in a bilayer system [2],
and the observation of a formation of an attractive di-polaron-like many-body correlated state [3]. This effect, which is due to the anisotropic nature of the dipole-dipole interaction, takes place in a new structure design allowing vertical coupling of dipolar exciton fluids, and is a first step towards realizing an exciton dipolar pair superfluid [4], an elusive and exotic collective phase.
Finally, I will introduce recent experiments with ‘flying’ electrically polarized dipolar-polaritons (‘dipolaritons’) in optical waveguides. These are electrically tunable exciton-photon super-position states that effectively act as “interacting photons” [5]. I will present a demonstration of a dipolar mirror and a dipolar transistor for such hybrid photons, a result promising for future implementations of a dipolar blockade and quantum circuitry of light.
Bibliography:
[1] See e.g., Fabian Böttcher, Jan-Niklas Schmidt, Matthias Wenzel, Jens Hertkorn, Mingyang Guo, Tim Langen, and Tilman Pfau “Transient Supersolid Properties in an Array of Dipolar Quantum Droplets” Phys. Rev. X 9, 011051 (2019), Tanzi, L., Roccuzzo, S.M., Lucioni, E. et al. “Supersolid symmetry breaking from compressional oscillations in a dipolar quantum gas” Nature 574, 382–385 (2019)
[2] Yotam Mazuz-Harpaz, Kobi Cohen, Michael Leveson, Ken West, Loren Pfeiffer, Maxim Khodas, and Ronen Rapaport "Dynamical formation of a strongly correlated dark condensate of dipolar excitons", Proc. Nat. Acad. Sci. 116 (37) 18328; (2019)
[3] Colin Hubert, Yifat Baruchi, Yotam Mazuz-Harpaz, Kobi Cohen, Klaus Biermann, Mikhail Lemeshko, Ken West, Loren Pfeiffer, Ronen Rapaport, and Paulo Santos, "Attractive dipolar coupling between stacked exciton fluids", Phys. Rev. X 9, 021026 (2019)
[4] Michal Zimmerman, Ronen Rapaport, Snir Gazit “Collective inter-layer pairing and pair-superfluidity
in vertically-stacked layers of dipolar excitons”, arXiv:2202.11754 (2022)
[5] Itamar Rosenberg, Dror Liran, Yotam Mazuz Harpaz, Ken West, Loren N. Pfeiffer, Ronen Rapaport “Strongly interacting dipolar polaritons”, Science Advances, 4, eaat8880 (2018)
Over the past decade, the unique properties of topological materials have generated huge excitement in the condensed matter physics community. Recently, high-quality thin films of cadmium arsenide (Cd3As2), a three-dimensional Dirac semimetal, have emerged as a promising platform for the observation of quantum transport phenomena from topological states and the realization of new topological states. In this talk, we will first discuss our recent progress in the growth of epitaxial thin films Cd3As2 by molecular beam epitaxy. We will then discuss the nature of the topological insulator-like states of thin (001) Cd3As2 films and the “half-integer” quantum Hall effect arising from these states. We will also discuss several measurements relevant for quantum information devices, including electronic interference experiments.
The dynamics of strongly interacting quantum matter is an exciting frontier in theoretical physics. In generic quantum many-body systems that are isolated from their environment, the system's dynamics explores the space of accessible states indiscriminately and is therefore dubbed "ergodic." However, this ergodicity can be subverted in quantum systems subjected to dynamical constraints arising from the interplay of different symmetries. In this talk, I will describe how this arises in a quantum spin model derived from a lattice gauge theory. In particular, I will show how dynamical constraints emerge in a certain limit and demonstrate that these constraints lead to a hierarchy of time scales for the dynamics of elementary excitations. I will also discuss recent experimental results that probe precursors of this physics in a closely related model of interacting fermions.
The collective modes in an itinerant electron system are a key manifestation of its quantum correlations. Recent advances in ultrafast pump-probe technologies allow us to access subtle properties of these modes and to manipulate them for control of quantum states and phases. I will demonstrate this by describing a nontrivial topological structure associated with correlation functions of 2D Fermi liquids, which manifests itself in unconventional collective modes, for example “hidden” zero-sound modes that don’t give rise to peaks in the spectral function, but do determine the long-time transient response of the Fermi liquid. I will also present a theory of controlling the soft collective mode that exists near a nematic quantum phase transition via nonlinear phonon excitation. The theory describes both quasi-equilibrium control and quantum quenches, and I will demonstrate its applicability to the unconventional superconductor FeSe.
Recently, entropy measurements of quantum dots where demonstrated based on charge sensitivity. These experiments require temperature variations, and extract entropy changes due to gate voltage variations using Maxwell relations. In addition to describing on going experimental puzzles, I will discuss new directions that these methods open, such as measurements of fractional entropy in frustrated Kondo systems. I will finally discuss a possible approach to measure entanglement entropy in mesoscopic systems, triggered by a relation between entanglement entropy - obtained by tracing out a subsystem - and thermal entropy - produced in the process of a projective measurement.
Hydrodynamics can be defined as a description of how many-particle systems evolve from local equilibrium to global equilibrium. An active question in recent years is how electrons might support new kinds of hydrodynamics in very clean materials. One-dimensional systems often show special behavior because of additional exact or approximate conservation laws, which can be verified by comparison to DMRG-type simulations. These can even lead to unexpected scaling behavior: we discuss the origin of unusual subdiffusive Kardar-Parisi-Zhang dynamics in the quantum Heisenberg spin-half chain and its recent experimental observation at fairly high temperatures in KCuF3. Similar behavior was observed with atoms in a quantum gas microscope. We then explain how even generic (non-integrable) metals in one dimension can show superdiffusive transport because of a competition between Luttinger liquid physics and thermalization. Some closing comments review other routes to unconventional hydrodynamics in experiment, including in higher dimensions.
The discovery of massless Dirac electrons in graphene and topological Dirac-Weyl materials has prompted a broad search for bosonic analogues of such Dirac particles. Recent experiments have found evidence for Dirac magnons in a two-dimensional CrI3 crystal and in a three-dimensional Heisenberg magnet Cu3TeO6. I will describe the results of an inelastic neutron scattering investigation on a stacked honeycomb lattice magnet CoTiO3, which is part of a broad family of ilmenite materials. I will argue that the magnon dispersion relation is well described by a simple magnetic Hamiltonian with strong easy-plane exchange anisotropy. Importantly, a magnon Dirac cone is found along the edge of the 3D Brillouin zone. However, the simplistic model does not capture the entire picture, therefore I will explain some required modifications. Lastly, I will present a perturbative formulation which allows generating an effective two-band bosonic model, similar to graphene, to analyze band touchings in bosonic Hamiltonians. Our results establish CoTiO3 as a model material to study interacting Dirac bosons in a 3D quantum XY magnet, but pose some yet to be resolved intriguing questions.
When cold electrons move in a perfect, static, and stable lattice, according to conventional wisdom, they move like almost free quasiparticles in flat space. Here, we present evidence that this is not entirely correct, and quasiparticles actually move in an emergent curved spacetime.
To this end, we discuss the second order electrical conductivity in materials lacking inversion and time-reversal symmetry, which exhibits a mixed axial-gravitational anomaly. We can explain this surprising result in terms of dynamical deformations of the semiclassical wavepacket as it moves through the periodic lattice potential, thereby establishing a common framework for the appearance of anomalous terms in many response functions. Our proposition that quasiparticles behave essentially like quantum cats has powerful implications for all types of quantum transport and may allow to probe synthetic gravitational fields in a bulk condensed matter setting.
The Landau levels and flat bands of twisted van der Waals heterostructures support a phenomenon known as quantum Hall ferromagnetism, where magnetic orders spontaneously develop in the spin or pseudospin sectors of a non-magnetic material due to strong Coulomb interactions. Quantum hall magnets support a variety of interesting low-energy collective excitations that reveal the nature of the ground states. In this talk, I will describe an all-electrical approach to obtain the dispersion relation ω(k) of spin wave excitations in bilayer graphene using transport devices that integrate a Fabry-Pérot cavity. Our measurements at the n=0 Landau level show evidence of gapless, linearly dispersing spin wave excitations, thereby providing direct evidence for an canted anti-ferromagnetic order with in-plane rotational symmetry. This technique is particularly suited to probe magnetic states formed under challenging experimental conditions such as low temperature and/or in a magnetic field. We anticipate its usage in studying other collective excitations of symmetry-broken ground states in van der Waals materials.
Fu et al, Phys. Rev. X 11, 021012 (2021)
Dynamical properties of a many-body system are determined by its properties as a quantum bath: the systems that thermalize act as an efficient bath, while integrable and many-body localized (MBL) systems fail to do so. I will describe a new approach to quantum many-body dynamics, inspired by the notion of the Feynman-Vernon influence functional (IF), which captures the properties of a quantum bath. I will consider interacting spin systems, and formulate an equation satisfied by their influence functionals. Surprisingly, this equation can be solved exactly for a class of many-body systems – perfect dephasers – which act as Markovian baths on their subsystems. More generally, I will show that, viewed as a fictitious wave function in the temporal domain, influence functional can be described by tensor-network methods. The efficiency of this approach is based on the behavior of temporal entanglement of the IF — a quantity that I will introduce — which remains low in very different physical regimes, including fast thermalization, integrability, and many-body localization. I will also discuss applications to describe dissipation effects. IF approach offers a new lens on many-body non-equilibrium phenomena, both in ergodic and non-ergodic regimes, connecting the theory of open quantum systems to quantum statistical physics.
Based on: Lerose, Sonner, Abanin, Phys. Rev. X 11, 021040 (2021); arXiv:2012.00777; arXiv:2103.13741; arXiv:2104.07607
In unconventional superconductors, the normal state electronic properties often strongly constrain the mechanism of superconductivity, and the electronic structure depends strongly on the crystal structure. I will discuss our recent angle-resolved photoemission (ARPES) studies on two materials where normal state electronic structure as well as crystal structure plays an important role in their propensity towards superconductivity. The first is LaNiGa2, a purported time-reversal-symmetry breaking superconductor, where recent availability of single-crystals has prompted a re-evaluation of the electronic structure and mechanism of superconductivity. The second is WTe2, a material that can be rendered superconducting in several different ways, most of which involve increasing electron density. We discuss changes in electronic and crystal structure upon electron doping.
Realizing topological phases at zero magnetic field has been a longstanding goal since Haldane’s theoretical proposal of the quantum anomalous Hall (QAH) state. My talk will focus on newly discovered QAH states that emerge in twisted bilayer and twisted monolayer-bilayer graphene (tMBG). In contrast to magnetically doped topological insulators, the QAH states in these moiré systems are driven by intrinsic strong interactions, which polarize the electrons into a single moiré miniband with Chern number of C = 1 or 2. Remarkably, the magnetization of these “orbital Chern insulators” (OCI) arises predominantly from the orbital motion of the electrons rather than the electron spin. I will discuss a novel effect originating from the curious magnetic properties of OCIs that enables non-volatile electrical switching of the magnetic and topological orders. Finally, I will present recent studies of the OCIs that emerge at half-fillings of the moiré superlattice unit cell in tMBG. Our observation suggests a topological charge density wave ground state that in addition to spin- and valley-ferromagnetism also spontaneously breaks the moiré superlattice symmetry.
Low-dimensional materials, such as 2D monolayers, 1D nanowires, and 0D quantum dots and molecules, are rich with many-body quantum phenomena. The reduced dimensionality, strong interactions, and topological effects lead to new emergent degrees of freedom of fundamental interest and promise for future applications, such as energy-efficient computation and quantum information. Thermal transport, which is sensitive to all energy-carrying degrees of freedom and their interactions, provides a discriminating probe to identify these emergent excitations. However, thermal measurement in low dimensions is dominated by lattice contributions, requiring an approach to isolate the electronic thermal conductance. In this talk, I will discuss how the measurement of nonlocal voltage fluctuations in a multiterminal device can reveal the electronic heat transported across a mesoscopic, low-dimensional bridge. We use 2D graphene as an electronic noise thermometer, demonstrating quantitative electronic thermal conductance measurement over a wide temperature range in an array of dimensionalities: 2D graphene, 1D nanotubes, 0D localized electron chains, and 3D, microscale bulk materials. I will discuss ongoing work revealing electron hydrodynamics, interaction-mediated plasmon hopping, spin waves in a magnetic insulator, and signs of a crossover from phonon to spin-related transport in a bulk spin liquid candidate material.
Inspired by the first principles results for a bilayer Fe3Sn2 [PRL 125, 026401 (2020)] we obtain low energy tight binding models for this class of materials and show that symmetry considerations generally allow for a low energy isolated band which may acquire a non-zero Chern number. We then introduce short ranged interactions in these models and use exact diagonalization and field theoretic arguments to demonstrate the possibility of realizing exotic phases of electrons like the fractional Chern insulator and spin liquid phases.
Bond-dependent interactions can lead to magnetic frustration even on simple bipartite lattices, which in turn leads to very rich quantum many-body physics as exemplified by the Kitaev model. We demonstrate the existence of such bond dependent interactions in CoNb2O6, a one-dimensional easy axis magnetic material. Through THz spectroscopy and theoretical calculations we find that the magnetic frustration results in rich domain wall dynamics in the low field regime. At higher fields and close to a quantum phase transition, the THz data provide the first direct experimental evidence for the Kramers-Wannier duality. Our work establishes that Co-based magnets provide a rich source of new materials where Kitaev model-like interactions may be explored.
When two van der Waals materials of slightly different orientations or lattice constants are overlaid, a moiré pattern emerges. The moiré pattern introduces a new length scale, many times the lattice constant of the original materials, for Bragg scattering of Bloch electrons in each layer.This gives rise to moiré minibands and rich emergent quantum phenomena. In this talk,I will discuss recent experiments on angle-aligned semiconductor heterobilayers, which exhibit remarkable correlated insulating states [1,2,3]. I will also discuss the prospect of using moiré superlattices as a quantum simulator.
1.Y. Tang et al., Nature 579, 353-358 (2020).
2.Y. Xu et al., Nature 587, 214–218 (2020).
3.C. Jin et al., Nat. Mater. 20, 940-944 (2021).
Weak interlayer coupling in 2-dimensional layered materials such as graphene gives rise to rich mechanical and electronic properties, in particular in the case where the two atomic lattices at the interface are rotated with respect to one another. The reduced crystal symmetry leads to anti-correlations and cancellations of the atomic interactions across the interface, leading to low friction1 and low interlayer electrical transport2. Using our recent nanomanipulation technology, based on atomic force microscopy, we show that combined electro-mechanical characterization can uniquely address open fundamental questions related to electronic charge transport2-3 through stacking faulted structures. To this end, we studied experimentally and theoretically the interlayer charge transport in twisted bilayer graphene systems separately for edges and bulk parts. We find that interlayer edge currents are several orders of magnitude larger than in the bulk and therefore govern the transport up until very large critical diameters depending on the potential across the adjacent layers and the angular mismatch angle. In addition, we show that the strong edge transport across the interface is governed by strong quantum mechanical interference effects as opposed to simple interlayer atomic interactions.
[1] E. Koren et al., Science, 6235 (2015) 679.
[2] E. Koren et al., Nature Nanotech., 9 (2016) 752.
[3] D. Dutta et al., Nature Comm, 11 (2020) 4746.
In this talk I will show how quantum geometric tensor or fidelity susceptibility can be used as a sensitive probe of quantum chaos. In particular, I will show that using this probe one can detect very small integrability breaking perturbations. I will also discuss that ergodic and integrable regimes are generically separated by an intermediate phase characterized by the maximum possible sensitivity of eigenstates to small perturbations and by exponentially slow in the system size dynamics. I will apply this probe to various clean and disordered models and in particular show a strong evidence that many-body localization is unstable in the thermodynamic limit.
Superconductivity is abundant near quantum-critical points, where fluctuations suppress the formation of Fermi liquid quasiparticles and the Bardeen-Cooper-Schrieffer theory no longer applies. Two very distinct approaches have been developed to address this issue: quantum-critical Eliashberg theory and holographic superconductivity. The former includes a strongly retarded pairing interaction of ill-defined fermions, the latter is rooted in the duality of quantum field theory and gravity theory. We demonstrate that both are different perspectives of the same theory. We derive holographic superconductivity in form of a gravity theory with emergent space-time from a quantum many-body Hamiltonian - the Yukawa SYK model - where the Eliashberg formalism is exact. Exploiting the power of holography, we then determine the dynamic pairing susceptibility of the model. Our holographic map comes with the potential to use quantum gravity corrections to go beyond the Eliashberg regime.
In the clean limit, continuous symmetry-breaking quantum phase transitions in 2D Dirac materials such as graphene and surfaces of 3D topological insulators are described by (2+1)D critical Gross-Neveu-Yukawa (GNY) models. In this talk, I will present recent results of the study of the effects of quenched random-mass disorder, both short- and long-range correlated, on the universal critical properties of the Ising, XY, and Heisenberg GNY models. The problem was studied via the application of the replica renormalization group combined with a controlled triple epsilon expansion below four dimensions. Among interesting results, we find new finite-disorder quantum critical and multicritical points and an instance of the supercritical Hopf bifurcation in the renormalization-group flow, which is accompanied by the birth of a stable limit cycle corresponding to discrete scale invariance.
Two-dimensional (2D) materials, composed of single atomic layers, have attracted vast research interest since the breakthrough discovery of graphene. One major benefit of such systems is the simple ability to tune the Fermi level through the charge neutrality point between electron and hole doping. For 2D Superconductors, this means that one may potentially achieve the regime described by Bose Einstein Condensation (BEC) physics of small bosonic tightly bound electron pairs. In my talk I will describe an experiment showing that single layer graphene, in which superconducting pairing is induced by proximity to a low density superconductor, can be tuned from hole to electron superconductivity through an ultra-law carrier density regime. We have studied both experimentally and theoretically the vicinity of this "Superconducting Dirac point" and found an unusual situation where reflections at interfaces between normal and superconducting regions within the graphene, suppress the conductance and, at the same time, Andreev reflections maintain a large phase breaking length. In addition, the Fermi level can be adjusted so that the momentum in the normal and superconducting regimes perfectly match giving rise to ideal Andreev reflection processes.
Graphene-based moiré systems have attracted considerable interest in recent years as a remarkably versatile venue for a variety of correlated phenomena. In addition to insulating and superconducting phases, there is growing evidence that a number of rotationally faulted graphene multilayer systems, such as twisted bilayer and double-bilayer graphene, exhibit electronic nematic order: the spontaneous breaking of lattice rotation symmetry. In this talk I will present recent reports of rotation symmetry in twisted graphene multilayers, focusing in particular on twisted double-bilayer graphene (tDBG), and will discuss a comparison with theory. I will show that a combination of symmetry-based analysis and a microscopic continuum model provides an understanding of the structure of the nematic phase observed in tDBG. Furthermore, I will discuss experimental manifestations, such as a surprising tunability of the orientation of the nematic director. In broader sense, this talk aims to highlight graphene moiré materials as a compelling experimental venue for studying a new type of nematic order distinct from the conventional Ising nematic order.
Motivated by far-reaching applications ranging from quantum simulations of complex processes in physics and chemistry to quantum information processing, a broad effort is currently underway to build large-scale programmable quantum systems. In this talk, I will demonstrate a programmable quantum simulator based on two-dimensional arrays of neutral atoms, featuring strong interactions controlled via excitation into Rydberg states. Using this approach, we realize a quantum spin model with tunable interactions for system sizes ranging from 64 to 256 qubits. We benchmark the system by creating and characterizing high-fidelity antiferromagnetically ordered states, and demonstrate the universal properties of an Ising quantum phase transition in (2+1) dimensions. We then create and study several new quantum phases that arise from the interplay between interactions and coherent laser excitation, experimentally map the phase diagram, and investigate the role of quantum fluctuations.
In the second half of the talk, we investigate non-equilibrium dynamics following rapid quenches in a many-body system. We probe coherent revivals corresponding to quantum many-body scars, and devise a universal model explaining their decay. Remarkably, we discover that scar revivals can be stabilized by periodic driving, which generates a robust subharmonic response akin to discrete time-crystalline order.
Offering a new lens into the study of complex quantum matter, these observations pave the way for investigations of exotic quantum phases, non-equilibrium entanglement dynamics, and hardware-efficient realization of quantum algorithms.
A time crystal should break time translation symmetry in the same sense as a spatial crystal breaks spatial translation symmetry. While this is an intuitively appealing idea, it also runs up against the tendency of macroscopic systems to equilibrate which is why perpetual motion machines are not commonly available. I will discuss the history of this idea and how, in the recent efflorescence of non-equilibrium quantum dynamics a phase with a family resemblance to this idea was discovered. It requires working out of equilibrium, generalizing the idea of a phase and breaking a discrete time translation symmetry - but at the end of it all we have a genuinely new macroscopic and surprisingly robust phenomenon. I will also report on the status of its experimental realization.
One should be amazed with an unreasonable effectiveness of random matrix theory to describe spectral fluctuations in simple non-integrable many-body systems, say one dimensional spin 1/2 chains with local interactions. I will discuss a class of Floquet (periodically driven) quantum spin chains - specifically, dual unitary Floquet circuits - where the random matrix result for the spectral form factor can be derived or even rigorously proven. Several other nontrivial exactly solvable features of the presented models, such as dynamical correlations or entanglement dynamics, will be discussed.
We study the entanglement dynamics and the possible phase transitions in a generic quantum automaton circuit subjected to projective measurements. We design an efficient algorithm which not only allows us to perform large-scale simulation for the Rényi entropy, but also provides a physical picture for the entanglement dynamics, which can be interpreted in terms of a classical bit-string model which belongs to the directed percolation universality class. We study the purification dynamics of a state formed by Einstein-Podolsky-Rosen pairs, and the growth of entanglement starting from a product state. In both cases, we verify numerically that the dynamics is in the universality class of classical directed percolation.
Andreev bound states (ABSs), the quantum many-body electronic states that are localized at Josephson weak-links, provide a platform to explore the interplay of superconductivity, spin-orbit interaction, Coulomb interaction, and magnetism, including in topological regimes. A feature of ABSs is that they carry supercurrent, which imbues the states with routable long-ranged electrodynamics. ABSs are thus suited to being probed by the well-developed circuit quantum electrodynamics (cQED) toolset, which offers microwave-domain measurement and manipulation of quantum states. In this talk, I will describe our implementation of cQED to reveal the spectrum, dynamics, and potential applications of quasiparticles trapped in ABSs hosted in a Josephson semiconductor nanowire. First, I will discuss the use of superconducting resonators for quantitative measurement of microwave response functions and the resulting insights on Coulomb interaction in our ABSs. Second, I will describe the influence of spin-orbit interaction on ABSs and how we leveraged that interaction to realize the Andreev spin qubit, a quantum-coherent supercurrent-carrying spin. Finally, I will argue that these experiments in “fermionic cQED” lay a foundation to explore the exotic physics of Majorana bound states and 2d quantum materials.
The effect of a measurement apparatus on a quantum system can be dramatic, a well known example is the Quantum Zeno Effect where by repeatedly measuring a quantum system it is possible to completely freeze its dynamics into a well defined state. In the many-body context the competition between unitary dynamics and stochastic measurements can give rise to new dynamical phases characterised by qualitatively different entanglement properties and to sharp phase transitions between them.
In this talk I will discuss this measurement-induced criticality in the context of a Quantum Ising chain with continuous monitoring of the transverse magnetization. I will compare different limits of the measurement problem and show how they provide a remarkably similar phenomenology as the measurement strength is increased, namely a sharp transition from a critical phase with logarithmic scaling of the entanglement to an area-law phase. I will argue how the essential features of this problem can be understood by looking at the associated non-Hermitian Quantum Ising model and its subradiance spectral transition.
Entanglement transitions are a new class of phase transitions between states of low (e.g. area-law) entanglement and high (e.g. volume-law) entanglement. The many-body localization transition separating localized eigenstates and thermal eigenstates is an example of such, and interest in MBL inspired the creation of more tractable models featuring entanglement transitions, such as ensembles of "random tensor network states", the focus of this talk. These allow for an exact mapping to statistical mechanics models, but with a difficult to analyze replica limit. I will discuss features of entanglement transitions in a subclass of these models in which the tensor networks have tree geometries and various patterns of disorder, allowing for easier numerical and analytical analysis, drawing connections to results seen in other models of entanglement transitions along the way.
The talk is focused on the origin of our recently observed interfacial-ferroelectricity in hexagonal Boron Nitride (hBN). We stack two layers of the hBN crystal in a parallel lattice orientation, unlike the natural symmetric stacking configurations. We find a stable dipole moment pointing out of the plane, and switch its orientation by scanning a biased tip at the surface. The switching involves a lateral sliding by one atomic spacing between the layers, which is observed directly by surface potential microscopy [1]. If time allows, I will also discuss our efforts to induce intrinsic electric and magnetic gauge-fields in graphene by particular strain-engineering schemes [2].
[1] https://arxiv.org/abs/2010.05182
[2] https://arxiv.org/abs/1909.09991
https://www.tau.ac.il/~moshebs
The quantum spin Hall (QSH) effect, originally observed in time reversal symmetry-protected HgTe quantum wells, is a unique phenomenon where charge carriers propagate along the (1-dimensional) edges of a 2-dimensional sample, their spin degree of freedom being locked to their direction of propagation. A similar system with counter-propagating, spin-polarized edge channels was predicted to occur in graphene in the presence of a perpendicular magnetic field, based on the specificities of Dirac fermions in the quantum Hall regime. Experimentally, the observation of such a state has been hindered for years by the presence of many-body effects stabilizing fully gapped (bulk and edges) phases.
By using substrate-screening engineering, we have recently modified the nature of electron-electron interactions in graphene and uncovered the underlying QSH effect. In this talk, I will introduce strategies to trigger a quantum spin Hall state in graphene from the quantum Hall effect. I will present magneto-transport measurements on high quality BN/graphene/BN/STO samples demonstrating the existence of a QSH state with the expected quantized resistances. The observed spin filtered, helical edge channel transport emerges at moderate magnetic fields (~ 1T), survives up to a temperature higher than 100 K over a micro long. I’ll finally present some recent scanning tunneling spectroscopy measurements that give direct access to the bulk gap related to substrate-screened exchange interaction and allows us to nail down the ground state order at the lattice scale.
L. Veyrat, C. Déprez, A. Coissard, X. Li , F. Gay, K. Watanabe, T. Taniguchi, Z. Han, B.A. Piot, H. Sellier, and B. Sacépé, Science 367, 781 (2020)
The effect of chiral molecules (CHM) adsorbed on conventional singlet-pairing s-wave superconductors was the subject of recent works. Scanning tunnelling microscopy measurements revealed in-gap zero bias conductance peaks (ZBCP) in tunnelling spectra acquired from CHM/Nb hybrid systems, suggesting an induced unconventional order parameter in the Nb surface. NbSe2 flakes with adsorbed chiral molecules demonstrated either ZBCPs or multiple in-gap peaks shifting with magnetic field as Shiba states. Our most recent work employs Muon spin rotation (µSR) measurements to measure the magnetic field depth profile on Nb films with and without adsorbed chiral molecules. The results of these measurements are consistent with triplet superconductivity arising due to a spin active interface between the chiral molecules layer and the Nb film. From all the above, it becomes evident that chiral molecules induce unconventional triplet superconductivity on conventional superconductors which originates on the surface but decays deep into the bulk.
Twisted bilayer graphene and related systems provide an exciting playground for correlated many-body physics. They are characterized by rich phase diagrams, featuring correlated insulating behavior, around integer filling fractions, surrounded by superconducting domes. Signs of additional ordering tendencies, such as high-temperature orders leading to “Dirac revivals” and nematicity, have also emerged. In this talk, I will review some of our recent efforts to explore the complex physics of twisted multilayer graphene systems. In particular, we will discuss the possibility that superconductivity and the correlated insulators of twisted-bilayer graphene are connected by Wess-Zumino-Witten terms. We classify the different possible microscopic realizations of this novel type of mechanism for superconductivity (and insulating behavior), in the presence and absence of additional high-temperature orders, which we classify as well. This leads to constraints on the phase diagram. In the second part of the talk, we will discuss evidence for nematic order based on STM data on twisted double-bilayer graphene and analyze implications for the microscopic form of the nematic order parameter. Finally, we will argue that twisted double-bilayer graphene allows for an unprecedented tunability of the orientation of the nematic director via electric fields.
The emerging field of twistronics, which harnesses the twist angle between two-dimensional materials, represents a promising route for the design of quantum materials, as the twist-angle-induced superlattices offer means to control topology and strong correlations. At the small twist limit, and particularly under strain, as atomic relaxation prevails, the emergent moiré superlattice encodes elusive insights into the local interlayer interaction. In this talk I will introduce moiré metrology as a combined experiment-theory framework to probe the stacking energy landscape of bilayer structures at the 0.1 meV/atom scale, outperforming the gold-standard of quantum chemistry. Through studying the shapes of moiré domains with numerous nano-imaging techniques, and correlating with multi-scale modelling, we assess and refine first-principle models for the interlayer interaction. We document the prowess of moiré metrology for three representative twisted systems: bilayer graphene, double bilayer graphene and H-stacked MoSe2/WSe2. Moiré metrology establishes sought after experimental benchmarks for interlayer interaction, thus enabling accurate modelling of twisted multilayers.
Quantum critical phenomena are believed to play a key role in many strongly correlated materials, including several families of high-temperature superconductors. Theoretically, the problem of quantum criticality in the presence of a Fermi surface has proven to be highly challenging. However it has recently been realized that many models used to describe such systems are amenable to numerically exact solution by quantum Monte Carlo (QMC) techniques, without suffering from the fermion sign problem. I will discuss the status of the understanding of metallic quantum criticality and the progress made by recent QMC simulations. I'll describe the results obtained so far and their implications for superconductivity, non-Fermi liquid behavior, and transport near metallic quantum critical points.
Topological superconductivity, realized as an intrinsic material property or as an emerging property of a hybrid structure, represents a phase of matter where topological constraints and superconductivity coexist. The exchange-statistics (braiding) of its quasiparticles is not bosonic nor fermionic but is rather non-Abelian. Due to their exchange-statistics as well as non-locality these excitations offer a promising route towards fault-tolerant quantum computation. The simplest non-Abelian anyon is the Majorana zero mode with an Ising order. However, since braiding of Ising anyons does not offer a universal quantum gate set, theoretical studies have introduced Parafermion zero modes (PZM), an array of which supports universal topological quantum computation. The primary route to synthesize PZMs involves inducing superconductivity on a fractional quantum Hall effect (FQHE) edge.
In this talk, I will introduce high-quality graphene-based van der Waals devices with narrow superconducting electrode (NbN), in which superconductivity and robust FQHE coexist. We find crossed Andreev reflection (CAR) across the superconductor separating two counterpropagating FQHE edges. Our observed CAR probability of the integer edges is insensitive to magnetic field, temperature, and filling, thereby providing evidence for spin-orbit coupling inherited from NbN enabling the pairing of the otherwise spin-polarized edges. FQHE edges notably exhibit a CAR probability higher than that of integer edges once fully developed. This FQHE CAR probability remains nonzero down to our lowest accessible temperature, suggesting superconducting pairing of fractional charges. These results provide a route to realize novel topological superconducting phases with universal braiding statistics in FQHE–superconductor hybrid devices based on graphene and NbN.
We image the spatial profile of the electric current by using a nitrogen-vacancy scanning tip. Using coherent quantum sensing, we obtain magnetic field resolution of ~10nT and spatial resolution of ~100nm. The current pattern we observe differs substantially from the flat profile of a normal metal, and indicates correlated flow through the semimetal. The pattern also shows non-monotonic temperature dependence, with hydrodynamic effects peaking at ~20 K.
We compare our results to a model which combines ab initio electron scattering rates and the electronic Boltzmann transport equation.
The model shows quantitative agreement with our measurement, allowing us to extract the strength of electron-electron interactions in our material. Furthermore, we conclude that electron interactions are phonon-mediated. This result opens a path for hydrodynamic flow and strong interactions in a variety of new materials.
In recent years, Van der Waals (2D) materials, have attracted increasing attention due to their distinctive physical properties. As layered materials, they have been considered for flexible electronics as they can sustain strain higher than 10% without breaking down, although they are only 1-3 atom thick. In addition, 2D materials usually possess a large density of defects that can govern many of their physical properties. In this talk I will present a specific material from the 2D materials family, transition metal dichalcogenides (TMDC) and the role of defects in these materials. I will show how we can apply non-uniform strain to a suspended Van der Waals material (WS2) and alter the dynamics of excitons and trions. Surprisingly, we find that as we increase the non-uniformity of the strain, we are able to convert the excitons into trions with almost 100% efficiency without any electrostatic gating, due to the presence of defects in TMDC. We also investigate the role of defects in gas sensing and reveal the real mechanism behind gas sensing in TMDC. Our results explain inconsistencies in previous experiments and pave the way towards new types of optoelectronic devices.
In this talk, I revisit topology in solid-state systems from a local perspective. This view highlights qualitative differences in the physical response to local perturbations between topological and trivial systems, from topological semimetals to insulators. We will discuss the role of crystalline symmetry and concretize it by analyzing the effect of local impurities in these different systems. I will show numerical results for electron localization in the presence of chemical potential disorder, where we find rich and unexpected behavior beyond the conventional theory of electron localization.
Elementary excitations in quantum magnets can be typically described in terms of long-lived quasiparticles, either simple magnons (spin waves), or more exotic fractionalized excitations such as spinons. In general, when multiple quasiparticles are present, they interact, and in a strongly correlated system, they interact strongly.
In this talk I will discuss signatures of interactions between quasiparticles that show up in the dynamical spin correlations of antiferromagnets in presence of a magnetic field. I will first focus on our study of the antiferromagnetic spin-1/2 chain, a paradigmatic model of strongly correlated systems which is known to host fractionalized spinon excitations. I will address both the low and high magnetization regimes. In the low magnetization regime, in the gapless phase, we find that the marginally irrelevant backscattering interaction between the spinons leaves a distinct signature in the transverse dynamical susceptibility creating a non-zero gap between two branches of excitations at small momentum. In the high magnetization regime, close to the saturation field, we show that interactions between magnons lead to a formation of two-magnon bound states which leave a sharp feature in the transverse correlations. I will then address higher-dimensional systems and argue that these observations are not unique to 1D.
Advances in controlling electron correlations in transition metal dichalcogenides have opened a new frontier of many-body physics in two dimensions. A field where these materials have yet to make a deep impact is antiferromagnetic spintronics—a relatively new research direction promising technologies with fast switching times, insensitivity to magnetic perturbations and reduced cross-talk. The theory behind the electrical switching of antiferromagnets is premised on the existence of a well-defined broken symmetry state that can be rotated to encode information. A spin glass is in many ways the antithesis of this state, characterized by an ergodic landscape of nearly degenerate magnetic configurations, freezing into its final distribution in a manner that is seemingly bereft of information.
In this talk, I will show that the coexistence of spin glass and antiferromagnetic order allows a novel mechanism to facilitate the switching of the intercalated transition metal dichalcogenide Fe1/3±δNbS2, which is rooted in the electrically stimulated collective winding of the spin glass. We find that remarkably low current densities of the order of 104 A/cm−2 can reorient the magnetic order in a single pulse activation. The local texture of the spin glass opens an anisotropic channel of interaction that can be used to rotate the equilibrium spin-orientation of the antiferromagnetic state. Moreover, I will present the first experimental observation of the predicted spin glass collective modes, known as Halperin-Saslow spin waves. The use of a spin glass’ collective dynamics to electrically manipulate antiferromagnetic spin textures has never been applied before, opening the field of antiferromagnetic spintronics to many more material platforms with complex magnetic textures.
Traditionally, magnetism in solids deals with ordering patterns of the electron magnetic dipole moment, as probed, for instance, via neutron diffraction. However, in quantum materials can also exhibit more complex forms of symmetry breaking, involving higher-order magnetic or electric multipoles. In this talk, I will discuss our recent theoretical proposal for Ising octupolar order in d-orbital systems, which explains a wide range of experiments in certain 5d transition metal oxides with spin-orbit coupling. Such multipolar symmetry broken states might serve as potential platforms for information storage. I will also highlight new experiments on realizing 2DEGs with j=3/2 fermions, which our theory suggests as candidates for multipolar stripe ordering.
The electrical resistivity of Fermi liquids (FLs) displays a quadratic temperature (T) dependence because of electron-electron (e-e) scattering. For such collisions to decay the charge current, there are two known mechanisms: inter-band scattering (identified by Baber) and Umklapp events. However, in two dilute metals: metallic strontium titanate (SrTiO3) and Bi2O2Se, resistivity was found to display T2 behavior in absence of either of these two mechanisms. In SrTiO3, the presence of soft phonons and their possible role as scattering centers raised the suspicion that T-square resistivity in SrTiO3 is not due to e-e scattering. More recently, Bi2O2Se, a layered semiconductor with only hard phonons, displayed T-square resistivity well below the degeneracy temperature, denying the possible role played by soft phonons. We observed a universal scaling between the prefactor of T2 resistivity and the Fermi energy in SrTiO3 and Bi2O2Se and across various FLs, which is an extension of the Kadowaki-Woods plot. Our results imply the absence of a satisfactory theoretical basis for the ubiquity of e-e driven T-square resistivity in Fermi liquids.
Recent years have seen a discovery of a number of polar metals, materials in which itinerant electrons coexist with an inversion-symmetry breaking order. In this talk I will discuss the properties of such systems when the polar ordering temperature is tuned to zero – i.e. at a polar quantum critical point. I will show that unlike other examples of quantum criticality, the soft modes of the polar order do not couple easily to electrons. I will consider the general mechanisms, by which the electrons can interact with the critical modes of the polar order and discuss the consequences of these interactions. In particular, I will demonstrate that exotic metallic phases, including non-Fermi liquid ones, can be realized in multiband systems in the vicinity of band crossings, such as nodal lines or Weyl points. Finally, I will discuss the role of nonlinear interactions in quantum critical polar metals and connect these to the yet unresolved mystery of superconductivity in doped SrTiO3.
We show that a one-dimensional topological superconductor can be realized in carbon nanotubes, using a relatively small magnetic field. Our analysis relies on the intrinsic curvature-enhanced spin-orbit coupling of the nanotubes, as well as on the orbital effect of a magnetic flux threaded through the nanotube. Tuning experimental parameters, we show that a half-metallic state may be induced in the nanotube. Coupling the system to an Ising superconductor, with an appreciable spin-triplet component, can then drive the nanotube into a topological superconducting phase. The proposed scheme is investigated by means of real-space tight-binding simulations, accompanied by an effective continuum low-energy theory, which allows us to gain some insight on the roles of different terms in the Hamiltonian. We calculate the topological phase diagram and ascertain the existence of localized Majorana zero modes near the edges. Moreover, we find that in the absence of a magnetic field, a regime exists where sufficiently strong interactions drive the system into a time-reversal-invariant topological superconducting phase.
In this talk I will unapologetically discuss two entirely different topics. Sorry.
In the first part, I will talk about a model of random dynamics in systems with topological degrees of freedom. Usually, coupling a many-body quantum system to a thermal environment destroys the quantum coherence of its state, leading to effectively classical dynamics at the longest time scales. I will discuss a system that avoids this classical fate because some of the information in its quantum state is topologically protected. I will show how a system of Majorana fermions coupled to a thermal bath and relaxing toward its ground state exhibits a new universality class of dynamics that can be described by an exact mapping.
In the second part, I will discuss the effect of "prestige bias" in job applications using a simple stat mech model. Using the language of Bayesian inference across iterated rounds of evaluation, I will show how there can be a first-order transition in the degree of advantage conferred by a prestigious affiliation.
The fractional quantum Hall (FQH) effect manifests when a large perpendicular magnetic field is applied to a two-dimensional electron system with sufficiently strong electron-electron interactions. The quasiparticle excitations of this highly correlated phase are neither fermions nor bosons; they are anyons possessing anyonic statistics. Braiding of non-Abelian anyons in certain FQH phases was an early proposal for topologically protected quantum computation.
A Fabry–Pérot (FP) interferometer of fractional quantum Hall edge modes may allow direct measurement of anyonic statistics [1] . Experimentally, interferometry in the QHE have been limited only to a few groups having access to high mobility GaAs materials. Recent developments in VdW heterostructures [2,3] have allowed us to cross
the mobility threshold and observe FQH states at moderate magnetic fields. However, since graphene is a gapless semiconductor, quantum point contact attempts have been difficult, slowing progress towards realizing interferometers.
I will present recent developments in our group: graphene-based gate-defined FP interferometers operating in the Quantum Hall effect. Using Landau Level (LL) gaps to direct edge states and create barriers, we demonstrate gate-tunable quantum point contacts (QPCs), which act as beam splitters of edge modes in the integer quantum Hall
(IQH) and FQH regimes. By cascading two QPCs in series, we formed FP interferometers and measure Aharanov-Bohm interference in the IQH regime owing to the unique electrostatic environment offered by our heterostructure. We extract edge mode velocities and phase coherence lengths of the first three electron-doped LLs and compare gate vs. etch defined interferometers.
[1] Nayak, C. et al. Non-Abelian anyons and topological quantum computation. Reviews of Modern Physics 80, 1083–1159 (2008)
[2] Zibrov, A. A. et al. Tunable interacting composite fermion phases in a half-filled bilayer-graphene Landau level. Nature 549,
360–364 (2017).
[3] J. I. A. Li. et al. Even denominator fractional quantum Hall states in bilayer graphene. Science 358, 648-652 (2017).
We study the nonperturbative pair production of particles induced by strong rotating electric fields. The excitations by tunneling become strongly chirality dependent due to nonadiabatic geometric effects. The threshold, i.e., Schwinger limit, even vanishes for particles with an optically allowed chirality. We explain these phenomena through the twisted Landau-Zener model proposed by M. V. Berry, and provide a quantitative understanding in terms of the geometric amplitude factor. As a condensed matter application, we make a nonperturbative analysis on the optically induced valley polarization in 2D Dirac materials. Furthermore, in 3D Dirac and Weyl materials with spin-orbit coupling, we predict the generation of a nonlinear spin or charge current in the direction of the laser propagation.
It is often thought that emergent phenomena in topological phases of matter are destroyed when tuning to a critical point. In particular, topologically protected edge states supposedly delocalize when the bulk correlation length diverges. We show that this is not true in general. Edge states of topological insulators or superconductors remain exponentially localized---despite a vanishing band gap---if the transition increases the topological index. This applies to all classes where the topological classification is larger than Z2, notably including Chern insulators. Moreover, these edge states are stable to disorder, unlike in topological semi-metals. This new phenomenon is explained by generalizing band (or mass) inversion---a unifying perspective on topological insulators---to kinetic inversion. In the spirit of the bulk-boundary correspondence, we also identify topological invariants at criticality, which take half-integer values and separate topologically-distinct universality classes by a multi-critical point. This work enlarges the scope of topological protection and stability by showing that bulk energy gaps can be unnecessary. Experimental probes and stability to interactions are discussed.
In a quantum spin liquid, the spins remain disordered down to zero temperature, and yet, it displays topological order that is stable against local perturbations. The Kitaev model with anisotropic interactions on the bonds of a honeycomb lattice is a paradigmatic model for a quantum spin liquid. I will discuss the effects of a magnetic field and report on our discovery of an intermediate gapless spin liquid sandwiched between the known gapped Kitaev spin liquid and a polarized phase. We show that the gapless spin liquid harbors fractionalized neutral fermionic excitations, dubbed spinons, that remarkably form a Fermi surface in a charge insulator. I will also discuss the current status of experiments on candidate Kitaev QSLs.
Reference: Patel and Trivedi, PNAS 116, 12199 (2019)
The information age we live in is supported on a physical under-layer of electronic hardware, which originates
in condensed matter physics research. The mighty progress made in silicon based technology seemed endless.
However, with the smallest feature size of transistors reaching down to mere 5 nm, this technology is reaching
an unavoidable physical limit. This calls for exploration of new alternatives.
Neuromorphic inspired systems are making fast progress. But this is based either on dedicated hardware made
with conventional electronics, or in software, such Deep Neural Networks, running in conventional computers.
Resistive switching phenomena opens the way to explore a technological disruptive solution, namely, implement
simple devices with the required functionalities to directly build neuromorphic systems. In this talk we shall describe
recent efforts towards making artificial neurons and synapses using transition metal oxides, including Mott strongly
correlated systems.
Refs
[Review] Challenges in materials and devices for resistive-switching-based neuromorphic computing; Journal of Applied Physics 124, 211101 (2018)
Subthreshold firing in Mott nanodevices; Nature, 569 (7756), pp.388-392 (2019).
An ultra-compact leaky-integrate-and-fire model for building spiking neural networks;; Scientific Reports, 9, 11123 (2019).
Biologically relevant dynamical behaviors realised in an ultra-compact neuron model; Frontiers in Neurosciences (accepted).
Graphene has been known as an excellent material for long-distance spin transport due to its weak spin-orbit coupling (SOC). However, the same reason makes graphene an adverse candidate for different spintronics applications in which strong SOC is required, such as the Datta and Das spin transistor proposal or spin-charge interconversions. It has recently been predicted theoretically that SOC can be induced in graphene so that exotic spin-orbit phenomena such as spin Hall effect (SHE) or Rashba-Edelstein effect can be obtained. In our work, by using van der Waals heterostructure-based lateral spin valves, we experimentally demonstrate spin-to-charge conversion (SCC) due to SHE in graphene via spin-orbit proximity with MoS2, a transition metal dichalcogenide (TMD)1,2. The combination of long-distance spin transport and large spin-to-charge conversion in a van der Waals heterostructure gives rise to a hitherto unreported efficiency for the spin-to-charge voltage output. We performed similar experiments in graphene in proximity with WSe2. Here we observed gate-tunable SHE with SCC efficiency larger than in some of the best SCC materials such as topological insulators. Using a similar approach, we observed large multidirectional SCC in Weyl semimetal MoTe23. Here, due to the low symmetry of MoTe2 crystal, we detect, along with the conventional SCC, an unconventional SCC where the spin polarization and the charge current are parallel. Our finding enables the simultaneous conversion of spin currents with any in-plane spin polarization in one single experimental configuration. All in all, these exceptional effects obtained by the unique properties of 2D materials open exciting opportunities in a variety of future spintronic applications.
- C. K. Safeer, et al. Nano Letters, 19, 2, 1074-1082 (2019)
- C.K. Safeer, et al. Nano Letters, doi: 10.1021/acs.nanolett.0c01428 (2020)
- C. K. Safeer, et al. Nano Letters, 19, 12, 8758-8766 (2019)
Gauge theories play a central role in the theoretical description of unconventional phases of matter that go beyond the standard paradigms of quantum statistical mechanics. While in high-energy physics, gauge fields correspond to fundamental particles, in condensed matter theory they are typically emergent and are invoked as an effective description of the low-energy degrees of freedom. Notable examples include spin-liquids, doped Mott insulators, and the fractional Hall effect, among others. In my talk, I will present a sign-problem free quantum Monte Carlo study of a lattice model hosting 'orthogonal' fermions coupled to an Ising-Higgs gauge theory. Our model provides a simple yet highly non-trivial example of electron fractionalization, which, crucially, remains numerically tractable. We uncover a particularly rich phase diagram arising from strong correlations between gauge and matter fields. In particular, we find that in the background of pi-flux lattice an orthogonal semi-metal (OSM) forms with gapless Dirac fermion excitations. With tuning of parameters, the OSM undergoes a confinement transition, in which symmetry breaking and confinement are coincident. We construct a field-theoretical description of the transition involving condensation of a matrix Higgs field. The critical theory is predicted to sustain an emergent and enlarged local (gauge) and global symmetries. We provide numerical evidence supporting this prediction. We also find that the physical (gauge-neutral) spectral function in the OSM phase comprises four fermion pockets, which smoothly evolve to a 'large' Fermi-surface upon approach to a Fermi liquid phase. The reconstruction of the Fermi-surface does not involve any form of translational symmetry breaking, in violation of the Luttinger sum-rule.
Symmetry protected topological (SPT) phases are gapped phases of matter that cannot be deformed to a trivial phase without breaking the symmetry or closing the bulk gap. Here, we introduce a new notion of a topological obstruction that is not captured by bulk energy gap closings in periodic boundary conditions. More specifically, given a symmetric boundary termination we say two bulk Hamiltonians belong to distinct boundary obstructed topological phases (BOTPs) if they can be deformed to each other on a system with periodic boundaries, but cannot be deformed to each other in the open system without closing the gap at at least one high symmetry surface. BOTPs are not topological phases of matter in the standard sense since they are adiabatically deformable to each other on a torus but, similar to SPTs, they are associated with boundary signatures in open systems such as surface states or fractional corner charges. In contrast to SPTs, these boundary signatures are not anomalous and can be removed by symmetrically adding lower dimensional SPTs on the boundary, but they are stable as long as the spectral gap at high-symmetry edges/surfaces remains open. We show that the double-mirror quadrupole model of [Science, 357(6346), 2018] is a prototypical example of such phases, and present a detailed analysis of several aspects of boundary obstructions in this model. In addition, we introduce several three-dimensional models having boundary obstructions, which are characterized either by surface states or fractional corner charges. We also provide a general framework to study boundary obstructions in free-fermion systems in terms of Wannier band representations (WBR), an extension of the recently-developed
band representation formalism to Wannier bands. WBRs capture the notion of topological obstructions in the Wannier bands which can then be used to study topological obstructions in the boundary spectrum by means of the correspondence between the Wannier and boundary spectra. This establishes a form of bulk-boundary correspondence for BOTPs by relating the bulk band representation to the boundary topology.
The quest for new functionalities in quantum materials has recently been extended to non-equilibrium states. In the context of superconductivity, examples have included the generation of transient superconductivity above the thermodynamic transition temperature, the excitation of coherent Higgs-mode oscillations, and the optical control of the interlayer phase in the cuprates. In this talk, I will propose that a prompt quench into a transient superconducting state from a metal induces large Higgs fluctuations of the order parameter, and will demonstrate that these fluctuations give rise to the amplification of light at frequencies below the superconducting gap. I will show new measurements on K3C60, in which these predictions are verified experimentally.
How nonequilibrium systems relax to steady states is a central topic in many-body dynamics. Of particular interest are universal features of such dynamics. In this talk I will discuss the decay rates of a chaotic quantum system coupled to noise. I will do so within a model where the Hamiltonian and (possibly) the system-noise coupling are described by random N x N Hermitian matrices. The focus will be on the spectral properties of the resulting Liouvillian superoperator that governs the time evolution of the system's density matrix. We find that the asymptotic decay rate to the steady state generically remains nonzero in the thermodynamic limit, i.e., the spectrum of the superoperator is gaped as N approaches infinity. However, the size of the gap depends nontrivially on the strength of the coupling to the environment. Initially, the gap increases with the dissipation strength but then reaches a maximum and declines upon additional strengthening of the coupling. Furthermore, a sharp spectral transition takes place: for dissipation beyond a critical value, the slowest decaying eigenvalues of the Liouvillian separate from the main cloud of eigenvalues and become isolated "midgap" states. I will discuss some of the implications of these findings.
n the seminar, I will give an introduction to unconventional 2D Ising superconductors. Two examples are: lead on a silicon substrate and monolayer transition metal dichalcogenides. I will detail the peculiar properties of the superconducting state that emerges in the presence of Ising spin-orbit coupling and an in-plane magnetic field. Orbital physics, singlet-triplet mixed Cooper pairs, and the effect of impurities will be presented.
The standard theory of bosonic superfluids assumes that vortices are analogous to monopole point charges, and are therefore characterized by quantum numbers that describe only their position and (vorticity) charge. This textbook description is derived from a cornerstone of fluid mechanics -- the Kelvin theorem -- which invokes the conservation of circulation in the dynamics of ideal incompressible fluids. However, in this talk I will show that in a broad class of superfluids, vortices are characterized by an additional degree of freedom, affecting significantly various physical phenomena. This new degree of freedom, which endows to vortices a dipole-like nature, stems from a non-analytic "core reconstruction" which takes place in superfluids with a large healing length, yielding a set of excited states of the vortex state with low energies and long lifetime. From a mathematical perspective, these new solutions of the Gross-Pitaevskii equations are “weak solutions”. Namely, they minimize the action but do not satisfy the corresponding variational equations over a set of zero measure. The consequences of the non-analytic core reconstruction regarding the transport properties of velocities in disorder media and the instability of Abrikosov’s lattice will be presented.
A new atomistic calculation methodology has been recently
developed which extends lattice dynamics to disordered and "real" solids,
called Nonaffine Lattice Dynamics (NALD), based on the concept of nonaffine
displacements [1], which are ubiquitous in all real materials (crystals
with defects and grain boundaries, glasses etc) and are deeply connected to
local topology of the lattice in terms of the statistical degree of local
centrosymmetry [2,3]. This framework also allows one to predict the
dynamical mechanical response of "real" materials from the underlying
vibrational spectrum (VDOS) across the entire time-scale spectrum, thus
providing a possible solution for the well known problem of bridging time
and length scales in the dynamical simulation of materials at the atomic
level. The method has been shown to be predictive on the example of a model
glassy material of Kremer-Grest polymer chains [4], and recent results
extend the description at the atomistic level for real polymer glasses
(polyethylene, pDCPD, pNBOH, etc) [5]. I will also present recent results
aiming at rationalizing the effect of disorder and anharmonicity on phonons
in solids [6,7], with implications for superconductivity in amorphous
materials [8].
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[2] R. Milkus and A. Zaccone, Phys. Rev. B 93, 094204 (2016).
[3] B. Cui, A. Zaccone, D. Rodney, J. Chem. Phys. 151, 224509 (2019)
[4] V.V. Palyulin, C. Ness, R. Milkus, R. Elder, T. W. Sirk, and A.
Zaccone, Soft Matter 14, 8475-8482 (2018).
[5] R. M. Elder, A. Zaccone, T. W. Sirk, ACS Macro Letters 8, 1160 (2019).
[6] M. Baggioli and A. Zaccone, Phys. Rev. Lett. 122, 145501 (2019).
[7] M. Baggioli and A. Zaccone, arXiv:1911.03351 (2019).
[8] M. Baggioli, C. Setty, A. Zaccone, arXiv:2001.00404 (2020).
Twisted bilayer graphene near the magic angle exhibits remarkably rich electron correlation physics, displaying insulating, magnetic, and superconducting phases. Here, using measurements of the local electronic compressibility, we reveal that these phases originate from a high-energy state with an unusual sequence of band populations. As carriers are added to the system, rather than filling all the four spin and valley flavors equally, we find that the population occurs through a sequence of sharp phase transitions, which appear as strong asymmetric jumps of the electronic compressibility near integer fillings of the moiré lattice. At each transition, a single spin/valley flavor takes all the carriers from its partially filled peers, "resetting" them back to the vicinity of the charge neutrality point. As a result, the Dirac-like character observed near the charge neutrality reappears after each integer filling. Measurement of the in-plane magnetic field dependence of the chemical potential near filling factor one reveals a large spontaneous magnetization, further substantiating this picture of a cascade of symmetry breakings. The sequence of phase transitions and Dirac revivals is observed at temperatures well above the onset of the superconducting and correlated insulating states. This indicates that the state we reveal here, with its strongly broken electronic flavor symmetry and revived Dirac-like electronic character, is a key player in the physics of magic angle graphene, forming the parent state out of which the more fragile superconducting and correlated insulating ground states emerge.
see attached file
The interface between the oxide insulators LaAlO3 and SrTiO3 hosts a gate tunable 2D electron gas that also becomes SC at low temperatures. It has been demonstrated that the 2DEG can be confined to create devices such as gate defined SQUIDs or a single electron transistor.
In effect the Physics of the SrTiO3 substrate play a major role in the behavior of the interface. SrTiO3 undergoes a structural phase transition at 105K resulting in a dense network of domains separated by nano-meter thick twin walls.
I will discuss our recent findings, where we used scanning SQUID microscopy to map the spatial distribution of conduction at the interface. Images of the interface showed channels of modulated current flow, superconductivity and magnetic signal. The domain walls change their location with thermal cycles and with the application of back gate voltage. In addition we observed that the domain wall behavior changes with application of stress. These findings open exciting possibilities for normal and superconducting devices based on domain walls
The generic existence of structural tunneling two-level systems (TLSs) in amorphous solids was postulated by the "Standard Tunneling Model" to explain the remarkable low temperature universality known by now to exist across the different classes of disordered solids. Being ubiquitous at low energies, TLSs dominate low energy noise, and as such restrict performance of quantum nanodvices including superconducting qubits, nanomechanical oscillators and photon detectors. Recently, the coupling of TLSs to superconducting qubits and microresonators has facilitated novel experimental studies of TLSs, including detailed studies of individual TLSs and studies of the TLS glass out of equilibrium. In this talk I will discuss some of these recent experiments, what insights they give with regard to the nature and characteristics of TLSs, and with regard to possibilities to mitigate the deleterious effects of TLSs in microresonators and qubits.
Topological Dirac and Weyl semimetals have an energy spectrum that hosts Weyl nodes appearing in pairs of opposite chirality. We will discuss the effects of inhomogeneities such as lattice deformations or a non-uniform magnetization on the response and the energy spectrum of such materials. These can arise either naturally or by engineering and can result, for example, in a space-dependent Weyl node separation which can be interpreted as an emergent axial vector potential. Consequently, emerging pseudo fields can derive equilibrium bound currents proportional to their strength that average to zero over the sample, and the interplay of pseudo fields and external fields and can redistribute charge or chiral charge in real as well as in momentum space via mechanisms such as the chiral anomaly. In addition, we will discuss how the topological surface states of Weyl semimetals, the Fermi arcs, can be re-interpreted as an n=0 pseudo-Landau level resulting from a pseudo-magnetic field confined to the surface, and how a bulk pseudo-magnetic field creates pseudo-Landau levels interpolating in real space between Fermi arcs at opposite surfaces. Hallmarks of pseudo-fields can appear in transport, and we discuss ways to detect and quantify them and contrast these with effects arising from external fields. Finally, we will discuss the manifestations of these ideas in metamaterials.
It is well known that quantum Hall conductivity in the presence of a constant magnetic field is expressed through the topological TKNN invariant. The same invariant is responsible for the intrinsic anomalous quantum Hall effect. We propose a generalization of these expressions to the QHE in the presence of non-uniform external fields. Our approach is based on purposely developed lattice Wigner-Weyl formalism, giving the Hall conductivity in terms of Weyl symbols of the two-point Green's function. It is shown to be topological invariant in the phase space of the system.
Periodic driving of quantum systems introduces new dimensions for controlling quantum states. In my talk I will show how a time-periodic drive could be considered as adding a dimension, and how simple driven systems could acquire topological features of higher-dimensional systems. Also, I will discuss how using several drives in parallel could produce new types of topological states supporting multiple Majorana modes.
In my talk, I will tell about my interest in emergent phenomena and how the extreme tunability of 2D materials is an unprecedented tool-box to investigate emergent and novel physical phenomena. I will discuss the importance of scanning probes and my plan to establish a new scanning platform based on 2D materials. I will then talk about my work in Columbia on an air-sensitive van der Waals superconductor 2H-NbSe2. How we established a new technique to contact and preserve the material. That we resolved a 20-year-old open question regarding the nature of an anomalous metallic state in the superconducting dome of thin-film superconductors (in collaboration with Danny Shahar, Weizmann). We found that in both 2H-NbSe2 and amorphous InOx, the anomalous metallic state was a non-equilibrium steady-state driven by electronic noise. With no added noise, I will present measurements of the dissipation phase diagrams of superconductivity in the two dimensional (2D) limit, layer by layer, down to a monolayer in the presence of temperature (T), magnetic field (B), and current (I) in 2H-NbSe2. Our results show that the phase-diagram strongly depends on the thickness, even in the 2D limit. At four layers we can define a finite region in the I-B phase diagram where dissipationless transport exists at T=0. At even smaller thicknesses, this region shrinks in area until, in a monolayer, it approaches a single point defined by I=B=T=0. In applied field, we show that time-dependent-Ginzburg-Landau (TDGL) simulations that describe dissipation by vortex motion, qualitatively reproduce our experimental I-B phase diagram. We show that by using non-local transport and TDGL calculations that we can engineer charge flow and create phase boundaries between dissipative and dissipationless transport regions in a single sample, demonstrating control over non-equilibrium states of matter.
If time permits, I will show new results which we understand as a blockade of vortex transport due to thermal fluctuations.
[1] Nano Letters 2018
[2] Science Advances 2019
[3] Nature Physics 2019
[4] In review PRL, arXiv:1909.08469
In systems with many local degrees of freedom, high-symmetry points in the phase diagram can provide an important starting point for the investigation of the broader phase diagram. In systems with both spin and orbital (or valley) degrees of freedom, SU(4)-symmetric models can serve as such a starting point.
In this talk I will discuss SU(4) quantum antiferromagnets on the triangular lattice, that arise from Mott-insulating phases of fermions with four flavors. I will consider different fillings of the SU(4) fermions, which lead to different representations of SU(4) on each site.
First, I will discuss the case of two fermions per site (i.e. half-filling), which corresponds to the 6-dimensional representation of SU(4). I will argue that in this case, the low energy properties of the model can be captured by an effective dimer model. I will then present exact diagonalization studies of the dimer model indicating that the ground state breaks translation invariance, forming a valence bond solid (VBS) with a 12-site unit cell.
In the second part of my talk, I will turn to the case of a single fermion per site, corresponding to the fundamental representation of SU(4). Based on numerical simulations using the density matrix renormalization group (DMRG) method, supported by field-theoretical arguments, I will provide evidence for a gapless liquid with an emergent Fermi surface in the ground state of the system.
I will conclude with a discussion of SU(4)-symmetry breaking perturbations in both cases.
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Mathematics, Physics and Chemistry meet most harmonically through symmetry. Since Noether’s groundbreaking theorem, which established the connection between symmetry and a physical property, physicists have relied on symmetry arguments to understand the natural laws. On the other hand, the role of topology has been to some extent under-appreciated and linked to specific exotic phenomena. However, the two are not far apart: topology regards constants of motion of the space of states as a whole, reflected in nonlocal (as opposed to local) properties of physical systems, and it is reflected by anomalous behavior at spatial defects such as boundaries or vortices. In particular topology in band structure theory remained elusive for the largest portion of its history. Only the last decade has witnessed an enormous effort to bridge this gap, with remarkable success in the prediction of new physical phenomena. In this talk, I will focus on the interplay between topology and crystal symmetry in band structure theory. I will show how spatial (unitary) symmetries which were once expected to lead to trivial extensions of already known topological phenomena, can, in fact, introduce complexities that often defy our intuition but at the same time can be used to make topological phenomena more accessible to technological devices. I will show examples of how crystalline topological phases may cease to exist with periodic boundaries; how crystalline symmetry (and its breaking) may lead to the confinement of topological bands on crystalline defects such as stacking faults; and explored how the local symmetric environment may be manipulated through intrinsic mechanisms to construct surface-based topological devices such as a Majorana interferometer to probe non-abelian statistics.
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I will present a variational method for studying the ground state of a quantum many-body Hamiltonian, which treats the correlation functions as variational parameters. This numerical approach is based on approximating the positivity of the density matrix in a controlled manner, and allows for obtaining the (approximate) ground state in polynomial time. Unlike the conventional variational principle which provides an upper bound on the ground-state energy, in this approach one obtains a lower bound on the ground-state energy. I will demonstrate the method on several one-dimensional spin 1/2 Hamiltonians. Possible extensions of the method will be discussed.
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Majorana zero modes are non-Abelian quasiparticles that emerge on the edges of topological phases of superconductors. Evidence of their presence have been reported in transport measurements on engineered superconducting-based nanostructures. In this talk I will discuss transport signatures of dynamical Majorana manipulation. In the first part of the talk I will show that adiabatic exchange of a pair of Majorana zero modes leads to quantized heat pumping. This feature is inherent to the presence of Majorana zero modes and I will discuss its robustness against temperature, voltage bias and the detailed coupling to the contacts. Next I will discuss conductance measurements of a Floquet Majorana wire. For this purpose I will relate the scattering matrix of the time dependent system to a fictitious stroboscopic scattering matrix. Using this simplified scattering problem I will calculate the conductance in the 4 topologically distinct phases that occur in the Floquet Kitaev wire.
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See attached file for abstract
Topological electronic materials host exotic boundary modes, that cannot be realized as standalone states, but only at the boundaries of a topologically classified bulk. Topological Weyl semimetals, whose bulk electrons exhibit chiral Weyl-like dispersion, host Fermi-arc states on their surfaces. The Fermi-arc surface bands disperse along open momentum contours terminating at the surface projections of bulk Weyl nodes with opposite chirality. Such reduction of the surface degrees of freedom by their splitting and segregation to opposite surfaces of the sample, that reoccurs in all topological states of matter and even exhibited by topological defects [1], provides topological protection from their surface elimination. We have confirmed the Weyl topological classification of both the inversion symmetry broken compound TaAs [2] and the time reversal symmetry broken Co3Sn2S2 [3] by spectroscopic visualization of their Fermi-arc surface states through the interference patterns those electrons embed in the local density of states. This has allowed us to examine their unique nature and level of protection against perturbations. In TaAs the Fermi arc bands are found to be much less affected by the surface potential compared to trivial bands that also exist on its surfaces. In contrast, in Co3Sn2S2 the dispersion of the topological Fermi-arc bands, and even their inter-Weyl node connectivity, are found to vary with the surface termination. This discrepancy seems to elude towards a tradeoff between momentum extended Fermi-arc bands, as those in Co3Sn2S2, which have more pronounced experimental signatures but are more susceptible to surface perturbations, to short extent ones, as those in TaAs, which are harder to detect but are more robust.
[1] Abhay Kumar Nayak et al, “Resolving the Topological Classification of Bismuth with Topological Defects” Science Advances (in press); arXiv:1903.00880
[2] Rajib Batabyal et al, “Visualizing weakly bound surface Fermi arcs and their correspondence to bulk Weyl fermions” Science Advances 2, e1600709 (2016)
[3] Noam Morali et al, “Fermi-arc diversity on surface terminations of the magnetic Weyl semimetal Co3Sn2S2” Science 365, 1286 (2019)
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A hallmark of the phase diagrams of correlated electronic systems is the existence of multiple electronic ordered states. In many cases, they cannot be simply described as independent competing phases, but instead display a complex intertwinement. A prime example of intertwined states is the case of primary and vestigial phases. While the former is characterized by a multi-component order parameter, the fluctuation-driven vestigial state is characterized by a composite order parameter formed by higher-order, symmetry-breaking combinations of the primary order parameter. This concept has been widely employed to elucidate nematicity in both iron-based and cuprate superconductors. In this talk, I will present a group-theoretical framework, supplemented by microscopic calculations, that extends this notion to a variety of phases, providing a general classification of vestigial orders of unconventional superconductors and density-waves. Electronic states with scalar and vector chiral order, spin-nematic order, Ising-nematic order, time-reversal symmetry-breaking order, and algebraic vestigial order emerge from this simple underlying principle. I will present a rich variety of possible phase diagrams involving the primary and vestigial orders, and discuss possible realizations of these exotic composite orders in different materials.
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The concept of electronic quasiparticles, as introduced by Landau, is one of the cornerstones of the theory of quantum many-body systems. However, a growing number of recent experiments in strongly correlated quantum materials have forced us to confront the existence of quantum matter for which the concept of electronic quasiparticles does not apply. Inspired by the rich phenomenology of the parent states of numerous high-temperature superconductors, I will describe some recent progress in our understanding of metallic states that do not admit a quasiparticle description but that nonetheless have a sharply-defined Fermi surface. I will present some experimental results on unconventional transport properties of magic-angle twisted bilayer graphene and comment on their possible connections with the rest of my talk.
An exact formula for the temperature dependent Hall number of metals is derived. It is valid for nonrelativistic fermions or bosons, with an arbitrary potential and interaction. This dc transport coefficient is proven to (remarkably) depend solely on equilibrium susceptibilities, which are more amenable to numerical algorithms than the conductivity. An application to strongly correlated phases is demonstrated by calculating the Hall sign in the vicinity of Mott phases of lattice bosons.
Charge transport in Anderson insulators is governed by disorder and Coulomb interactions and their competition leads to glassy behavior. This has been experimentally observed in heavily-doped semiconductors where the disorder necessary to render them Anderson-insulators is large enough to reduce electronic relaxation rates many decades below inter-sites transition-times associated with their conductivity. In this talk, use will be made of this characteristic feature to demonstrate another inherent feature of disordered insulators; a time-dependent local perturbation may induce a nonlocal response over scales that may extend much further than the localization length.
Defects in the atomic lattice of solids are sometimes desired. For example, atomic vacancies, single ones or more elaborated defective structures, can generate localized magnetic moments in a non-magnetic crystalline lattice. Increasing their density to a few percent magnetic order can appear. Furthermore, usual graphite samples, independently whether they are from natural sources, Kish or highly oriented pyrolytic graphite, bulk or powder samples, have interfaces. These interfaces occur between twisted regions with Bernal or rhombohedral or between Bernal and rhombohedral stacking orders. I discuss old and new results - from single interfaces (bilayer graphene) to multigraphene and bulk or powder graphite samples - that speak for the existence of superconductivity with a broad distribution of critical temperatures. Especial emphasis will be given to new results, which indicate the existence of granular superconductivity even above room temperature.
In the Hofstadter problem, an orbital flux of the order of a flux quantum goes through each unit cell. There are some special features when one considers a honeycomb lattice. For a certain class of hopping problems obeying a nonunitary "chiral " symmetry, when 1/q quanta of flux penetrate each unit cell, the central two bands touch at 2q Dirac points. These touchings are protected by lattice symmetries and the "chiral" symmetry. When simple short-range interactions are introduced, we find a plethora of phases which have charge, magnetic, and/or bond order. I will compare this to Kharitonov's phase diagram in the continuum limit for tiny fields.
Over the last decade we have introduced and perfected so-called spatiotemporal encoding (SPEN) methodologies to collect multidimensional NMR spectra and images in a single scan. This talk will focus on the new spin physics underlying these methodologies, and on the potential of these techniques to deliver superior imaging information, particularly in comparison with established methods such as spin-echo EPI in the realm of diffusion MRI, and with fast-spin-echo/RARE in multi-shot anatomical MRI. Achievements that will be described include the acquisition of diffusion in vivo images at <100µm in-plane resolutions in challenging preclinical areas, hitherto unavailable characterizations of cancerous tissues in animals and in patients, and sub-mm anatomical measurements in humans with unprecedented acceleration factors.
Nonequilibrium conditions are traditionally seen as detrimental to the appearance of quantum coherent many-body phenomena in condensed matter systems, and much effort is often devoted to their elimination. Recently this approach has changed: It has been realized that driven-dissipative Markovian dynamics of the Lindblad type could be used as a resource. By proper engineering of the reservoirs and their local couplings to a system, one may drive the system towards desired quantum-correlated steady states, even in the absence of internal Hamiltonian dynamics.
An intriguing category of nontrivial equilibrium many-particle phases are those which are distinguished by topology rather than by symmetry. Natural questions thus arise: Which of these topological states can be achieved as the result of purely dissipative Lindblad-type dynamics? Could they display novel behavior, with no equilibrium analogues? Besides the fundamental importance of these issues, they may offer novel routes to the realization of topologically-nontrivial states in quantum simulators, especially ultracold atomic gases.
In our previous work we have provided a no-go theorem determining which Gaussian (noninteracting) topological states are achievable as the unique steady states of local dissipative dynamics, as well as a general recipe for creating, identifying and classifying the achievable states in ultracold atoms and related systems. After reviewing this work I will discuss two newer developments:
1. What are the resulting transport properties, such as persistent currents and conductivity? We find that, in contrast with equilibrium systems, the usual relation between the Chern topological number and the Hall conductivity is broken. We explore the intriguing edge dynamics and elucidate under which conditions the Hall conductivity is quantized.
2. We show that dissipation-induced topology is robust against weak disorder, but may break down under strong enough disorder, with a critical point separating the two regimes. Surprisingly, disordered dissipation leads to a critical point similar to the equilibrium one, while disordered Hamiltonian in the presence of dissipation leads to a novel critical point with significantly different critical exponents.
The study of the magnetic-field driven superconductor-insulator transition in thin superconducting films at low temperatures reveals an unusual insulator whose conductivity seems to approach zero at a finite temperature, while its current-voltage characteristics are bistable, indicating that the electrons decouple from the phonon system, In parallel, the superconducting state at lower magnetic fields exhibits a broad range where metallic behavior is seen down to the lowest temperatures.
As a motivation I review several recent experiments on transition metal dichalcogenides.
Then I will present what is known about these materials and what makes there response to the magnetic field special.
I will then review the spin-orbit coupling based on symmetry considerations.
Then I'll focus on the singlet to triplet conversion induced by the Zeeman field.
Finally I'd discuss the effect this has on the critical field.
I will describe our experiments probing the tunneling spectra of the layered superconductor NbSe2 using van-der-Waals tunnel junctions. The junctions, characterized by a hard gap,enable the measurement of high resolution spectra and the probing of sub-gap states. Usingthis method, we make a number of key findings: (i) We find a clear signature of a 2nd , low-energy gap, in NbSe2 . By measuring the spectra vs. in-plane and out-of-plane fields, we can place values on its diffusion constant and coherence length. (ii) We probe NbSe2 spectra at the ultrathin limit. We find that thin NbSe2 retains its gap up well above the Pauli limit, a consequence of Ising protection. (iii) Finally, we find that van-der-Waals tunnel barriers host defects which undergo proximity with the underlying superconductor, giving rise to Andreev bound states. As NbSe2 survives very high in-plane fields, we can track the defect state energy which exhibits a field-dependent singlet-doublet transition. I will discuss possible origins of the singlet-ground state behavior of defect-related Andreev bound states.
In quantum Hall edge states, charge fractionalization can occur due to injection of charge pulses, which decompose into eigenmodes propagating at different velocities. The method of non-equilibrium bosonization allows to distinguish the regimes of quasi-particle creation and local equilibration, and to characterize the final prethermalized state. Comparison with experimental results gives good agreement and allows to determine the velocities of edge modes.
We present a self-consistent theory of the steady-state electron distribution in metals under continuous-wave illumination which treats, for the first time, both thermal and non-thermal effects on the same footing. We show the number of non-thermal electrons (i.e., the deviation from thermal equilibrium) is a very small effect, namely, that the power that ends up generating these non-thermal electrons is many orders of magnitude smaller than the amount of power that leads to regular heating.
Using this theory, we re-examine the exciting claims on the possibility to enhance chemical reactions with these non-thermal electrons. We identify a series of errors in the temperature measurements in some of the most famous paper on the topic which led their authors to under estimate regular heating effects. As an alternative, we show that a very simple theory, based on just simple heating, can explain the published experimental data with excellent accuracy.
Cavity quantum electrodynamics (CQED) is the study of the interaction between matter and photons confined in a cavity. In the Jaynes-Cummings model the matter is described using the two-level approximation, and only a single cavity mode is taken into account. The interaction has a relatively large effect when the ratio E/ℏω between the energy gap E separating the two levels and the cavity mode photon energy ℏω is tuned close to unity.
The talk is devoted to the study of the light-matter interaction in the nonlinear regime using three different CQED systems. In the first experiment a Josephson flux qubit serves as a two-level system and a superconducting resonator as the cavity [1]. We experimentally find that the cavity response exhibits higher order resonances (called super-harmonic resonances) in the nonlinear regime when the ratio E/ℏω is tuned close to an integer value larger than unity. Moreover, we observe is significant narrowing in the cavity resonance that is induced by qubit driving, and which is attributed to quantum jumps. In the second experiment the interaction between a spin ensemble of diphenylpicrylhydrazyl (DPPH) molecules and a superconducting resonator is explored in the region where E/ℏω≫1 [2]. We find that the cavity response is significantly modified when the spins are intensively driven close to their Larmor frequency. Retardation in the response of the spin ensemble gives rise to effects such as cavity mode cooling and heating. In the third experiment the interaction between nitrogen-vacancy (NV) and nitrogen substitutional (P1) spin defects in diamond and a superconducting resonator is studied [3]. We find that nonlinearity imposes a fundamental limit upon sensitivity of CQED-based spin detection. Moreover, multi-photon resonances are observed when the ratio between the Larmor frequency and the driving frequency is tuned close to an integer value and the CQED system is tuned close to its resonance E/ℏω=1. In addition, other experimentally observed multi-photon resonances are attributed to the dipolar coupling between NV and P1 defects.
References
1. Eyal Buks, Chunqing Deng, Jean-Luc F.X. Orgazzi, Martin Otto and Adrian Lupascu, Phys. Rev. A 94, 033807 (2016).
2. Hui Wang, Sergei Masis, Roei Levi, Oleg Shtempluk and Eyal Buks, Phys. Rev. A 95, 053853 (2017).
3. Nir Alfasi, Sergei Masis, Roni Winik, Demitry Farfurnik, Oleg Shtempluck, Nir Bar-Gill and Eyal Buks, Phys. Rev. A 97 (2018).
At its core, Quantum Mechanics is a theory developed to describe fundamental observations in the spectroscopy of solids and gases. Despite these practical roots, however, quantum theory is infamous for being highly counterintuitive, largely due to its intrinsically probabilistic nature. Neural networks have recently emerged as a powerful tool that can extract non-trivial correlations in vast datasets. They routinely outperform state-of-the-art techniques in language translation, medical diagnosis and image recognition. It remains to be seen if neural networks can be trained to predict stochastic quantum evolution without a priori specifying the rules of quantum theory. I will show how we trained a recurrent neural network to infer the individual quantum trajectories associated with the evolution of a superconducting qubit under unitary evolution, decoherence and continuous measurement from raw observations only. The network extracts the system Hamiltonian, measurement operators and physical parameters. It is also able to perform tomography of an unknown initial state without any prior calibration. This method has potential to greatly simplify and enhance tasks in quantum systems such as noise characterization, parameter estimation, feedback and optimization of quantum control.
Arxiv:1811.12420
Quantum fluids of matter with long range, anisotropic interactions display rich emergent collective phenomena. A prominent example is the dipole-dipole interaction, which has recently been addressed by a growing community, both from atomic physics as well as from condensed matter physics, with the latter being focused on dipolar quantum fluids of two-dimensional excitons, and very recently, on the introduction of interacting dipolar polaritons. These strongly interacting dipolar exciton and polariton systems offer opportunities to explore new collective phenomena which are currently inaccessible with atomic dipolar gases, and to demonstrate new types of quantum devices on the level of two-particle interaction.
In this talk I will present several recent results in systems of dipolar excitons and polaritons. These include strong experimental evidence for the dynamical formation of a robust dark quantum liquid phase of dipolar excitons in a bilayer system. This observation is corroborated by a surprising theory predicting a remarkable stabilization of a dense dark-spin exciton Bose-Einstein condensate, driven by particle correlations due to the strong dipolar interactions. Also, I will report on the first observation of a formation of an attractive polaron-like many-body correlated state of vertically coupled dipolar exciton fluids. Finally, I will introduce recent experiments showing formation of flying electrically polarized dipolar-polaritons (dipolaritons) in optical waveguides, resulting in a very large, electrically tunable enhancement of the polariton-polariton interactions, a result promising for future implementations of a dipolar polariton blockade.
Over the last decade we have introduced and perfected so-called spatiotemporal encoding (SPEN) methodologies to collect multidimensional NMR spectra and images in a single scan. This talk will focus on the new spin physics underlying these methodologies, and on the potential of these techniques to deliver superior imaging information, particularly in comparison with established methods such as spin-echo EPI in the realm of diffusion MRI, and with fast-spin-echo/RARE in multi-shot anatomical MRI. Achievements that will be described include the acquisition of diffusion in vivo images at <100µm in-plane resolutions in challenging preclinical areas, hitherto unavailable characterizations of cancerous tissues in animals and in patients, and sub-mm anatomical measurements in humans with unprecedented acceleration factors.
Many body systems involving strongly interacting electrons exhibit various rich and interesting physical states, such as Mott insulators, superconductors, heavy Fermions and etc. Optical properties serve as an important tool to study these correlations and their resulting collective excitations. In my talk I will briefly review several examples of our experimental observations regarding: I. Pairing symmetry, energy gap, superfluid stiffness and phase collective modes in superconducting thin films [1,2,3]. II. Hybridization of localized and conduction energy bands and their optical plasmons in heavy Fermion systems [4]. III. The interplay between the coherent and incoherent sectors of the dynamic conductivity of doped Mott insulator systems [5]. I will then dwell into the latter example and show the in-plane dynamic and static charge conductivity of electron doped Sr2IrO4 using optical spectroscopy and DC transport measurements. I will demonstrate the similarity of the optical signature for a pseudo-gap in several systems. Based on these similarities, and the absence of a correlation between superconductivity and pseudo-gap in the doped iridate compound I will argue that the pseudo-gap is a signature of the presence of residual correlations inherited from the insulating anti-ferromagnetic state.
1. U. S. Pracht, N.B. et al., Phys. Rev. B 93, 100503(R) (2016).
2. U. S. Pracht et al., Phys. Rev. B 96, 094514 (2017).
3. N.B. et al., EPL (Europhysics Letters) 104, 67006 (2013).
4. N.B. et al., Phys. Rev. B 94, 235101 (2016).
5. K. Wang, N.B. et al., Phys. Rev. B 98, 045107 (2018).
We will describe our theoretical and experimental work towards understanding the joint activated dynamics exhibited by two superconducting quantum oscillators. The open quantum systems approach can describe a cavity mode oscillator which is strongly coupled to a superconducting qubit in the strongly coherently driven dispersive regime. We will start by introducing previous work ranging from the quantum optical stochastic methods, exact solutions in phase space and methods of non-equilibrium field theory. Next we will explain the challenge of extending the application of these methods to two oscillators and to the non-semi-classical regime. We will survey our early work on this problem which started by earlier theoretical work on the use of strongly driving pulses for quantum state detection [1,2,3]. Recent collaboration with experiments have encouraged us to apply a range of methods and we uncovered some surprising dynamical behaviour of the including a joint bistability of the generalised Jaynes-Cummings model [4,5], meta-stable dark states [6] and critical slowing down of the system [7] which are properties of the bistability in two degrees of freedom.
[1] E. Ginossar and Lev S Bishop and D. I Schuster and S. M Girvin, Protocol for high fidelity readout in the photon blockade regime of circuit QED,Phys. Rev. A 82,022335 (2010)
[2] Lev S. Bishop, Eran Ginossar, and S. M. Girvin, Response of the Strongly Driven Jaynes-Cummings Oscillator, Phys. Rev. Lett. 10 , 100505 (2010)
[3] M. D. Reed, L. DiCarlo, B. R. Johnson, L. Sun, D. I. Schuster, L. Frunzio, R. J. Schoelkopf, High-Fidelity Readout in Circuit Quantum Electrodynamics Using the Jaynes-Cummings Nonlinearity, Phys. Rev. Lett. 173601 (2010)
[4] Th. K. Mavrogordatos, G. Tancredi, M. Elliott, M. J. Peterer, A. Patterson, J. Rahamim,2 P. J. Leek, E. Ginossar, and M. H. Szymanska, Simultaneous Bistability of a Qubit and Resonator in Circuit Quantum Electrodynamics, Phys. Rev. Lett. 040402 (2017)
[5] Matthew Elliott and Eran Ginossar, Applications of the Fokker-Planck equation in circuit quantum electrodynamics, Phys. Rev. A 94, 043840 (2016)
[6] Th. K. Mavrogordatos, F. Barratt, U. Asari, P. Szafulski, E. Ginossar, and M. H. Szymańska ,Rare quantum metastable states in the strongly dispersive Jaynes-Cummings oscillator, Phys. Rev. A 97, 033828 (2018)
[7] P. Brooks, G. Tancredi et al., in preparation.
The most obvious and distinctive feature of an amorphous solid is its heterogeneous microscopic structure. A central issue is how such disorder governs the elastic properties of an amorphous solid so that it has different behavior from its crystalline counterpart. I will show how such disorder on the microscale determines the elastic properties on long length scales. This theoretical approach ultimately allows us to control a material’s elastic properties and to understand how a material ages and stores memories.
I start by studying the change in an amorphous solid’s elastic properties upon the removal of a single bond. I show that the change in moduli, which has a broad and universal shape, is uncorrelated for different imposed strains.Thus, by selectively removing a small number of bonds, the precise global and local elastic behavior of the solid can be controlled. This in turn suggests that small changes in bond properties, which occur naturally as a solid ages, can dramatically alter the solid’s elastic response; the history of imposed strains is encoded in the non-linear response and the aging process, usually considered to be detrimental, can be harnessed to design materials with novel desired properties.
In this talk I shall review our work on the gas-liquid transition of 2D excitons in GaAs quantum wells. Below a critical temperature and above a critical density this system undergoes a phase transition to a liquid state. By employing a variety of optical spectroscopy tools we map the phase diagram of the system, and show that the liquid is a Bose Einstein condensate of dark excitons (exciton state which is not coupled to light).
We find that the liquid phase is fragmented below Tc and the spatial fluctuations decay as the temperature is lowered, manifesting a continuous transition from a Bose glass to BEC.
Interactions between electrons in solids are responsible for a wide variety of physical phenomena such as magnetism, superconductivity Mott insulators and more. Understanding interactions between electrons, and manipulating them to stabilize desired electronic phases have been the research focus of the strongly correlated electrons community in the past few decades. Ultrafast optics is a unique experimental tool where strong ultrashort pulses of light can be used both to probe a multitude of electronic phenomena, and to excite and manipulate the properties of the electronic system, driving it away from its equilibrium state. In this talk I will show how ultrafast optical techniques can be used to probe spin spin correlations and modify magnetic interactions in a Van der Waals ferromagnet. CrSiTe3 is composed of van der Waals bonded sheets of ferromagnetically interacting Heisenberg spins that, in isolation, would be impeded from long range order by the Mermin-Wagner theorem. I will show that CrSiTe3 evades thislaw via a two-step crossover from two- to three-dimensional magnetic short range order above its Curie temperature (Tc = 31 K), manifested through two previously undetected totally symmetric distortions at T2D ~ 110 K and T3D ~ 60 K serving as a direct probe for measuring intarlayer and interlayer spin-spin correlations. Having understood the interplay between short range correlations and the magnetoelastic distortions I will show how optically induced electron transfer could be used to enhance the magnetic super-exchange interaction and how this manipulation can be detected by optically probing generation of coherent phonons.
Electronic nematicity – spontaneous quadrupole deformation of the electronic dispersion – has been discovered in a variety of strongly correlated quantum materials. Of special interest are metals near a nematic quantum critical point, such as iron-based superconductors. I will discuss how the non-conservation of a nematic order parameter links long-range quantum fluctuations and short-range anisotropy. This link appears in the nonzero uniform dynamical susceptibility in polarized Raman scattering, and reveals important microscopic details about the iron-based superconductors. The interplay of long- and short- scales also gives rise to a unique form of superconducting pairing and leads to a highly anisotropic superconducting gap that evolves strongly with temperature.
The many body localized phase provides the first and only example of a generic quantum interacting system that does not reach thermal equilibrium, and thereby violates the most fundamental principles of statistical physics. In the last decade, an enormous theoretical effort was invested in understanding the nature of this phase. It has attracted a similar deal of attention also within the experimental community, as it has the potential of storing information about initial states for long times and it allows the application of driving protocols without heating the system to an infinite temperature.
A key ingredient for achieving the MBL phase is randomness. The roots of this phase lie within the phenomenon of Anderson localization, where non-interacting particles form a localized non-ergodic phase. It is the question regarding the fate of Anderson localization in the presence of interactions that plants the seed for the discovery of the MBL phase.
We pose the question whether randomness is indeed an essential ingredient in achieving generic non-ergodic interacting phases. We propose the idea that the essential ingredient for MBL is localization, which does not necessarily mean disorder. We analyze the spectral and the dynamical properties of one-dimensional interacting fermions and spins in the presence of both disorder and linear potential. We show that by considering these two different localizing mechanisms, i.e., disorder and linear fields, one may construct a two-dimensional phase diagram which hosts a connected non-ergodic (MBL) phase.
We also examine the effect of periodic driving on the dynamics of many-body systems and show how such driving provides a general framework for controlling the transport properties in the system, as well establish mobile composite particles. We demonstrate that by including successive driving terms, it is possible to completely suppress the motion of particles, and effectively localize the many-body system, without the presence of disorder.
While at this point we can not make conclusive statements about the nature of this phase in higher dimensions, the lack of randomness and the low sensitivity to dimensionality may render these systems more accessible to a theoretical investigation in dimensions larger than one. Furthermore, we make steps towards employing well studied machine learning techniques to address the issue of finite size. Although we don’t show explicitly if it is possible to use such techniques to numerically solve larger system sizes, we show that a mapping of the disorder realization to the level statistics is easily learned.
1. Y. Baum, Evert P. L. van Nieuwenburg and Gil Refael, ”From Dynamical Localization to
Bunching in interacting Floquet Systems”, SciPost Phys. 5, 017 (2018).
2. Y. Baum, Evert P. L. van Nieuwenburg and Gil Refael, ”From Bloch Oscillations to Many
Body Localization in Clean Interacting Systems”, arXiv:1808.00471 (2018).
Plasma is a partially of fully ionized gas, comprising the 99 % of the visible matter in the universe. The physical plasma is scarce in nature on Earth, however is continuingly gains popularity in the science and technology fields, since being described by Langmuir in 1920s. Currently, multiple directions of plasma related research are prominent in a variety of scientific and technological fields: from the quest for fusion energy, through plasma propulsion for space-travel, to plasma medicine, agriculture and material fabrication and functionalization. The potential for applications is indeed vast, however so are the scientific challenges that emerge in the context of the above applications. In this talk I will concentrate on two exciting areas: plasma-assisted synthesis of nanomaterials and the plasma interactions with liquids, biological tissues and cells. Both of these research areas are incredibly complex, owing to multiple different interacting species with a wide variety of energies, sizes and chemical potentials, interacting across all the possible phases of matter, from gas to liquid, solid and crystal phases, presenting many exciting physical challenges. Notably, both these areas consider multiphase environment, posing many scientific challenges, such as: large density and temperature, unknown plasma characteristics, the control of the reactivity transfer at the plasma–liquid/solid interface, interfacial charging, droplet transport and self-organization of plasmas in contact with liquid/solid. Similar challenges occur in the interface of plasma with biological tissues and cells. The phenomenon of plasma self-organization into coherent structures and patterns is particularly interesting, because it modulates the characteristics of the plasma, influencing the density and the energy of the charged particles, chemical composition, species transport along and across the plasma-tissue/liquid interface, radiation and electric fields. The aim of this seminar is to introduce these selected frontiers of plasma science, discuss their promise, challenges and the experimental approach to their investigation.
Periodic drives are a common tool to control physical systems, but have a limited applicability because time-dependent drives generically lead to heating. How to prevent the heating is a fundamental question with important practical implications. We address this question by analyzing a chain of coupled kicked rotors, and find two situations in which the heating rate can be arbitrarily small: (i) marginal localization, for drives with large frequencies and small amplitudes, (ii) linear stability, for initial conditions close to a fixed point. In both cases, we find that the dynamics shows universal scaling laws that allow us to distinguish localized, diffusive, and super-diffusive regimes. The marginally localized phase has common traits with recently discovered pre-thermalized phases of many-body quantum-Hamiltonian systems, but does not require quantum coherence.
Time-periodic driving serves as a promising tool for exploring new phases of quantum matter In particular, it can be used to induce topological phenomena in conventional, non-topological systems. In this talk, I will discuss semiconductors driven with a resonant coherent light – necessary ingredients for the creation of Floquet topological insulators. I will analyze the steady-states emergent in such systems in the presence of natural dissipation mechanisms including phonon-relaxation, photo-emission and Auger processes, as well as coupling to external leads. I will show that despite the highly non-equilibrium nature of these systems, by judicially choosing the properties of the external environments, their steady states can exhibit some of the hallmark phenomena of contemporary condensed matter: topological transport and phase transitions.
The analysis of topological excitations in interacting many-body systems often provides important insights into quantum phases and phase transitions. Seminal examples are the (thermal) BKT transition as well as the (quantum) superfluid-Mott insulator transition, which are naturally described in terms of vortices. More recently, such a ‘dual’ formulation has proven extremely illuminating in the study of exotic phases of matter that host fractional excitations, e.g., in topological phases and quantum magnets. In my talk, I will review the dual description of conventional phases of matter and explain how exotic, fractionalized phases are captured within this approach. I will then generalize these dualities to fermionic systems, and discuss the implications for the half-filled Landau level and strongly interacting surfaces of topological insulators. Finally, I will describe how two-dimensional Dirac fermions (and their symmetries) can be mapped onto interacting bosons.
The large variety of phenomena exhibited by the interface between two band-insulators LaAlO3 and SrTiO3 has attracted intense research activity in recent years. In this talk, I discuss the results of electronic transport properties of two-dimensional electron gas formed at the LaAlO3/SrTiO3 heterointerface.
LaAlO3/SrTiO3 interface exhibits superconductivity at low temperatures. Transport studies reported in this system seem to indicate that the superconductivity is two dimensional in nature, with a charge carrier density dependent TBKT. We have experimentally studied low frequency resistance fluctuations (noise) and its higher order statistics near the superconducting transition region by varying the temperature, gate voltage and magnetic field. From the analysis of the higher order statistics of resistance fluctuations, we find large non-Gaussian components (NGC) in resistance fluctuations appear near TBKT, which signifies strong correlations among interacting vortices in the system. The NGC are found to be completely absent above TC. Theoretical simulations indicate that the large non-Gaussian resistance fluctuations are manifestation of a percolative transition of a Josephson-coupled superconducting network.
LaAlO3/SrTiO3 interface exhibits coexistence of superconductivity and ferromagnetism, and they are gate tunable. In general, superconductivity and ferromagnetism are antagonistic to each other. So, the appearance of two co-existing phase at the interface has opened up a new direction of research in condensed matter physics. In this talk, I discuss the results of perpendicular magnetic field dependence of sheet resistance above superconducting transition temperature (T/TBKT = 2). From our experiments, we identify a gate voltage tunable Lifshitz transition in this system. We observed a novel transient superconducting state (TSS) in the presence of a magnetic field applied perpendicular to the interface. We find that the TSS appeared concomitantly with a Lifshitz transition as a consequence of the interplay between ferromagnetism, superconductivity and the finite relaxation time of the in-plane magnetization in this system.
We demonstrate that the resistance fluctuations (noise) have strikingly different features on either side of the Lifshitz transition. Below the Lifshitz transition, noise is dominated by carrier density fluctuations arising from trapping-detrapping of charge carriers from defects in the underlying SrTiO3 substrate. Above the Lifshitz transition, we propose that the noise presumably originate from the scattering of carriers from different available conduction channels.
We employed multiple in situ characterization techniques in a dilution refrigerator to probe superconductivity in single crystals of SrTi1-xNbxO3 under continuously tunable strain. We find dramatic changes in the superconducting transition temperature. Microscopic scenarios of this phenomenon will be discussed.
The doped 1D Kondo Lattice describes complex competition between itinerant and magnetic ordering. The numerically computed wave vector-dependent charge and spin susceptibilities give insights into its low-energy properties. Similar to the prediction of the large N approximation, gapless spin and charge modes appear at the large Fermi wave vector. The highly suppressed spin velocity is a manifestation of “heavy” Luttinger liquid quasiparticles. A low-energy hybridization gap is detected at the small (conduction band) Fermi wave vector. In contrast to the exponential suppression of the Fermi velocity in the large-N approximation, we fit the spin velocity by a density-dependent power law of the Kondo coupling. The differences between the large-N theory and our numerical results are associated with the emergent magnetic Ruderman–Kittel–Kasuya–Yosida interactions.
Quantum mechanics sets an upper bound on the amount of charge flow as well as on the amount of heat flow in ballistic onedimensional channels. The two relevant upper bounds, which combine only fundamental constants, are the quantum of the electrical conductance, Ge=e2/h, and the quantum of the thermal conductance, Gth=κ0T=(π2kB2/3h)T. Remarkably, the latter does not depend on the particles charge, particles exchange statistics, and is expected also to be insensitive to the interaction strength among the particles. However, unlike the elative ease in observing the quantization of the electrical conductance, measuring accurately the thermal conductance is more challenging.
The universality of the Gth quantization in 1D ballistic channels was demonstrated for weakly interacting particles: phonons [1], photons [2], and in an electronic Fermi-liquid [3]. I will describe our recent experiments with heat flow in a strongly interacting system of 2D electrons in the fractional quantum Hall regime. In the lowest Landau level we studied particle-like states (v<½) and the more complex hole like states (½<v<1), which carry counter propagating neutral (zero net charge) modes [4]. We found quantization of Gth=κ0T in all these abelian states. In the first-excited Landau level (2<v<3), we concentrated on the even-denominator v=5/2 state, and found fractional quantization of the thermal conductance, Gth=½κ0T, a definite mark of a non-abelian state harboring Majorana excitations [5].
1. K. Schwab, et al., Nature 404, 974 (2000)
2. M. Meschke, et al., Nature 444, 187 (2006)
3. S. Jezouin, et al., Science 342, 601 (2013)
4. M. Banerjee et. al., Nature 545, 75 (2017)
5. M. Banerjee et. al., arXiv: 1710.00492
A new method to measure the superconducting stiffness tensor , without subjecting the sample to magnetic field, is applied to La1.875Sr0.125CuO4 (LSCO). The method is based on the London equation
, where
is the current density and
is the vector potential. Using rotor free
and measuring
via the magnetic moment of superconducting rings, we extract
. The technique, named Stiffnessometer, is sensitive to very small stiffness, which translates to penetration depth on the order of a few millimeters. We apply this method to two different LSCO rings: one with the current running only in the CuO2 planes, and another where the current must cross planes. We find different transition temperatures for the two rings, namely, there is a temperature range with two-dimensional stiffness. The Stiffnessometer results are accompanied by Low Energy mSR measurements on the same sample to determine the stiffness anisotropy at favorable temperatures.
In this talk I shall review our work on the gas-liquid transition of 2D excitons in GaAs quantum wells. Below a critical temperature and above a critical density this system undergoes a phase transition to a liquid state. By employing a variety of optical spectroscopy tools we map the phase diagram of the system and show that the liquid is a Bose Einstein condensate. We find that the excitons condense in the lowest energy state which is not coupled to light (“dark excitons”).
Strongly disordered superconductors in a magnetic field display many characteristic properties of type-II superconductivity— except at low temperatures where an anomalous linear T-dependence of the resistive critical field Bc2 is routinely observed. This behavior violates the conventional theory of superconductivity, and its origin remains a long-standing puzzle. In a combined experimental and theoretical effort, we conducted systematic measurements of the critical magnetic field and current on amorphous indium oxide films of various levels of disorder. Surprisingly, our measurements show that the Bc2 anomaly near zero-temperature is accompanied by a clear mean-field like scaling behavior of the critical current. Theoretically, we show that these are consequences of the vortex-glass ground state and its thermal fluctuations. This theory further predicts the linear-T anomaly to occur in films as well as bulk superconductors with a slope that depends on the normal-state sheet resistance—in agreement with experimental data. Thus, our combined study reveals universal low-temperature behavior of Bc2 in a large class of disordered superconductors.
The past decade has witnessed intensive research aimed at investigating the role of band-structure topology and discovering topological materials. The topology provides a new aspect to understand the band-structure and reveal exotic phenomena, such as topological surface states. In this talk, I will focus on recently-discovered Weyl semimetals. Besides the unique Fermi arc surface states, I will introduce the transport and optical phenomena in realistic materials, which include the linear response (e.g. anomalous Hall and spin Hall effects) and nonlinear response (second-harmonic generation and dc photocurrents) induced by the Weyl band structure.
Refs. Annu. Rev. Condens. Matter Phys. 8, 337-354 (2017); Nature Physics 11, 728 (2015); arXiv:1708.08589, and arXiv:1803.00562.
I shall give theoretical explanations of two surprising experimental observations of universal behaviour in semiconductor systems.
The fist concerns the dependence of photoconductivity $G$ upon light intensity $I$. It is typically found that $G=I^\gamma$. Simple kinetic theory indicates that we should expect $\gamma=1$ or $\gamma=1/2$, but experimentally values close to $\gamma =3/4$ or $\gamma=2/3$ are often observed, with $I$ varying over several decades. I shall present a new explanation for these universal exponents.
The second universality concerns exciton spectroscopy in heterostructures. The linewidth $W$ of the absorption line and the Stokes shift $S$ of the luminescence peak relative to the absorption peak are found to be related by $S/W=0.6$ in most systems for which both values are published. This ratio is independent of the degree of disorder and of the composition of the semiconductors forming the heterostructure, with $W$ varying over two decades. I shall also give a quantitative explanation of this result.
Finally I point out what these two phenomena have in common.
The two dimensional electron liquid formed at the (111) interface between SrTiO3 and LaAlO3 is
a laboratory for studying electronic properties in tunable correlated hexagonal systems.
Symmetry changes imposed by the interface and by various structural transitions in the bulk can
affect the electronic properties at the interface. In addition, this system can be smoothly tuned
from the superconductor deep into the insulator regime.
The normal state properties of the (111) LaAlO3/SrTiO3 interface are indicative of contributions
from electron-type and hole-type charge carriers. The latter are also consistent with the polar
structure of this interface. Upon applying gate voltage to add electrons, a band with a higherspin
state gets populated, resulting in a six-fold anisotropic magnetoresistance [1].
Superconductivity is observed in a dome-shaped region in the carrier density – temperature
phase diagram. The upper critical field is strongly anisotropic and exceeds the Clogston-
Chandrasekhar limit. This suggests strong spin-orbit interaction so. Surprisingly, so is also
nonmonotonic as a function of gate voltage as found both from analysis of the superconducting
properties and of the weak antilocalization measurements [2].
Finally, in the depleted region we can probe the highly insulating regime, where the sheet
resistance is significantly larger than the quantum one. Despite the large resistance, the
interface exhibits the sharp increase in resistance under applied magnetic field characteristic of
a superconductor to insulator transition. By use of electrostatic gating and magnetic fields, the
sample is tuned from the metallic region, where supeconductivity is fully manifested, deep into
the insulating state. Through examination of the field dependence of the sheet resistance and
comparison of the response to fields in different orientations we show that vortex-like
fluctuations are responsible for the transition in this material and that these fluctuations persist
deep into the insulating state [3].
[1] P.K. Rout, I. Agireen, E. Maniv, M. Goldstein, Y. Dagan Physical Review B 95 (24), 241107
[2] P.K. Rout, E. Maniv, Y. Dagan, arXiv:1706.01717 (Accepted in Phys. Rev. Lett.)
[3] M. Mograbi et al. To be published somewhere but not in Nature.
The combination of interactions and static gauge fields plays a pivotal role in our understanding of strongly correlated quantum matter. Cold atomic gases endowed with a synthetic dimension are emerging as an ideal platform to experimentally address this interplay in quasi-one-dimensional systems. A fundamental question is whether these setups can give access to pristine two-dimensional phenomena, such as the fractional quantum Hall effect, and how. We show that unambiguous signatures of bosonic and fermionic Laughlin-like states can be observed and characterized in synthetic ladders. We theoretically diagnose these Laughlin-like states focusing on the chiral current flowing in the ladder, on the central charge of the low-energy theory, and on the properties of the entanglement entropy.
The essence of superfluidity is the possibility to witness metastable flow states. In the standard classical stability analysis, one finds that flow states whose rotation velocity is less than a critical velocity are metastable (“Landau criterion”). In this talk I will show that the standard superfluidity criteria fail in low-dimensional circuits and that a proper determination of the superfluidity regime-diagram requires a quantum chaos perspective. In particular, I will explain the drastic differences between three-site rings and rings that have more than three sites. In the former, instability of flow states is due to a swap of separatrices, while in the latter it has to do with a web of non-linear resonances.
NanoSQUIDs residing on the apex of a quartz tip (SOT), suitable for scanning probe microscopy with record size, spin sensitivity, and thermal sensitivity are presented[1,2] . We have developed SOT with an effective diameter smaller than 50 nm, spin sensitivity better than a single electron spin and thermal sensitivity better than 1 μK/Hz1/2 . This technique is used to study nanoscale magnetism present in systems such as atomically sharp LaMnO3/SrTiO3 (LMO/STO) heterostructures[3] and to study dissipation mechanism in quantum system such as hBN-encapsulated graphene[2,4] . Magnetic imaging of LMO/STO revealed a superparamagnetic behavior resulting from an electronic phase separation leading to nucleation of metallic ferromagnetic islands in an insulating antiferromagnetic matrix. Thermal imaging of hBN-encapsulated graphene reveals a fascinating atomic-scale dissipation mechanism providing visualization and control of phonon emission from inelastic electron scattering off individual atomic defects[4] , opening the door to direct imaging and spectroscopy of dissipation processes in quantum systems.
[1] D. Vasyukov, Y. Anahory, L. Embon, D. Halbertal, J. Cuppens, L. Neeman, A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, M. E. Huber, and E. Zeldov, Nature Nanotech. 8, 639 (2013)
[2] D. Halbertal, J. Cuppens, M. Ben Shalom, L. Embon, N. Shadmi, Y. Anahory, H. R. Naren, J. Sarkar, A. Uri, Y. Ronen, Y. Myasoedov, L. S. Levitov, E. Joselevich, A. K. Geim, and E. Zeldov, Nature 539, 407 (2016).
[3] Y. Anahory, L. Embon, C. J. Li, S. Banerjee, A. Meltzer, H. R. Naren, A. Yakovenko, J. Cuppens, Y. Myasoedov, M. L. Rappaport, M. E. Huber, K. Michaeli, T. Venkatesan, and E. Zeldov, Nature Communications 8, 85 (2016)
[4] D. Halbertal, M. Ben Shalom, A. Uri, K. Bagani, A.Y. Meltzer, I. Marcus, Y. Myasoedov, J. Birkbeck, L.S. Levitov, A.K. Geim, and E. Zeldov, Science 358, 1303 (2017).
With the rapid development of quantum technologies and the ability to manipulate individual atoms and ion, thermodynamics faces new challenges. The main question is not whether thermodynamics is valid, but whether it is relevant. Does it provide useful predictions on quantities of interest in the microscopic world? By introducing the concept of global passivity we derive an extension of the second law of thermodynamics that can properly handle initial quantum correlations (i.e., entanglement, and quantum discord) between the system and the environment. This extension is very important in nanoscopic setups where the environment may be small and strongly interacting with the system of interest. A second main finding of this framework is a family of additional thermodynamic relations that involve measurable quantities that were so far not constrained by thermodynamics. In particular, we use these relations to set upper and lower bounds on the buildup of system-environment correlation in quantum dephasing scenarios (decoherence). As a second example, we study the evolution of energy covariance between an atom and an optical cavity. Our findings are highly relevant for various modern experimental setups such as ion traps, atoms in an optical cavity or in optical lattices, and more.
Layered materials can be exfoliated to very thin films. In our group we utilize a technique called “mechanical transfer”, which allows stacking of such materials into new types of heterostructures which can involve layers of distinct functionalities. In my talk I will describe this fabrication technique, and demonstrate how we build a new type of device which consists of a layered superconductor (NbSe2), on which we deposit an ultra-thin layer of the semiconductor WSe2. Together, these two materials form a tunneling device, which can be cooled to extremely low temperatures (20 mK) in a dilution cryostat fitted with special filters which allow very sensitive energy spectroscopy. Our tunneling spectra allow tracking the evolution of the superconductor order parameter at the presence of strong magnetic fields, and quantifying its stability. Specifically, the depairing energy scale can be evaluated, and its origin – orbital or spin – identified. Thus, we are able to identify which of the two bands participating in NbSe2 superconductivity is associated with this material’s remarkable stability against in-plane magnetic field.
I plan to make a short review of the features of the energy spectrum of carriers in the quantum wells of a gapless semiconductor HgTe, in which, depending on the thickness of the well, the "normal", inverted and semimetallic spectra are realized. I will review the edge, topologically protected states, that should arise in structures with an inverted spectrum (which is realized at a quantum well thickness greater than d_c = 6.3 nm.
In more detail, I will discuss the current state of experimental results on the study of the energy spectrum, the role of edge states in kinetic effects. For a real understanding of any experimental results the reliable knowledge of the energy spectrum is necessary. Theoretically, it has been studied quite well and in detail. However, by the present time, quite a lot of contradictions have accumulated between theory and experiment. Therefore, our main attention will be paid to our experimental studies of the spectrum of the valence band in structures with a well thickness close to d_c and with d> d_с [1,2,3]. It will be shown that interface inversion asymmetry leads to a large spin-orbit splitting, which radically changes the spectrum of the valence band.
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1. G. M. Minkov, A. V. Germanenko, O. E. Rut, A. A. Sherstobitov, M. O. Nestoklon, S. A. Dvoretski, and N. N. Mikhailov, Spin-orbit splitting of valence and conduction bands in HgTe quantum wells near the Dirac point, , Phys. Rev. B 93, 155304 – Published 11 April 2016.
2. G. M. Minkov, V. Ya. Aleshkin, O. E. Rut, A. A. Sherstobitov, A. V. Germanenko, S. A. Dvoretski, and N. N. Mikhailov, Valence band energy spectrum of HgTe quantum wells with an inverted band structure, Phys. Rev. B 96, 035310 (2017) - Published 26 July 2017
3. G.M. Minkov et al, Conductance of a Lateral p–nJunction in TwoDimensional HgTe Structures with an Inverted Spectrum: The Role of Edge States JETP Letters, 2015, Vol. 101, No. 7, pp. 469–473, 2015
Topological insulators have been in the focus of condensed-matter research in recent years. In particular, much effort was dedicated to optically excite and control currents involving the topological surface states. We image the unoccupied bandstructures of a topological insulator as it evolves following an optical excitation, and observe the signature of a photo-induced current. By applying a time-mapping analysis we gain a complete view on the occupied and unoccupied electronic states, and how they are coupled by the optical excitation. This enables us to determine that photocurrents are excited only via the resonant optical transitions, paving the way to optical control of currents in topological insulators.
We study dynamical and steady-state dissipative quantum phase transitions of a driven Bose Hubbard model between an incoherent Mott-like insulating phase and a non-equilibrium superfluid. We highlight the crucial role of pumping scheme and the unique features that emerge in the driven- dissipative case as compared to the well known equilibrium quantum criticality. These results might be relevant for the upcoming generation of circuit QED arrays experiments aiming at realizing Mott Insulator of Polaritons and its transition into a nonequilibrium superfluid.
Generic interacting, disordered and isolated systems were shown to break ergodicity in a dynamical transition, called the many-body localization transition. In this talk I will survey the anomalous dynamical properties of the ergodic phase and will present a phenomenological picture which was proposed to explain them. I will provide evidence which suggest that this picture might not be complete.
Magnetic skyrmions are topologically protected, nanoscale, whorls and hedgehogs of magnetization. Because of their topological properties skyrmions also give rise to novel phenomena such as the topological Hall effect. Low temperature magnetic force microscopy (MFM) provides us with a tool for both imaging and control of skyrmions. Here I will describe recent experiments on multilayer Ir/Fe/Co/Pt stacks which can host Néel (hedgehog) skyrmions. We use MFM to image the magnetic structure in the films as a function of temperature and magnetic field. This allows us to explore the correspondence between the topological Hall effect and the magnetic textures that we image. For the analysis we rely on a new simple model for the magnetic field from a skyrmion.
The spin liquid phase is one of the prominent strongly interacting topological phases of matter whose unambiguous confirmation is yet to be reached despite intensive experimental efforts on numerous candidate materials. The challenge is derived from the difficulty of formulating realistic theoretical models for these materials and interpreting the corresponding experimental data. Here we study a theoretical model with bond-dependent interactions, directly motivated by recent experiments on two-dimensional correlated materials with strong spin-orbit coupling. We show numerical evidence for the existence of an extended family of quantum spin liquids, which are possibly connected to the Kitaev spin liquid state. These results are used to provide an explanation of the scattering continuum seen in neutron scattering on α-RuCl.
We study magnetotransport in a disordered Weyl semimetal taking into
account localization effects. In the vicinity of a Weyl node, a single
chiral Landau level coexists with a number of conventional non-chiral
levels. Disorder scattering mixes these topologically different modes
leading to peculiar localization effects. Similar interplay of topology
and localization occurs at the edge of a two-dimensional topological
insulator and in carbon nanotubes. We develop a general theory
describing transport phenomena in all these cases. Our theory yields
conductance, shot noise power, and full counting statistics of the
charge transfer. In the case of a Weyl semimetal, we find that
localization is greatly enhanced in a strong magnetic field with the
typical localization length scaling as 1/B. This situation is typical
for all topological conductors with broken time-reversal symmetry.
Systems with preserved time-reversal symmetry (e.g., carbon nanotubes),
sustain at most one topologically protected channel. For this case, we
derive exact distribution function of transmission probabilities based
on the mapping to a certain random-matrix model.
Over the past 5 years or so, superconductivity has been observed in a number of different materials with an extremely low density of charge carriers. Some of these are even among the most dilute metals that exist, such as SrTiO3 and bismuth. In this limit, the screening of the Coulomb repulsion is poor, and moreover, the conventional phonon mechanism for superconductivity is completely irrelevant. This raises the question: how can these materials be superconducting? I will propose two different mechanisms for superconductivity in low-density metals, based on dynamically screened Coulomb interactions and fluctuations near a structural quantum critical point. I will further discuss the prospects of these mechanisms for topological superconductivity and superconductivity in two-dimensional materials.
The Josephson effect in conventional superconductors has led to a variety of extensions and applications from SQUID magnetometers to pairing symmetry detection using planar junctions. It is therefore interesting to extend the discussion to topological Josephson junctions where Majorana fermions, and not just Cooper pairs, can tunnel through the junction. In this talk I will consider a setup made of a ring of topological superconducting wires separated by Junctions and coupled to a quantum dot. The coupling to the quantum dot serves as a knob which tunes the periodicity of the current in the junction as a function of external flux. The periodicity can change from one flux quantum (h/2e), expected from a conventional junction, to that of a topological junction, h/e. This tuning ability can distinguish between a topological junction and a dirty non-topological junction. I will also discuss phase slips and the suppression of 2pi phase slips in topological Josephson junctions.
Bilayer graphene subjected to magnetic and electric fields exhibits a plethora of phases with radically distinct behaviors. Most prominently, the \nu=0 quantum Hall state which develops at zero doping manifest a rich phase diagram marked by changes in the electrical conductance, and is tunable by either of the external fields. This behavior is attributed to the variety of many-body states with broken symmetry, which can form due to the multi-component nature of the discrete degrees of freedom (spin, valley and orbital isospin). In particular, in the presence of a perpendicular electric field D, at least two distinct insulating phases are identified: a canted antiferromagnet (CAF) at low D, and a fully layer polarized (FLP) phase at low D. However, at intermediate values of D and the magnetic field B, a finite conductance is developed, potentially indicating the formation of novel intermediate phases. I present a theoretical study motivated by these observations, which accounts for the interplay between lattice-scale interactions, inherently anisotropic in the spin-valley-orbital manifold, and a (formerly overlooked) trigonal warping effect in the electronic band-structure. Employing a Hartree-Fock calculation, we find several competing many-body states characterized by unusual spin-valley entangled correlations, which are promising candidates for the ground-state at intermediate fields. Most interestingly, we find a regime of parameters where the emergent intermediate phase is characterized by a simultaneous breaking of two distinct U(1) symmetries. I further discuss the implications on transport properties; most remarkably, the emergence of high conductance is a possible signature of robust collective edge modes associated with textures in the spin/valley manifold, which are topological in nature.
The anomalous Hall effect arises in systems with both spin-orbit coupling and magnetization.
We study the minimal model of the anomalous Hall effect based on the massive Dirac Hamiltonian and consider two
limiting cases of weak (Gaussian) and strong (Poisson) impurities.
The standard diagrammatic approach to the problem is limited to computation of ladder diagrams.
We demonstrate that this is insufficient in the case of Gaussian disorder. An important additional contribution comes from
scattering on pairs of closely located defects and essentially modifies previously obtained results for anomalous Hall conductivity.
n the case of Poisson disorder, we go beyond semiclassical limit and calculate weak
localization corrections. Unlike the case of ordinary Hall effect, we identify a finite quantum correction to anomalous Hall resistivity.
Depending on the structure of impurities, this correction can have any sign and interpolates smoothly between universal orthogonal
(localization) and symplectic (antilocalization) limits.
Recent experiments have provided mounting evidence for the existence of Majorana
bound states (MBSs) in condensed-matter systems. Until the long-term goal of braiding MBSs is
achieved, one is prompted to ask: what is the next step in the study of topological
superconductivity and MBSs? In my talk I will discuss two topics relating to this question. In the
first part I will examine the possibility of, not only detecting the Majoranas, but also witnessing
some of their exotic properties. In particular their non-local nature, or in other words, the fact that
the MBS is half a fermion whose occupation is encoded in a nonlocal way. I will show that current
cross correlations in a T-junction with a single MBS exhibit universal features, related to the
Majorana nonlocality. This will be contrasted with the case of an accidental low-energy Andreev
bound state. In the second part I will discuss the possibility of realizing a different topological
phase hosting MBSs in currently available experimental platforms. This will be a topological
superconducting phase which is protected by time-reversal symmetry, and which is characterized
by having a Kramers’ pair of MBSs at each end. As I will discuss, repulsive interactions are a
necessary ingredient for the realization of this phase. I will present a mechanism, based on the
interplay between repulsive interactions and proximity to a conventional superconductor, which
drives the system into the topological phase. The effect of interactions is studied analytically using
both a mean-field approach and the renormalization group. We corroborate our conclusions
numerically using DMRG.
The current that flows in a ring can be a signature of fundamental and topological properties of quantum states of charge-carrying particles. Applying a magnetic flux through a ring creates a phase gradient, in response to which a current flows, creating magnetic fields that we measure with a scanning SQUID microscope. I will take you on a tour of currents and phases in common and exotic quantum materials. Gold rings are normal metals with finite resistance, but remarkably, they carry persistent currents whose sign and magnitude confirm the quantum behavior of disordered metals. Aluminum rings superconduct at low temperatures, and are an ideal model system to study superconducting fluctuations. The strong agreement of theory and experiment in conventional metals and superconductors sets the stage to study superconducting rings interrupted by a single Josephson junction. This geometry allows us to measure a fundamental and informative property of the junction, called the current-phase relation. In junctions made of topological materials, the current could theoretically be 4pi-periodic rather than 2pi-periodic as a function of the phase winding in the ring. I will report on progress towards this smoking-gun signature for Majorana modes.
I will compare the effects of quantum and thermal fluctuations in a spin chain by calculating the probability distribution for spin fluctuations in a segment. The calculation will use the concept of an "entanglement Hamiltonian." The entanglement Hamiltonian can be used to identify topological phases, but I will show that it is helpful for long-wavelength correlations as well as topological ones. The entanglement Hamiltonian is an imaginary system that describes the correlations of the ground state. It cannot be measureddirectly, but it is related to the statistics of the fluctuations, so measuring the spin fluctuations of the atoms on the sites of an optical lattice is an indirect way of measuring the entanglement Hamiltonian.
In this talk, I will address the question of the existence of gapless topological superconducting phases in one dimension. I will present a general approach to treat charge-conserving one-dimensional systems with pairing interactions, and argue that in presence of symmetries the notion of a topological phase can be well defined although the system is gapless. I will demonstrate our approach on systems with time-reversal symmetry and on systems with SO(3) symmetry, where a gapless analogue of the Haldane phase in spin-1 chains can be realized.
Jun Ye
JILA, National Institute of Standards and Technology and University of Colorado, Boulder, Colorado 80309-0440, USA
Engineered spin–orbit coupling (SOC) in cold atom systems can shed light to synthetic materials and complex condensed matter phenomena. We demonstrate that SOC between fermions can be engineered to occur naturally in an optical lattice clock. SOC is both generated and probed using a direct ultra-narrow optical clock transition between two electronic orbital states in 87Sr atoms. We use clock spectroscopy to prepare lattice band populations, internal electronic states and quasi-momenta, and to produce spin–orbit-coupled dynamics. The exceptionally long lifetime of the excited clock state eliminates decoherence and atom loss from spontaneous emission at all relevant experimental timescales, allowing subsequent momentum- and spin-resolved in-situ probing of the SOC band structure and eigenstates. We use these capabilities to study Bloch oscillations, spin–momentum locking and Van Hove singularities in the transition density of states. Furthermore, many-body correlations arising from the interplay of interactions and SOC have been observed recently.
E-mail: Ye@JILA.colorado.edu
We study the density of states (DOS) and the transition temperature Tc in a dirty superconducting film with rare classical magnetic impurities of an arbitrary strength described by the Poissonian statistics. We take into account that the potential disorder is a source for mesoscopic fluctuations of the local DOS, and, consequently, for the effective strength of magnetic impurities. We find that these mesoscopic fluctuations result in a non-zero DOS for all energies in the region of the phase diagram where without this effect the DOS is zero within the standard mean-field theory. This mechanism can be more efficient in filling the mean-field superconducting gap than rare fluctuations of the potential disorder (instantons). Depending on the magnetic impurity strength, the suppression of Tc by spin-flip scattering can be faster or slower than in the standard mean-field theory.
Energy dissipation is a fundamental process governing the dynamics of physical systems. In condensed matter physics, in particular, scattering mechanisms, loss of quantum information, or breakdown of topological protection are deeply rooted in the intricate details of how and where the dissipation occurs. Despite its vital importance, direct imaging of dissipation in quantum systems is currently impossible because the existing thermal imaging methods lack the necessary sensitivity and are unsuitable for low temperature operation.
We developed a scanning nanoSQUID with sub 50 nm diameter that resides at the apex of a sharp pipette [1] that can act simultaneously as nanomagnetometer with single spin sensitivity and as nanothermometer providing cryogenic thermal imaging with four orders of magnitude improved thermal sensitivity of below 1 µK/Hz1/2 [2]. The non-contact non-invasive thermometry allows thermal imaging of very low nanoscale energy dissipation down to the fundamental Landauer limit of 40 fW for continuous readout of a single qubit at 1 GHz at 4.2 K. These advances enable observation of changes in dissipation due to single electron charging of individual quantum dots in carbon nanotubes. Our thermal imaging study of hBN encapsulated graphene reveals a fascinating dissipation mechanism due to resonant localized states at the edges of graphene providing the first visualization of inelastic electron scattering mechanism on the nanoscale, opening the door to direct imaging of microscopic dissipation processes in quantum matter.
[1] Vasyukov et al., Nature Nanotech. 8, 639 (2013).
[2] Halbertal et al., Nature 539, 407 (2016).
Spectroscopy is a powerful tool to probe matter. By measuring the spectrum of excitations, one can reveal the symmetries and topological properties of a physical system. Mesoscopic devices offer a unique possibility to both engineer and investigate novel excitations. Unfortunately, conventional spectroscopy techniques suffer several drawbacks for coupling radiation to mesoscopic systems and detecting their small absorption signals.
We propose an on-chip, Josephson-junction based spectrometer which surpasses state-of-the-art instruments and is ideally suited for probing elementary excitations in mesoscopic systems. It has a SQUID-based design providing uniform wideband coupling from 2-2000 GHz, low background noise, high sensitivity, and narrow linewidth. We describe the operating principle and design of the spectrometer, show preliminary results demonstrating proof-of-concept, and outline experiments which exploit the spectrometer to investigate unconventional Andreev states.
I will present two projects, based on the use of coupled superconducting resonators, to observe quantum effects:
1. Multiple bosons undergoing coherent evolution in a coupled network of sites constitute a so-called quantum walk system. The simplest example of such a two-particle interference is the celebrated Hong-Ou-Mandel interference. When scaling to larger boson numbers, simulating the exact distribution of bosons has been shown, under reasonable assumptions, to be exponentially hard. We analyze the feasibility and expected performance of a globally connected superconducting resonator based quantum walk system, using the known characteristics of state-of-the-art components. We simulate the sensitivity of such a system to decay processes and to perturbations and compare with coherent input states.
2. Atomic sized two-level systems (TLSs) in dielectrics are known as a major source of loss in superconducting devices, particularly due to frequency noise. However, the induced frequency shifts on the devices, even by far off-resonance TLSs, is often suppressed by symmetry when standard single-tone spectroscopy is used. We introduce a two-tone spectroscopy on the normal modes of a pair of coupled superconducting coplanar waveguide resonators to uncover this effect by asymmetric saturation. Together with an appropriate generalized saturation model this enables us to extract the average single-photon Rabi frequency of dominant TLSs to be Ω0/2π≈79 kHz. At high photon numbers we observe an enhanced sensitivity to nonlinear kinetic inductance when using the two-tone method and estimate the value of the Kerr coefficient as K/2π≈−1×10−4Hz/photon. Furthermore, the life-time of each resonance can be controlled (increased) by pumping of the other mode as demonstrated both experimentally and theoretically.
Periodically driven quantum systems, such as semiconductors subject to light and cold atoms in optical lattices, provide a novel and versatile platform for realizing topological phenomena. Among these are analogs of topological insulators and superconductors, attainable in static systems. However, some of these phenomena are unique to the periodically driven case. I will describe how the interplay between periodic driving, disorder, and interactions gives rise to new steady states exhibiting robust topological phenomena, with no analogues in static systems. Specifically, I will show that disordered two dimensional driven systems admit an “anomalous" phase with chiral edge states that coexist with a fully localized bulk. This phase serves as a basis for new far-from-equilibrium quantized transport phenomena. Specifically, I will discuss the quantization of magnetization and two terminal currents in this anomalous phase.
The superconductor-insulator transition (SIT) is a prototype of a quantum phase transition which is very versatile experimentally: varying a non-thermal tuning parameter such as disorder, thickness, composition, magnetic field or gate-voltage causes the system to switch from a superconductor to an insulator at zero temperature.
Unlike their classic counterparts, quantum phase transitions are governed by quantum rather than thermal fluctuations at low temperatures. The direct experimental study of such fluctuations close to the SIT is rather challenging. So far research has mainly concentrated on dc resistivity based measurements such as transport and magnetoresistance and on global and local tunneling spectroscopy. These cannot provide explicit information on the critical behavior through the transition.
In my talk I will describe two experimental efforts designed to measure signatures of quantum fluctuations close to quantum criticality. The first experiment utilizes a unique, highly sensitive, setup to measure the specific heat of ultrathin Pb films through the SIT. The specific heat is found to increase considerably as the sample is pushed towards the SIT thus signaling quantum criticality. The second experiment is a scanning squid measurement able to probe local susceptibility in TiNbN films close to the quantum transition. These studies reveal electronic superconducting granularity which fluctuates in time and space at temperatures well below Tc. The temperature regime of these fluctuations grows as the SIT is approached indicating their quantum nature.
I will discuss the significance of these results and their contribution to understanding the electronic processes in the vicinity of the quantum phase transition.
Precise timekeeping is critical to metrology, forming the basis by which standards of time, length and fundamental constants are determined. Stable clocks are particularly valuable in spectroscopy as they define the ultimate frequency precision that can be reached. In quantum metrology, where the phase of a qubit is used to detect external fields, the clock stability is defined by the qubit coherence time, and therefore determines the spectral linewidth and frequency precision. I will present a demonstration of a quantum sensing protocol for oscillating fields where the spectral precision goes beyond the sensor coherence time and is limited by the stability of a classical clock. Using this technique, we observe a precision in frequency estimation scaling as 1/T^{3/2} for classical fields. The narrow linewidth magnetometer based on single quantum coherent spins in diamond is used to sense magnetic fields with an intrinsic frequency resolution of 607µHz, 8 orders of magnitude narrower than the qubit coherence time.
Transition metal oxides (TMOs) are complex electronic systems which exhibit a multitude of collective phenomena. Two archetypal examples are VO2 and NdNiO3, which undergo a metal-insulator phase-transition (MIT), the origin of which is still under debate.
We have discovered a new kind of memory effect in both systems, manifest through an increase of resistance at a specific temperature, which is set by reversing the temperature-ramp from heating to cooling during the MIT, thus we call it ‘Ramp Reversal Memory’.
The characteristics of this memory effect do not coincide with any previously reported history or memory effects in similar systems. From a broad range of experimental features, supported by theoretical modelling, we claim that the main ingredients for the effect to arise are the spatial phase-separation of metallic and insulating regions during the MIT and the coupling of lattice strain to the local critical temperature of the phase transition. We predict that similar ramp-reversal effects exist also in other systems.
Search engines, e-mail, watch-on-demand websites, cyber-security and smartphones are examples of how information technologies have changed our lives in the past decade. However, the growing usage of these technologies has arrived with a cost—a sharp increase in global electrical-energy consumption. Likewise, the inability to manage heat has already become crucial for mobile-phones. Hence, novel materials that will replace the existing technologies are being sought continuously. Superconductors and ferroelectrics are functional solid-state systems that exhibit unique collective electron behavior, which renders these systems for energy-efficient nano switching devices. Nevertheless, to date the understanding and controllability of the emergence of collective-electron behavior in these materials, which is also at the nanoscale, has remained elusive to us, hindering the realization of their great technological potential.
We developed and utilized methods to expose the collective electron behavior in superconductors and ferroelectrics at the deep submicron scale. For instance, using advanced atomic force microscopy (AFM) and transition electron microscopy (TEM) techniques we have tailored the multiscale ferroelectric behavior (Fig. 1). Likewise, using advanced nano-fabrication designs, we demonstrated how the superconducting behavior can be controlled at the nanoscale. Thanks to the strong correlation between electron collectiveness and functionality in these materials, not only have we illuminated the origin of collective electron interactions in solid-state systems, but we also facilitated novel technologies, ranging from single-photon detectors to quantum switching devices.
[1] Bar Lev and Reichman, Phys. Rev. B 89, 220201(R) (2014).
Practical quantum information processing relies on the ability to protect qubits against errors. This is often achieved by encoding every qubit in multiple two-level systems (“physical qubits”), thus forming a redundantly encoded “logical qubit”. However, implementing gates between such logical qubits requires a large number of operations between pairs of physical qubits.
In this talk, I will present a scheme for an entangling gate between two redundantly encoded logical qubits, and its recent experimental realization. The scheme relies on an alternative approach for realizing logical qubits, which uses the multilevel structure of a single cavity, instead of multiple two-level systems. A single ancillary transmon is used to induce nonlinear coupling between two cavities, thereby generating an effective interaction between the photonic states. The gate is controlled using parametric pumping on the transmon, resulting in an entangling rate between the cavities that is three orders of magnitude larger than the cavity decoherence rate and parasitic interactions. We characterize the gate by full quantum process tomography, and measure a reduction in fidelity per gate application as low as ~1%, on par with state-of-the-art gates between two-level physical qubits. These results demonstrate the potential of cavity-encoded logical qubits beyond the robust storage of quantum information, and open a route towards their use in larger quantum networks.
Photonic cluster states are a resource for quantum computation based solely on single-photon measurements. We use semiconductor quantum dots to deterministically generate long strings of polarization-entangled photons in a cluster state by periodic timed excitation of a precessing matter qubit. In each period, an entangled photon is added to the cluster state formed by the matter qubit and the previously emitted photons. In our prototype device, the qubit is the confined dark exciton, and it produces strings of hundreds of photons in which the entanglement persists over five sequential photons.
*Work done in collaboration with Ido Schwartz, Dan Cogan, Emma Schmidgall, Yaroslav Don, Liron Gantz, Oded Kenneth, and Netanel Lindner [Schwartz et al Science 354, 434, (2016)].
We have previously demonstrated that as the 1D confinement potential is weakened the electron wavefunctions relax in the second dimension and can form a two-row system, we now show that a zig-zag formation can be observed in electron focusing experiments. In a similar way we have investigated the dynamics of a strongly confined 1D electron gas and will show that a spin separation occurs. We present direct evidence that this spontaneous spin polarization is responsible for the 0.7 anomaly and can rule out other explanations.
I will open my talk by explaining the fundamental concept of parametric resonance and giving examples of some practical application (Paul traps, Kapitza pendulum). I will then clarify why the existence of this phenomenon in many-body systems is still debated. Although it is believed that large systems cannot display true parametric resonances, I will overview some exceptions to this general rule. In the last part of the talk, I will describe some new results on the dynamics of periodically-driven many-body quantum systems.
Synopsis : I will argue that quantum mechanics endows us with resources that may boost thermodynamic performance: heat-machine power, cooling speed or work, but the basic thermodynamic bounds are still adhered to.
The debated rapport between thermodynamics and quantum mechanics will be addressed in the framework of the theory of periodically driven/controlled quantum thermodynamic machines. The basic models studied by us are a two-level system (TLS) or a harmonic oscillator, whose energy is periodically modulated while the system is coupled to two distinct thermal baths. When the modulation interval is short compared to the bath memory time, the system–bath correlations are affected, thereby causing cooling or heating of the TLS, depending on the interval [1,2]. This setup constitutes the simplest (minimal) quantum heat machine (QHM) that may operate as either an engine or a refrigerator, depending on the modulation/driving rate [3]. It is used by us to scrutinize basic thermodynamic principles in the quantum domain:
- It is possible to surpass the Carnot bound when the driving is done by a quantum device (piston) in an appropriate state [4-6]. However, such enhanced efficiency does not mean that quantum effects invalidate the Carnot bound: their classical analog is a non-thermal bath whose temperature is controlled by its state. By contrast, engines powered by non-thermal (e.g. squeezed) baths may cease to be heat machines and act instead as devices exchanging work rather than heat with the baths, so that the Carnot bound is inapplicable to such devices [7].
- An extension of the TLS model of quantum heat machines to multiple entangled TLS shows that the power output can be boosted by the quantum cooperativity of these systems, but the efficiency is still bounded by Carnot [6] .
- The refrigeration effected by such QHMs persists as the temperature approaches absolute zero for certain quantized baths, e.g., magnons, thus challenging the Third Law (Nernst's unattainability principle) [2].
Thus, we may conclude that quantum machines powered by thermal and non-thermal baths may benefit from hitherto unexploited quantum resources, but they still comply with traditional thermodynamic rules, possibly with the exception of the Third Law.
[1] N.Erez et al., Nature 452, 724 (2008).
[2] M. Kolar et al. PRL 109, 090601 (2012).
[3] D. Gelbwaser-Klimovsky et al. PRE 87, 012140 (2013).
One of the major and long-standing challenges of oxide 2DEGs is still to understand how to enhance the electron mobility. The mobility is still orders of magnitude lower than that of the conventional semiconductors and with the current fabrication method we still cannot fully control the charge at the interface. Here, I will present the recent activities of our group in this area where we try to push the mobility to record high values, i.e. the usage of the so-called modulation-doping technique has significantly increased the carrier mobility to mobility of 70.000 cm2/Vs and realization of quantum Hall effect in these films. These findings pave the way for studies of mesoscopic physics with complex oxides and design of high-mobility all-oxide electronic devices.
Zero-resistance in superconductors is protected at finite temperatures by the global phase coherence, which by virtue of the uncertainty principle guarantees the dissipationless flow of Cooper pairs. In two-dimensional critically disordered films dual superinsulating state can form. In superinsulators coherent fluctuations of phase localize Cooper pairs leading to infinite-resistance at finite temperatures. I will present an experimental observation of the magnetic field-driven superconductor-superinsulator transition in disordered thin NbTiN films. Appearance of the superinsulator is detected by the abrupt upturn from the Arrhenius-type temperature dependence of the resistance, evidencing formation of the zero-conducting state at finite temperature. At relatively low magnetic fields we have found the critical divergent behavior of the resistance characteristic to charge Berezinskii–Kosterlitz–Thouless (BKT) transition, which is dual to the vortex BKT transition in superconducting state.
This seminar has been cancelled in order not to overlap with Assa's birthday conference:
Using exact diagonalization for non-interacting systems and density matrix renormalization group for interacting systems we show that Li and Haldane’s conjecture [1] on the correspondence between the low-lying many-particle excitation spectrum and the entanglement spectrum holds for disordered ballistic one-dimensional many-particle systems. We observe and explain the presence of an unexpected shell structure in the excitation spectrum [2]. The low-lying shells are robust and survive even for strong electron-electron interactions.
[1] Li H. and Haldane F. D. M., Phys. Rev. Lett., 101(2008) 010504.
[2] S. Leiman, A. Eisenbach and R. Berkovits, EPL, 112 (2015).
One of the defining properties of electrons is their mutual Coulombic repulsion. In solids, however, this basic property may change. A famous example is that of superconductors, where coupling to lattice vibrations makes electrons attract each other and leads to the formation of bound pairs. But what if all degrees of freedom are electronic? Is it still possible to make electrons attractive via their repulsion from other electrons? Such a mechanism, termed ‘excitonic’, was proposed fifty years ago by W. A. Little, aiming to achieve stronger and more exotic superconductivity, yet despite many experimental efforts, direct evidence for such ‘excitonic’ attraction is still lacking. Here, we demonstrate this unique attraction by constructing, from the bottom up, the fundamental building block of this mechanism. Our experiments are based on quantum devices made from pristine carbon nanotubes, combined with cryogenic precision manipulation. Using this platform we demonstrate that two electrons can be made to attract using an independent electronic system as the binding glue. Owing to its large tunability, our system offers crucial insights into the underlying physics, such as the dependence of the emergent attraction on the underlying repulsion and the origin of the pairing energy. We also demonstrate transport signatures of ‘excitonic’ pairing. This experimental demonstration of ‘excitonic’ pairing paves the way for the design of exotic states of matter.
Probing Odd-Triplet Contributions to the Proximity Effect by Scanning Tunneling Spectroscopy
S. Diesch1, M. Wolz1, P. Machon1, C. Sürgers2, W. Belzig1, A. Di Bernardo3, Y. Gu3, J. Linder4, M. G. Blamire3, J. W. A. Robinson3, E. Scheer1
1Department of Physics, University of Konstanz, 78457 Konstanz, Germany
2Physical Institute, Karlsruhe Institute of Technology, 76049 Karlsruhe, Germany
3Department of Material Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom
4Physics Institute, Norwegian Technical University, 7491 Trondheim, Norway
Abstract. In this talk we will address the superconducting proximity effect between a superconductor (S) and a normal metal (N) linked by a spin-active interface. With the help of a low-temperature scanning tunneling microscope [1,2] we study the local density of states of trilayer systems. The first example consists of aluminum (S), the ferromagnetic insulator (FI) EuS, and the noble metal silver (N) for varying thickness of the FI. In several recent studies it has been shown that EuS acts as ferromagnetic insulator with well-defined magnetic properties down to very low thicknesses [3]. For very thin FI with dFI = 2 nm we find a strong enhancement of the induced minigap at the normal side. For intermediate thickness we observe pronounced subgap structures that vary from contact to contact. For dFI = 10 nm the spectra are in agreement with the diffusive theory for S/N structures (without FI) as confirmed in earlier studies [2]. We discuss our findings in the light of recent theories of odd-triplet contributions created by the spin-active interface [4,5,6].
The second example uses the ferromagnetic metal (FM) Ho between niobium (S) and gold (N) [7]. These measurements reveal pronounced changes to the Nb sub-gap superconducting density of states on driving the Ho through a metamagnetic transition from a helical antiferromagnetic to a homogeneous ferromagnetic state for which a conventional BCS gap is recovered. The results directly verify odd frequency spin-triplet superconductivity at superconductor / inhomogeneous magnet interfaces [8].
References
[1] C. Debuschewitz, F. Münstermann, V. Kunej, E. Scheer
A compact and versatile scanning tunnelling microscope with high energy resolution for use in a 3He Cryostat, J. Low Temp. Phys. 147, 525 (2007)
[2] M. Wolz, C. Debuschewitz, W. Belzig, E. Scheer
Evidence for attractive pair interaction in diffusive gold films deduced from studies of the superconducting proximity effect with aluminium, submitted
[3] J. Linder, A. Sudbø, T. Yokoyama, R. Grein, M. Eschrig, Phys. Rev. B 81, 214505 (2010)
[4] B. Li, N. Roschewsky, B. A. Assaf, M. Eich, M. Epstein-Martin, D. Heiman, M. Münzenberg, and J. S. Moodera, Phys. Rev. Lett. 110, 09700 (2013)
[5] A. Cottet, W. Belzig, Phys. Rev. B 72, 180503R (2005);
[6] P. Machon, W. Belzig, in preparation
[7] W. A. Robinson, J. D. S. Witt, M. G. Blamire, Science 329, 59 (2010).
[8] A. Di Bernardo, S. Diesch, Y. Gu, J. Linder, M.G. Blamire, E. Scheer, J. W. A. Robinson, Nature Comm. 6, 8053 (2015)
Glass is a unique state of matter which appears in many physical systems, including amorphous SiO2 (window glass), spin glass, polymer glass, etc., all characterized by the lack of internal order and extremely slow dynamics. The electron glass is a newcomer to this field. In these systems strong Coulomb interactions between localized electrons result in a slow relaxation of the conductance towards equilibrium.
In my talk I will present experimental results obtained on discontinuous metallic films which were fabricated at cryogenic temperatures, below the glass temperature. The conductance dynamics in these samples depend not only on the ambient temperature, but also on the maximal temperature they were exposed to, Tmax. Based on a model of hopping between metastable states, I will claim that this dependence on Tmax may provide insight into the dynamics of many glassy systems.
Superconducting qubits based on Josephson junctions are a promising platform for quantum computation,
reaching quality factors of over one million.
On one hand, such high quality factors enable the implementation of quantum error correction; on the other, they make possible the investigation of decoherence mechanisms with high accuracy. An intrinsic decoherence process originates from the coupling between the qubit degree of freedom and the quasiparticles that tunnel across Josephson junctions. In this talk I will review briefly the general theory of quasiparticle effects, valid bothfor equilibrium and non-equilibrium quasiparticles, and discuss recent experiments with transmon and fluxonium qubits.
In a transmon, tunneling of a single quasiparticle is associated with a change in parity; I will compare the theory of the parity-switching rate with recent measurements and comment on the implications for the dephasing rate. In qubits that can be tuned by magnetic flux, such as the fluxonium, the quasiparticle-induced decoherence rate depends on the flux; this provides a way to differentiate quasiparticle tunneling from other sources of decoherence. Taking advantage of this flux dependence, we have recently solved a 40-year-old puzzle in the physics of Josephson junctions.
Superconducting qubits based on Josephson junctions are a promising platform for quantum computation, reaching quality factors of over one million. On one hand, such high quality factors enable the implementation of quantum error correction; on the other, they make possible the investigation of decoherence mechanisms with high accuracy. An intrinsic decoherence process originates from the coupling between the qubit degree of freedom and the quasiparticles that tunnel across Josephson junctions. In this talk I will review briefly the general theory of quasiparticle effects, valid both for equilibrium and non-equilibrium quasiparticles, and discuss recent experiments with transmon and fluxonium qubits. In a transmon, tunneling of a single quasiparticle is associated with a change in parity; I will compare the theory of the parity-switching rate with recent measurements and comment on the implications for the dephasing rate. In qubits that can be tuned by magnetic flux, such as the fluxonium, the quasiparticle-induced decoherence rate depends on the flux; this provides a way to differentiate quasiparticle tunneling from other sources of decoherence. Taking advantage of this flux dependence, we have recently solved a 40-year-old puzzle in the physics of Josephson junctions.
The talk will have three parts, discussing electronic transport in three different one-dimensional systems: (i) A junction of three interacting quantum wires has many interesting and distinct transport properties compared to a single quantum wire with a scattering centre. Classification of Y-junctions and interesting results on conductance of Y-junctions will be mainly discussed. (ii)-Periodically driven one-dimensional quantum wires, can pump charge. Results on charge pumping in closed and open systems will be contrasted. Further, pumping on the edge of quantum spin Hall insulators will be discussed. (iii) A recent proposal to enhance crossed Andreev reflection in “normal metal - superconductor - normal metal” will be discussed.
References:
- A. Soori and D. Sen, EPL 93, 57007 (2011) & A. Soori and D. Sen, PRB 84, 035422 (2011).
- A. Soori and D. Sen, Phys. Rev. B 82, 115432 (2010) & A. Soori, S. Das, and S. Rao, Phys. Rev. B 86, 125312 (2012).
- A. Soori and S Mukerjee, arxiv: 1603.00363 (2016).
Ultrafast laser pulses have been used to manipulate complex quantum materials and to induce dynamical phase transitions. One of the most striking examples is the transient enhancement of superconductivity in several classes of materials upon irradiating them with high intensity pulses of terahertz light. Motivated by these experiments we analyze the Cooper pairing instabilities in non-equilibrium electron-phonon systems and demonstrate that the light induced non-equilibrium state of phonons results in an enhancement of Cooper pairing. This opens the possibility of transient light induced superconductivity at temperatures that are considerably higher than the equilibrium transition temperature
They tried to kill us. We survived. Let's eat!
ESRSTM (electron spin resonance scanning tunneling microscopy )is a technique capable of detecting single spins by recording the power spectral density at the Larmor frequency - in the tunneling current. Description of previous results will be provided. Recently a Si(111)7 × 7 surface exposed to 0.1 L of O_2 and the carbonized Si(111) surface are investigated by ESR-STM using frequency sweeps and magnetic field sweeps. Only after oxidizing the clean Si(111)7 × 7 or by using the carbonized Si(111), spatially averaged ESR-STM spectra exhibit several peaks and dips around the frequencies corresponding to g = 2. The energy difference between these features is close to the known hyperfine splitting of A ≅ 9 MHz for vacancies in SiC interacting with next-nearest neighbor ^29 Si. The final proof that the hyperfine spectrum is indeed observed, is the observation of the nuclear Zeeman transition of a single nuclear spin that were observed by an ENDOR type experiment. The results of this experiment reveals the gyromagnetic ratio of 29^Si nuclei. ESRSTM was observed also on Cu atoms. The ENDOR experiment will be carried out to detect the gyromagnetic ratios of the two types Cu nuclei. These results were performed in frequency domain. In this mode the technique is time consuming as the different frequency components are detected one after the other. In time domain, the acquisition is orders of magnitude faster. Results on adsorbed Tempo molecules will be presented. These results display the whole spectrum from 0-0.75GHz. The results reveal the expected Larmor frequency components of Tempo (f1) and additional higher frequency components that are interpreted as tip spin centers (f2). In addition strong peaks appear at a frequency (f1-f2)/2 which may point to a possible mechanism of interference between electrons that are propagating through tip and sample spin centers, respectively.
Perovskite oxides of transition metals offer scientists and engineers a theme park of new physics, with complex structure-property relations and a wide scope of useful phenomena. Advances in epitaxial growth techniques bring unprecedented possibilities of atomic engineering of these oxides and their interfaces, with exciting prospects of unravelling their underlying physics and harnessing it towards useful applications. However, fundamental material challenges inhibit the introduction of these materials onto semiconductors and thus into the microelectronics technology.
The potential of the oxides’ functionalities to evolve out of the labs and into technology has received a considerable boost with the pioneering of epitaxial growth approaches for perovskites directly on semiconductors. This combination opens a route to bridge between oxide functionalities and microelectronics. Moreover, interface engineering of oxides on semiconductors can lead to new functionalities, made possible by the coupling between the dissimilar materials.
The formation of a 2 dimensional electron gas (2DEG) is a prominent example of an unexpected phenomenon occurring at the interface of two insulating oxides, with complex underlying physics and potential for technological applications. We demonstrate the integration of high carrier density oxide 2DEGs on silicon, which brings the useful properties of 2DEGs closer to application in microelectronics. In order to put this claim to the test, we assess the performance of oxide 2DEGs on silicon as potential channels.
Additional functionality can be obtained by coupling of oxides to semiconductors. This concept will be presented with examples for the investigation of electronic transport across the oxide-semiconductor interface and its prospects for photoelectrocatalysis. Another example is the coupling between a monolayer of a non-ferroelectric oxide and silicon, which leads to interface-induced ferroelectric functionality.
With these examples I hope to provide a glimpse into the prospects and promise of combining functional oxides with semiconductors, and address some of challenges lying ahead.
N/A
In this talk I will present a new experimental technique capable of measuring the shape of the electrons in (super)conducting materials.
One-dimensional helical edge of a two-dimensional topological insulator is believed to be an ideal conductor, i.e. to be characterized by the universal conductance. This means that in contrast with ordinary one-dimensional particles, which are known to be always localized by arbitrary weak disorder, the states of chiral electrons are extended regardless of disorder. I will discuss the effect of localized spins - Kondo impurities. Provided that the electron-spin couplings is anisotropic this system can be mapped to the problem of the pinning of the ordinary charge density wave by a disordered potential. This mapping proves that arbitrary weak anisotropic disorder in coupling of chiral electrons with spin impurities eliminates effects of the topology and leads to the Anderson localization of the edge states. I will analyze existing experimental results in view of this conclusion.
We study an effective model of two interacting species of bosons in two dimensions, which is amenable to sign problem free Monte Carlo simulations. In addition to conventional ground states, we access a paired superfluid which is also a topological phase, protected by the remaining unbroken symmetry. This phase arises from the condensation of a composite object, the bound state of vortices and anti-vortices of one species, to a boson of the second species. We introduce a bulk response function, the Ising analog of the quantized Hall effect, to diagnose the topological phase. The interplay of broken symmetry and topology leads to interesting effects such as fractionally charged vortices in the paired superfluid. Possible extensions towards realistic models of cold atomic bosons will be discussed
We present a novel technique to image superparamagnetic iron oxide nanoparticles via their fluctuating magnetic fields. The detection is based on the nitrogen-vacancy (NV) color center in diamond, which allows optically detected magnetic resonance (ODMR) measurements on its electron spin structure. In combination with an atomic-force-microscope, this atomic-sized color center maps ambient magnetic fields in a wide frequency range from DC up to several GHz [1], while retaining a high spatial resolution in the sub-nanometer range
[2]. We demonstrate imaging of single 10 nm sized magnetite nanoparticles using this spin noise detection technique. By fitting simulations (Ornstein-Uhlenbeck process) to the data, we are able to infer additional information on such a particle and its dynamics, like the attempt frequency and the anisotropy constant [3]. This is of high interest to the proposed application of magnetite nanoparticles as an alternative MRI contrast agent or to the field of particle-aided tumor hyperthermia.
[1] E. Schäfer-Nolte et al., Phys. Rev. Lett. 113, 217204 (2014)
[2] P. Maletinsky et al., Nat. Nanotech. 7, 320 (2012)
[3] D. Schmid-Lorch et al., Nano Lett. 15, 4942 (2015)
Liquid helium 4 undergoes a phase transition into a superfluid at low temperatures. Owing to its quantum nature, the solid phase of helium 4 has been suggested to host a similar type of Bose-condensed state, supporting frictionless mass flow. This unique phenomenon has been termed supersolidity. In the past decade, the existence of supersolidity has been alternately confirmed and disproved experimentally. The controversy originates from the fact that a small supersolid signal can be overshadowed by large elastic effects arising from an anomaly in the shear modulus of the solid. Using a novel experimental approach, with specially-designed multi-mode torsional oscillators, we were able to accurately distinguish between the different contributions for the first time, and discovered a small frequency-independent signal, as expected in the presence of a supersolid phase. I will discuss the significance of our results in the broader context of the field's current state.
Research in graphene has evolved into a stage in which other isolated atomic planes of materials can now be reassembled in a chosen sequence, as in building with Lego where the blocks are defined with one-atomic-plane precision [1]. Due to their weak interlayer forces “Van der Waals” crystals can be separated to individual layers of a single crystallographic domain before reorienting and transferring them to form a new stack. The first part of the talk is aimed to give a wide review of the recent research done in Manchester in this field, with emphasize on ways to study materials which are unstable in ambient conditions [2].
The second part will focus on hetrostructures in which graphene is stacked between hexagonal boron nitride crystals (hBN), where electrons can travel for many micrometres before experiencing any scattering or de-coherence; I will first describe the extremely high superconducting currents which can be driven along graphene by placing it in proximity to a superconductor [3]. Remarkably, these currents persist even in the presence of high magnetic fields such that graphene can, in principle, be tuned into the Quantum Hall regime. The proximity-induced supercurrent, however, is found to be mediated by mesoscopic trajectories at the edges and its cut-off is prior to the onset of the QH. Next, I will discuss the nature of current flow when breaking the inversion symmetry of the system and opening an energy gap. Topological valley currents are predicted when placing the chemical potential in the gap. Lastly, I will present experimental observations of hydrodynamic-like flow and formation of electron-whirlpools in this system at higher temperatures [4].
[1] "Van der Waals heterostructures." Nature499.7459 (2013): 419-425.
[2] “Quality heterostructures from two dimensional crystals unstable in air by their assembly in inert atmosphere” Nano letters 15(8),4914 (2015).
[3] "Proximity superconductivity in ballistic graphene, from Fabry-Perot oscillations to random Andreev states in magnetic field." arXiv:1504.03286.
[4] "Negative local resistance due to viscous electron backflow in graphene." arXiv:1509.04165.
I will describe nonequilibrium dynamics in interacting quantum systems
mainly using the protocol of quenching the systems and following their evolution in
time. I'll discuss the evolution Lieb-Liniger system, a gas of interacting bosons
moving on the continuous infinite line and interacting via a short range potential.
Considering first a finite number of bosons on the line we find that for any value of
repulsive coupling the system asymptotes towards a strongly repulsive gas for any
initial state, while for an attractive coupling, the system forms a maximal bound
state that dominates at longer times. In either case the system equilibrates but does
not thermalize, an effect that is consistent with prethermalization. Then
considering the system in the thermodynamic limit - with the number of bosons and
the system size sent to infinity at a constant density and the long time limit taken
subsequently- I'll discuss the equilibration of the density and density-density
correlation functions for strong but finite positive coupling and show they are
described by GGE (generalized Gibbs ensemble) for translationally invariant initial
states with short range correlations. If the initial state is strongly non translational
invariant the system does not equilibrate. I will give some examples of quenches:
from a Mott insulator initial state or from a domain wall configuration or a Newton’s
Cradle. I also will show that if the coupling constant is negative the GGE fails for
most initial states in the Lieb-Liniger model. If time permits I shall discuss also the
quench dynamics of the XXZ Heisenberg chain.
Metal nanostructures are very attractive systems for confining and guiding light at the nanoscale. Being able to control their plasmon resonance could pave the way for realizing optical detectors and devices with both great spatial and spectral sensitivity. In this talk I will demonstrate local control of plasmons using two different nano-devices.
The first device consists of a single CdSe/ZnS quantum dot which is placed at a distance of a few nanometers from the surface of a gold nanoparticle. We control the distance between these two objects with a sub-nanometer precision using a technique where a short DNA strand is used as a spacer. By measuring the change in the exciton properties we were able to locally probe the plasmon resonance. We show that exciton absorption becomes sensitive to the excitation polarization and that the absorption and emission rates of the quantum dot can be enhanced by up to two orders of magnitude by the presence of the gold nanoparticle.
The second device consists of a single gold nanoparticle biased by two nanoscale electrodes. In this device, we control the emitted light spectrum by applying a bias voltage. Applying a large voltage induces inelastic electron tunneling in the device and generates plasmons which are detected by their radiative decay. We study the voltage dependence of the emitted spectrum and show a quantitative agreement with theory.
"Exciton–Plasmon Interactions in Quantum Dot–Gold Nanoparticle Structures" Eyal Cohen-Hoshen, Garnett W. Bryant, Iddo Pinkas, Joseph Sperling, and Israel Bar-Joseph Nano Letters 2012 12 (8), 4260-4264
"Fano Resonance in an Electrically Driven Plasmonic Device" Yuval Vardi, Eyal Cohen-Hoshen, Guy Shalem, and Israel Bar-Joseph, Nano Letters 2015 Submitted.
The presence of disorder in a non-interacting system can
localize all the energy eigenstates, a phenomena dubbed Anderson
localization. Many-body localization is an extension of this phenomena
to include interactions. Effects of interactions show up in the
logarithmic growth of entanglement after a global quench. We perform a
systematic study of the evolution and saturation of entanglement and
show that it can be used to detect the localization transition. We
consider the bipartite fluctuation which also captures the transition
and is promising as an experimental probe. We compare these results to
that of a non-interacting system and note important differences
between the two.
Interactions in Bose gas are usually modeled by a delta function potential
(i.e., each pair of particles interacts only where the positions of the two
particles coincide). This description ignores the range of interaction. We
calculated the effects of the finite range of the interactions. Inspired by
Van-der Waals potential, we proposed a one dimensional interaction potential of 3
delta functions, the central one is repulsive and the two peripheral ones
are attractive. This model introduces a length scale for the interaction
without giving up the mathematical simplicity of using delta functions. By
generalizing the work of Lieb and Liniger, we found an approximate Bethe-ansatz
solution for the spectrum of N interacting Bosons moving on a ring and showed
that the effect of interaction range might be crucial in some cases.
Joint work with Shmuel Fishman and Wolfgang Ketterle.
Charge density waves (CDWs) and superconductivity are canonical examples of
symmetry breaking in materials. Both are characterized by a complex order parameter –
namely an amplitude and a phase. In the limit of weak coupling and in the absence of
disorder, the formation of pairs (electron-electron for superconductivity, electron-hole for
CDWs) and the establishment of macroscopic phase coherence both occur at the
transition temperature Tc that marks the onset of long-range order. But, the situation may
be drastically different at strong coupling or in the presence of disorder. We have
performed extensive experimental investigations on pristine and intercalated samples of
2H-NbSe2, a transition metal dichalcogenide CDW material with strong electron-phonon
coupling, using a combination of structural (X-ray), spectroscopic (photoemission and
tunnelling) and transport probes. We find that Tc(δ) is suppressed as a function of the
intercalation-concentration δ and eventually vanishes at a critical value of δ=δc leading to
quantum phase transition (QPT). Our integrated approach provides clear signatures that
the phase of the order parameter becomes incoherent at the quantum/ thermal phase
transition, although the amplitude remains finite over an extensive region above Tc or
beyond δc. This leads to the persistence of a gap in the electronic spectra in the absence of
long-range order, a phenomenon strikingly similar to the so-called pseudogap in
completely different systems such as high temperature superconductors, disordered
superconducting thin films and cold atoms
Semiconducting nanowires have captured vast scientific attention ever since the first detection of a zero-bias conductance peak – possibly signifying a Majorana mode. It is therefore surprising how little information we gained in the time that have passed since that report. A major experimental obstacle is the brittleness and reactivity of the nanowires, that oxidize once brought into ambient conditions. By tackling this technological challenge we have been able to study spectroscopically the one-dimensional electronic states in bare InAs semiconducting nanowires. We visualize the one dimensional channels through the scattering of the electrons from point impurities on the surface of the nanowire. A series of stacking faults in the crystallographic structure form a Fabry-Perot interferometer that quantizes these states into resonances and allows us to extract the relaxation rate of these electrons as well as the strength of stacking fault as scatterers. Finally, at the end of the nanowire, where the Majorana modes should reside in the topological superconducting case, we find plethora of electronic states. These include quantized quantum-dot states at the tip of the nanowire that leak into the nanowire with strong electron-hole asymmetry, and a one-dimensional state that decays into the nanowire revealing an uncharted nonmonotonic regime in the decoherence of electrons in one dimension.
The design and synthesis of molecules that operate like microscopic machines is of fundamental importance. Such systems can be modelled theoretically by stochastic dynamics in which the system makes thermally activated transitions between a finite set of coarse-grained states. Artificial molecular machines can be driven away from thermal equilibrium in ways not found in biological molecular motors, in particular by time variation of external parameters. Such systems are often termed stochastic pumps. We demonstrate that a seemingly natural protocol of driving such systems does not work. We argue that this result holds also for systems of several particles with zero range interactions.
Recent experimental techniques have allowed to realize, in the so-called cold atomic systems, the quantum-coherent dynamics of many particles undergoing an arbitrary perturbation. Among the many application, one which has attracted a lot of interest is the possibility to study the evolutionof a non-equilibrium state of the Hamiltonian: the so-called quantum-quench protocols. When the spectrum of the Hamiltonian obeys some quite general conditions, local observables relax to a steady state described by the so-called “diagonal ensemble density matrix”, strictly depending on the eigenstates of the Hamiltonian. A question dating back to the origins of quantum mechanics is when the steady state is a thermal equilibrium one. It results, indeed, that this happens when the eigenstates are random states obeying the so-called Eigenstate Thermalization condition, but there is no general theory telling us precisely when this happens. This is quite different from classical thermalization induced by ergodic microscopic dynamics.
After having reviewed these results, clarifying the meaning of the words “integrable” and “ergodic”, I will discuss my recent work aimed to generalize them to the case of periodically driven systems. Noteworthy, even in this case, under conditions very similar to those of the quantum
quench, local observables relax to a steady condition described by the so-called “Floquet-diagonal density matrix”, very similar to its quenched counterpart. Here the observables in the steady condition are time-periodic and the energy-eigenstates are replaced by the Floquet states, which are the eigenstates of the stroboscopic dynamics. I will show numerical results on the Quantum Ising chain, which never thermalizes being integrable, and on its fully-connected counterpart, the Lipkin model. This quantum system has a well defined classical limit and it thermalizes to T = ∞ whenever the classical dynamics is ergodic. This thermalization is induced by the Floquet states obeying ETH and being extended in the Hilbert space. I will conclude the seminar moving from the discussion of local observables, with a well-defined classical limit, to truly quantum non-local objects like the fidelity.
References
[1] A. Russomanno, A. Silva, and G. E. Santoro, Phys. Rev. Lett 109, 257201 (2012), arxiv:1204.5084.
[2] A. Russomanno, A. Silva, and G. E. Santoro, J. Stat. Mech. p. P09012 (2013), arxiv:1306.2805.
[3] S. Sharma, A. Russomanno, G. E. Santoro, and A. Dutta, EPL 106, 67003 (2014).
[4] A. Russomanno, R. Fazio, and G. E. Santoro, arXiv (2014), arxiv:1412.0202
Sensitive and rapid detection of fluorescent dyes at low concentrations is a well-known challenge in many biological applications. Fluorescently labeled biosensors are widely used in specific DNA sequences detection and protein-protein interactions. Conventional genomic DNA (or RNA) detection methods predominantly rely on polymerase chain reaction (PCR) to achieve high sensitivity. However, PCR is costly, time consuming, and requires expertise in molecular biology. Hence, it is difficult for use in real time measurements. An early diagnosis lab tool should be compact, easy to operate, sensitive, and provide high throughput.
Here I present a novel technology termed Magnetic Modulation Biosensing (MMB) for rapid and highly sensitive detection of biomarkers. MMB relies on coupling magnetic beads to fluorescent labeled probes. Using two external magnetic poles, which are positioned on opposite sides of a small glass cell, the magnetic beads are aggregated into the detection area and set in a 1-D periodic motion, in and out of an orthogonal laser beam. The resulting periodic fluorescent light is collected by a photomultiplier and demodulated using a lock-in amplifier. The method offers high sensitivity since the magnetic field attracts the fluorescently labeled probes from the entire solution volume into a small detection area. The modulation and the narrow detection bandwidth separate the signal from the background noise of the non-magnetized solution. Moreover, the aggregation and modulation of the magnetic beads eliminate the need for separation and washing steps usually incorporated in heterogeneous assays, thereby shortening and facilitating the detection assay.
The MMB system is a generic device that has many applications. In general, any approach that produces fluorescent light as a result of the biorecognition event can utilize the MMB system to detect low concentrations of labeled probes. The ability to detect, for example, specific DNA sequences, rapidly, simply, and at low concentrations, is important for the development of a portable, high-throughput tool that enables early detection of pathogens and biomarkers.
We introduce notions concerning locally preferred structures and discuss
recent experimental and numerical results on metallic fluids that suggest
the onset of cooperative dynamics as a liquid is supercooled to form a
glass.
We will further suggest that certain quantum effects may emerge in the
high temperature limit of general "classical fluids". Towards this end, we
will invoke the WKB approximation, extend standard kinetic theory by
taking into account a possible minimal quantum time scale, apply ideas
from transition state theory, and relate (via Planck's constant) the
thermodynamic entropy to periods of semi-classical trajectories. Taken
together, these will suggest that, on average, the extrapolated high
temperature viscosity of general liquids may tend to a value set by the
product of the particle number density n and Planck's constant h.
Experimental measurements of an ensemble of 23 metallic fluids indicate
that might indeed be the case; the extrapolated high temperature viscosity
of each of these liquids divided (for each respective fluid) by its value
of nh veers towards a Gaussian with an ensemble average value that is
close to unity up to an error of size 0.6%. We invoke similar ideas to
discuss other transport properties to suggest how simple behaviors may
appear including resistivity saturation and linear T resistivity may
appear naturally. This approach suggests that minimal time lags may be
present in general fluid dynamics.
I review Raman data on iron pnictide and chalcogenides high-Tc superconductors.Below Tc one or two sharp well defined in-gap modes are observed for cross polarized Raman geometry. Next, I constrain the possible pairing symmetry relying on this data as well as ARPES, and neutron scattering.The in-gap modes are interpreted as Bardasis-Schrieffer and particle-hole excitons respectively consistent with the generalized s-wave pairing. Next I'll talk about the normal state data on parent compounds, AFe2As2, with A=Sr,Eu. The theory that yields a quasi-elastic scattering in terms of spin/orbital nematic correlations is presented.
Periodically driven quantum systems, such as semiconductors subject to light and cold atoms in optical lattices, provide a novel and versatile platform for realizing topological phenomena. Some of these are analogs of topological insulators and superconductors, attainable also in static systems; others are unique to the periodically driven case. I will describe how periodic driving, disorder, and interactions can conspire to give rise to new robust steady states, with no analogues in static systems. In disordered two-dimensional driven systems, a phase with chiral edge states and fully localized bulk states is possible; this phase can realize a non-adiabatic quantized charge pump. In interacting one dimensional driven systems, current carrying states with excessively long life times can arise.
In recent years we have been involved in the search for Majorana fermions (MF) in topological superconducting thin films [1-3]. The idea was to induce superconductivity in the doped topological insulator Bi2Se3 by the proximity effect when it is sandwiched in a bilayer with an s-wave superconductor. For this we deposited bilayer thin films of Bi2Se3-NbN from which we prepared junctions of Bi2Se3-NbN-Au. Conductance spectra results of these junctions showed robust zero bias conductance peaks, which indicate unconventional superconductivity, but did not provide a conclusive evidence for MF. In the past year we investigated ultra-thin bilayers of Bi2Se3-NbN, and compared their properties to those of bare NbN reference films of the same thickness [4]. In my talk I shall discuss some of our recent results, in particular, gating effects of these bilayers, where significant magnetoresistance structures are observed also above the superconducting transition temperature.
[1] G. Koren, T. Kirzhner, E. Lahoud, K. B. Chashka, and A. Kanigel, Phys. Rev. B 84, 224521 (2011).
[2] G. Koren and T. Kirzhner, Phys. Rev. B 86, 144508 (2012).
[3] Gad Koren, Tal Kirzhner, YoavKalcheim and Oded Millo, EPL 103, 67010, (2013).
[4] Gad Koren, Supercond. Sci. Technol. 28, 025003 (2015).
The Fermi surface - a discontinuity in the momentum distribution - is a hallmark of fermionic ensembles. Its existence even for interacting systems forms the basis for Landau’s celebrated Fermi liquid theory. Understanding under what circumstances the Fermi surface collapses and Fermi liquid theory breaks down is one of the long-standing challenges in condensed matter physics. We study this question experimentally with a strongly interacting ultracold Fermi gas in the BCS-BEC crossover regime. The nature of the normal state of the gas in the BCS-BEC crossover is an intriguing and controversial topic, both experimentally and theoretically. While the many-body ground state is always a superfluid condensate of paired fermions, the normal state must evolve from a Fermi liquid to a Bose gas of molecules as a function of the interaction strength. We measure the distribution of single-particle energies and momenta (spectral function) in a homogeneous gas above Tc. Fits to data taken for different interaction strengths reveal the onset of pairing and decreasing spectral weight (or quasiparticle residue, Z) for the Fermi liquid. We extract the effective mass m, pair correlation length, and Tan’s contact. Surprisingly, we find that Z vanishes abruptly and not gradually with increasing interaction, which signals a sudden breakdown of a Fermi liquid description and the disappearance of the Fermi surface.
Spintronics is expected to play an important role towards the quest for more power efficient electronics, by utilizing besides the charge degree of freedom also the electron spin. Key physical phenomena to enable efficient charge-to-spin current conversion, and vice versa, are spin Hall effects, which generate transverse spin current from charge currents even in non-magnetic conductors. In order to gain insight into the underlying physical mechanism and to identify technologically relevant materials, it is important to quantify the spin Hall angle g, which is a direct measure of the charge-to-spin conversion efficiency [1]. We developed a measurement approach based on spin pumping, which enables us to quantify even small spin Hall angles with high accuracy [2,3]. Spin pumping utilizes microwave excitation of a ferromagnetic layer adjacent to a normal metal to generate a homogeneous dc spin current over a macroscopic area. Thickness dependent measurements also allow the determination of spin diffusion lengths, which are essential for a proper quantification of spin Hall effects [4,5]. This approach can be used to detect spin Hall effects in a wide variety of materials, including metallic antiferromagnets [6]. Furthermore, we have shown how magnetic proximity effects can reduce spin Hall conductivities in one of the most widely used material for spin current detection: Pt.
In the second part of my presentation, I will discuss how spin Hall effects can be used for the manipulation of spin textures. In magnetic films with perpendicular anisotropies, magnetostatic interactions can stabilize magnetic skyrmion bubbles. By adding an additional layer with strong spin-orbit coupling to the ferromagnet, it is possible to generate an interfacial chiral Dzyaloshinskii Moriya interaction, which stabilizes the skyrmion spin structure in the magnetic bubble domain wall. Using spin Hall effects these magnetic skyrmion bubbles can then be electrically manipulated. This is demonstrated for completely metallic systems, where we can generate skyrmions through inhomogeneities of electric charge currents in a process that is remarkable similar to the droplet formation in surface-tension driven fluid flows. This provides a practical approach for skyrmion formation on demand.
refs in pdf
The recent discovery of signatures consistent with Majorana fermions in topological superconductors may open the door to their future use in qubit devices. In this talk I will describe theoretical advances in the field of hybrid superconducting circuits that combine semiconductors and superconductors to form new types of topologically protected qubits. Given the time I will also survey experimental progress in these directions.
The state formed at the interface between two materials often offers unique and exciting properties. For instance, modern electronics is heavily based on such interfaces. A decade ago, a new conducting interface was discovered between the two insulating materials LaAlO3 and SrTiO3. The interface was soon found to host many interesting properties including superconductivity, magnetism, resistive switching and the possibility to easily create nanowires at the interface. This spurred a lot of interest in the system, but despite the extensive research, fundamental questions remains such as what the origin of the emergent properties is, and how appealing interface properties can be found and tailored.
Here, we show that replacing LaAlO3 with different complex oxide films opens up new avenues to answer these questions. By the right choice of substitutes for LaAlO3, one can end up with heterostructures where the origin of the interface conductivity is unambiguous and simple. In addition, more perfect heterostructures can be synthesized, giving rise to e.g. enhanced electron mobility and the emergence of quantum magnetotransport properties at low temperatures.
We have studied the electronic transport in GaAs and InAs one dimensional quantum wires. We have observed that in disordered wires the electronic transport is strongly affected by the Coulomb interactions. The experimental results are well described in the framework of so called Luttinger liquid theory. We find the value of the Luttinger interacting parameter for our wires. The conductance peaks observed in Coulomb blockade regime in InAs wires reduce their values at low temperature in contrast with the Coulomb blockade peaks reported so far in all the experiments in other material systems. This phenomenon was predicted theoretically [1] for the materials with the Luttinger Interaction parameter g<1/2.
In the presentation, I will give few possible explanations why the effective Luttinger parameter g in the InAs wires is smaller than the one observed in other 1D systems, like Carbon nanotubes, or GaAs quantum wires reported so far.
[1] A. Furusaki, Phys. Rev. B 57, 7141 (1998).
The study of electrical properties of semiconducting nanocrystals (NCs) poses fundamental as practical difficulties. While control of the optical characteristics of these objects by size and shape is well understood, precise modification of the electronic properties, beyond the band-gap, is still challenging. We have taken a combined macroscopic-nanoscopic approach to investigate the electronic properties of Cu2S NCs. First, a semiconductor-metallic hybrid structure, comprising a Cu2S NC encapsulated by a Ruthenium cage-like shell. The growth of a metallic component induces semiconductor metallic synergistic behavior and unique negative differential conductance features in the tunneling spectra. Second, we discovered a new impurity free mechanism for NC doping, by thermal treatment at moderate temperatures, thus creating vacancies leading to free charge carriers. This thermal doping method is applied to Cu2S-NC arrays, where Cu vacancies easily form, resulting in p-type doping and achieving up to 6 orders of magnitude conductance enhancement. We succeeded also in demonstrating local thermal doping via a focused laser beam serving as the heating source, opening an innovative route for low-temperature patterned doping of NC arrays.
- “Thermal doping by vacancy formation in copper sulfide nanocrystal arrays”, K.Vinokurov, S.Keren-Zur, I. Hadar, Y. Schilt, U. Raviv, O.Millo, U.Banin, NanoLetters,2014, DOI:10.1021/nl4043642
- “Periodic negative differential conductance in a single metallic nanocage”
Y.Bekenstein, K.Vinokurov, TJ.Levy, E.Rabani, U.Banin, O.Millo
Physical Review B 86 (8), 085431(2012) - “Electronic properties of hybrid Cu2S/Ru semiconductor/metallic-cage nanoparticles”
Y.Bekenstein, K.Vinokurov, U.Banin, O.Millo
Nanotechnology 23 (50), 505710(2012) - “Charge transport in Cu2S nanocrystal arrays a study of crystal size and ligand length”, O.Elimelech, K.Vinokurov, O.Millo, U.Banin Zeitschrift für Physikalische Chemie (2014)
In disordered systems, the electronic ground state is the result of a competition between Coulomb interactions, disorder, which eventually leads to localization of charge carriers, and, when relevant, superconductivity. In this conflict between antagonistic forces, dimensionality plays a special role and determines what ground states are allowed. Indeed, in three-dimensional systems, two distinct quantum phase transitions, the metal-to-insulator transition (MIT) and the superconductor-to-metal transition, separate the three possible ground states. By contrast, in two dimensions, the system can only exhibit a direct superconductor-to-insulator transition (SIT), since metals are theoretically forbidden in the absence of strong electron-electron interactions. One important question is then to understand how the three ground states (superconducting, metallic, and insulating) that are possible in bulk systems evolve when the thickness is reduced.
Thin alloy films provide interesting systems to address this question. I will report on the disorder-induced quantum phase transitions in amorphous NbxSi1-x thin films. The disorder is here tuned by three different parameters: the Nb composition, the thickness and the annealing temperature. Low temperature DC measurements reveal unexpected dissipative states after the destruction of the superconducting long range order. These states then evolve towards an insulating state. Our results provide an insight on the entanglement between the MIT and the SIT.
Flying insects can perform a wide array of extreme aerial maneuvers with exquisite accuracy and robustness, outmaneuvering any man-made flying device. As a physical system, a flapping insect is strongly nonlinear with fast-growing mechanical instabilities that must be controlled to allow flight. Hence, similar to balancing a stick on one's fingertip, flapping flight is a delicate balancing act made possible only by ever-present, fast corrective actions. Understanding the underlying mechanisms of insect flight is a major challenge, since this graceful behavior is highly coupled to complex fluid flows and arises from the concerted operation of physiological functions across multiple length and time scales. As such, Insect flight research involves basic concepts from nonlinear dynamics, fluid mechanics, neurobiology and control theory, and has direct application to the development of small flapping robots.
Here we show how flies control their rotational degrees of freedom: yaw, pitch and roll. We focus on their body roll angle, which is unstable and most sensitive degree of freedom. We glue a magnet to each fly and apply a short magnetic pulse that rolls it in mid-air. Fast video shows that flies fully correct for perturbations of up to 100o within 30-+7 ms. The roll correction maneuver consists of a stroke-amplitude asymmetry that is well described by a linear PI controller. For more aggressive perturbations, we show evidence for nonlinear and hierarchical control mechanisms. Flies respond to roll perturbations within a single wing-beat, or 5 ms, making this correction reflex one of the fastest in the animal kingdom.
We have investigated electron transport in a quasi-one dimensional electron gas where the confinement potential can be progressively weakened so resulting in the energy levels decreasing in energy relative to each other. However the decline occurs at different rates which is attributed to the electron-electron repulsion varying with the shape of the wavefunction. When the former ground state crosses the higher levels we find missing plateaux of quantised conductance, this can also be observed in the increase in the capacitance between the confining gates and the electron gas. Studies of the movement of the levels shows that both crossings and anti-crossings can be observed, this will be discussed along with other consequences of the way the levels interact. The role of the interactions in giving rise to the spin dependent 0.7 structure will be presented.
Many-body localization is a peculiar dynamical transition between ergodic and non-ergodic phases, which may occur at any temperature and in any dimension. Current theory suggest that for temperatures below the transition the system is non-ergodic and localized, such that conductivity vanishes at the thermodynamic limit, while for temperatures above the transition the system is thermal and conductive. In this talk I will present a study of ergodicity and dynamical properties of the many-body localization transition using a combination of non-equilibrium diagrammatic techniques and numerically exact methods. I will present the dynamical phase-diagram at infinite temperature, and provide evidence of existence of a novel phase, which is ergodic yet has a vanishing DC conductivity.
Over recent years, a growing interest in the properties of molecular junction, typically described by coupling of individual molecules (quantum dots) to macroscopic electrodes under nonequilibrium conditions, has raised fundamental and conceptual issues regarding the physics of nanometer scale systems, as well as those of the low dimensional mesoscopic systems in which they share certain qualities. Theory faces several important challenges: first, the existence of more than one steady-state in these type of systems has been a matter of debate, both in the context of simple impurity models and in the case of inelastic tunneling channels. Second, characterization of the relevant time scales under the role of molecular vibrations, environmental disorder and dissipation is not yet fully understood. In order to address these issues, I will present an exact theory which enable us to study both dynamical and steady state properties. The theory, based on the Nakajima-Zwanzig equations, relies on the fact that out of equilibrium system posses a so called memory terms. Apparently, this memory term, also known in the literature as a non-Markovian effect, holds a significant amount of information regarding transient properties as well as those of the system's relaxation. This would facilitate our understanding of elastic and inelastic transport phenomena, where the nature of both coherent and incoherent transport is crucial for a complete picture.
NanoSQUIDs residing on the apex of a quartz tip (SOT), suitable for scanning probe microscopy with record size, spin sensitivity, and operating magnetic fields, are presented[1]. We have developed SOT made of Pb with an effective diameter of 46 nm and flux noise of Φn = 50 nΦ0/Hz1/2 at 4.2 K that is operational up to unprecedented high fields of 1 T[2]. The corresponding spin sensitivity of the device is Sn = 0.38 μB/Hz1/2, which is about two orders of magnitude more sensitive than any other SQUID to date. A limitation which is common to all scanning SQUID systems is their sensitivity to only one component of the magnetic field. Recently, a new device that is fabricated by pulling a quartz tube with a “θ” shaped cross section overcomes this limitation [3]. This geometry gives rise to two parallel SQUID loops sharing a common branch. Using a focused ion beam, we then etch the tip so that the two SQUID loops become oblique with respect to each other. As a result of the 3D structure, the SQUID can be tuned in-situ to be sensitive to two orthogonal components of the magnetic field.
We use this technique to study vortex matter in superconductors. At low vortex density and low currents, we measure the fundamental dependence of the elementary pinning force of multiple defects on the vortex displacement. The outstanding magnetic sensitivity of the SOT allows probing vortex displacements as small as 10 pm. This study reveals rich internal structure of the pinning potential and unexpected phenomena such as softening of the restoring force and abrupt depinning. The results shed new light on the importance of multi-scale random disorder on vortex dynamics and thermal relaxation. At high vortex density and high currents, we image the flow patterns of moving lattice revealing dynamic instabilities, plastic flow, and ordering.
[1] A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, L. Neeman, D. Vasyukov, E. Zeldov, M. E. Huber, J. Martin and A. Yacoby, Nano Lett. 10, 1046 (2010)
[2] D. Vasyukov, Y. Anahory, L. Embon, D. Halbertal, J. Cuppens, L. Neeman, A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, M. E. Huber, and E. Zeldov, Nature Nanotech. 8, 639 (2013).
[3] Y. Anahory, J. Reiner, L. Embon, D. Halbertal, A. Yakovenko, Y. Myasoedov, M.L. Rappaport, M. Huber and E. Zeldov, Nano Lett. (in press)
The study of interacting many-body physics is traditionally focused on condensed matter, where electrons interact via Coulomb repulsion and create a rich panoply of correlated states. Observing these delicate quantum states demands pristine environments and locally-defined potentials. The challenges of realizing and probing materials systems, however, has driven the creation of quantum simulators, which are clean and tunable by design, but are typically composed of bosonic constituents with weak interactions. Can we make a condensed matter system with the cleanliness and tunability of a quantum simulator? In this talk, I will describe how a new technique allows us to use carbon nanotubes to create pristine 1D electron systems with unparalleled tunability, and how these devices serve as ideal scanning probes to image the spatial correlations of delicate 1D quantum states. These capabilities open the possibility for a new generation of condensed matter experiments exploring interacting electrons, spins, and nano-mechanics in precision-designed potential landscapes.
In this talk I will discuss the phenomena of dephasing and phase lapses as they occur in two setups operating in the quantum Hall regime. Both setups consist of a quantum dot and an electronic Mach-Zehnder interferometer. Dephasing, i.e. loss of coherent transport, and phases lapses, i.e. abrupt jumps in the phase of the transmission amplitude, turn out to be intimately related in these setups.
In the first setup, transport through a chiral channel is affected by charge fluctuations in a nearby quantum dot [1]. We have studied this setup both in the integer and in the fractional quantum Hall regimes with filling factors 2 and 4/3, respectively [2]. It is found that the regime of operation strongly affects the amount of dephasing and the occurrence (or absence) of phase lapses. Evaluation of the concurrence in the system sheds light on the absence of full dephasing observed in the fractional regime. Recent experimental results will be also discussed [3].
In the second setup transport through a quantum dot operating in the quantum Hall regime with filling factor 2 is analyzed [4]. Similarly to the zero magnetic field case [5], phase lapses occur also here. However, the mechanism responsible for their occurrence is substantially different. It will be shown that certain degrees-of-freedom of the quantum dot act as a dephasor to the coherent transport through the quantum dot.
I will present recent results obtained both theoretically and
experimentally on fractal spectral properties of a polariton gas in a
Fibonacci quasi-periodic potential. The observed spectrum is
accurately reproduced from a theoretical model that we shall present.
We have observed for the first time log-periodic oscillations and the
opening of mini-gaps following the gap labeling theorem. These results
illustrate the potential of cavity polaritons as a quantum simulator
in complex topological geometries.
Graphene and topological insulators are both materials where electronic transport properties are dictated by relativistic energy-momentum dispersion relations. Several topological insulators, such as Bi2Se3, have a layered structure, with weak out-of-plane van-der-Waals bonds, allowing for easy exfoliation into thin flakes. In the talk I will describe how layered materials such as graphene and Bi2Se3 can be vertically stacked, creating a new kind of heterostructure. In electronic devices fabricated from such heterostructures, each material is contacted individually, and electronic properties of the interface are measured at low tempertures. The measurements reveal that the interface is a tunnel junction of exceptionally good quality. This tunnel junction can then be used to perform mutual inelastic spectrocsopy, revealing the phonon spectra of both materials, and to measure the density-dependent spectra of graphene and bilayer graphene.
Since the initial discovery of skyrmion lattices in chiral magnets [1], there has been a tremendous growth in this field as an increasing number of compounds are found to have extended regions of stable skyrmion lattices [2] even close to room temperature [3]. These systems have significant promise for applications due to their size scale and the low currents or drives needed to move the skyrmions [4]. Another interesting aspect of skyrmions is that the equations of motion have significant non-dissipative terms or a Magnus effect which makes them unique in terms of collective driven dynamics as compared to other systems such as vortex lattices in type-II superconductors, sliding charge density waves, and frictional systems. We examine the driven dynamics of skyrmions interacting with random and periodic substrate potentials using both continuum based modelling and particle based simulations. In clean systems we examine the range in which skyrmion motion can be explored as a function of the magnetic field and current and show that there can be a current-induced creation or destruction of skyrmions. In systems with random pinning we find that there is a finite depinning threshold and that the Hall angle shows a strong dependence on the disorder strength. We also show that features in the transport curves correlate with different types of skyrmion flow regimes including a skyrmion glass depining/skyrmion plastic flow region as well as a transition to a dynamically reordered skyrmion crystal at higher drives. We find that increasing the Magnus term produces a low depinning threshold which is due to a combination of skyrmions forming complex orbits within the pinning sites and skyrmion-skyrmion scattering effects. If the skyrmions are moving over a periodic substrate, with increasing drive the Hall angle changes in quantized steps which correspond to periodic trajectories of the skyrmion that lock to symmetry directions of the substrate potential.
[1] S. Muhlbauer et al Science 323 915 (2009).
[2] X. Z. Yu et al. Nature 465, 901–904 (2010).
[3] X.Z. Yu et al Nature Materials, 10, 106 (2011).
[4] A. Fert, V. Cros, and J. Sampaio Nature Nanotechnology 8, 152 (2013).
What are random matrices? Why are they useful in physics? What is
their predictive power? These questions will be answered in the first
part of the talk, and the connection between random matrices and
chaotic systems shown.
Random matrices find applications in such diverse fields of physics as
disordered systems, many-body quantum systems, chiral symmetry
breaking in QCD, and even beyond physics in number theory. Some of
these will be discussed in the remainder of the talk.
We have measured the electronic-structure of FeSe$_x$Te$_{1-x}$ above and below T$_c$ using ARPES. In the normal state we find multiple bands with remarkably small values for the Fermi energy $\varepsilon_F$. Yet, below T$_c$ we find a superconducting gap $\Delta$ that is comparable in size to $\varepsilon_F$, leading to a ratio $\Delta/\varepsilon_F\approx 0.5$ that is much larger than found in any previously studied superconductor. We also observe an anomalous dispersion of the coherence peak which is very similar to the dispersion found in cold Fermi-gas experiments and which is consistent with the predictions of the BCS-BEC crossover theory.
During the last 20 years, the possibility of fabricating nanoscale samples has opened the possibility to study critical phenomena in reduced dimension systems. This includes strong thermal fluctuations, fluctuations of the order parameter, the weakening of the interactions involved and the onset of a competition between surface and volume effects; the major pending question being the existence of singularities of thermodynamic functions at the phase transition in 0D, 1D or 2D magnetic or superconducting systems. Thanks to innovative experimental development based on suspended sensors made at Institut Néel, a very sensitive thermal experiment permits the measurement of specific heat signatures of very small mass sample (down to the nanogram) [1].
In this presentation, I will detail recent experiments demonstrating the power of the specific heat tool to study phase transitions at low dimension. Various illustrations will be given starting from magnetic materials like CoO down to very small thickness (few nm) where a significant reduction in the Néel temperature of CoO ultra-thin films has been revealed. It is found that the films consist of weakly coupled antiferromagnetic (AF) grains. The TN reduction from large to small grain samples scales with the grain size reduction, according to the Binder theory of critical phenomena in systems of reduced dimensions.
Then, I will present measurements performed on submicron mesoscopic superconductors (ring, disk) where phase transitions between different vortex states have been evidenced as a function of applied magnetic field [2,3]. Finally, I will review recent experimental development allowing the measurement of in situ cold deposited superconducting thin films close to the insulating regime. A quench condensed apparatus has been mounted permitting the deposition of the sample directly on the Si membrane sensors at low temperature. Heat capacity measurement can be done continuously as a function of the thickness in granular or ultra-thin film through the insulator to superconducting transition [4]. This innovative equipment paves the way to new measurement of specific heat anomaly in low dimensional systems, in the presence of fluctuations or close to a Quantum Phase Transition.
The disorder-driven superconductor to insulator transition (SIT) is considered to be a prototype of a quantum phase transition at zero temperature. Lately, there has been a renewed interest in this field due to the experimental observations of a number of dramatic features near the SIT of amorphous superconducting materials such as indium oxide and niobium nitride. These novel features included the simple activated temperature dependence of the resistance in the insulating side, a large peak in the magneto resistance, peculiar I-V characteristics and traces of superconductivity at temperatures above Tc. We present experimental results from tunneling spectroscopy, Terahertz spectroscopy and transport measurements that shed light on the physical mechanism governing this phase transition. Our key observations are: 1 - Superconducting gap in the insulating side of the transition. 2 - A possible experimental evidence for collective modes (namely the "Higgs" amplitude mode) in such disordered films close to the SIT. 3 - The possibility to tune the quantum phase transition simply by screening the e-e interactions. These results and their possible consequences will be discussed.
Careful transport measurements near the superconductor-insulator transition in amorphous indium-oxide reveal a symmetry relating states in the insulator to those in the superconducting phase. I will discuss this symmetry and its consequences, and show that it is violated upon entering the strong insulator at low temperatures.
Although solid He does not exhibit bulk supersolidity, there is evidence that grains of solid can move inside a solid matrix.
To observe a flow of solid directly, we constructed a "microphone" embedded in solid He contained inside a torsional oscillator.
The microphone can detect vibrations down a few percent of a lattice constant.
Our idea was that if the solid He flows past the microphone, the atomic corrugation of its surface should generate vibrations at a
frequency f = flow speed/lattice constant. We indeed found that solid He can flow while maintaining its solid structure.
At our lowest temperature of 0.5K, a frictionless flow of solid was observed, in agreement with the prediction that grain boundaries can be superfluid.
Why are the pyrochlore crystals R2Ti2O7 "quantum" magnets? Beginning with
this question,this talk will introduce concepts of quantum magnetism on
the pyrochlore lattice. The symmetry of the lattice and of quantum
mechanical spin states will be described. Using symmetry arguments, the
most general form of a spin=1/2 quantum exchange model for the pyrochlore
crystal can be written down. Symmetry considerations also yield a map
between rare earth spin states and spin=1/2 states, leading to a universal
model of "anisotropic exchange" for rare earth pyrochlores. This model
contains four coupling constants, which have been determined
experimentally for various rare earth pyrochlores.
I review the concept of thermalization of light in systems of coupled nonlinear optical waveguides within the framework of the tight-binding model with nonlinearity.
In many cases of interest the energy and power conservation laws enable the formulation of the equilibrium properties of such systems in terms of the Gibbs measure with positive temperature.
As a particular example I will consider the statistics of two nonlinearly coupled vector fields relevant to the propagation of polarized light in discrete waveguides in the presence of the four-wave mixing. In the limit of the
large nonlinearity an analytical expression for the distribution of Stokes parameters can be obtained which is found to be dependent only on the statistical properties of the initial polarization state and not on the strength of nonlinearity.
The interest in magnetization reversal in nanostructures mainly focuses on two phenomena: the superparamagnetic behavior at a temperature range below the Curie temperature and the expected crossover between thermally-activated reversal to reversal dominated by macroscopic quantum tunneling at sufficiently low temperatures. Here we explore both phenomena by monitoring individual reversals in nanostructures of SrRuO3 and obtain novel insight of both phenomena. In particular, we show for the first time the applicability of the Langevin equation to superparamagnetic fluctuations of an individual volume [1] and we find compelling indication for magnetization reversal dominated by macroscopic quantum tunneling below a record high temperature of 10 K [2].
References
- O. Sinwani, J. W. Reiner, and L. Klein (2014). Monitoring superparamagnetic Langevin behavior of individual SrRuO3 nanostructures. Phys. Rev. B 89 020404(R).
- O. Sinwani, J. W. Reiner, and L. Klein (2012). Indication for macroscopic quantum tunneling below 10 K in nanostructures of SrRuO3. Phys. Rev. B 86 100403(R).
Superconducting quantum interference devices (SQUIDs) are among the most sensitive
sensors for magnetic field, and nanoscale SQUIDs, in particular, also have high
sensitivity to magnetic dipole fields. The sensitivity of SQUIDs with submicron sensor
areas is better than ~100 µB/√Hz, where µB is the Bohr magneton, the magnetic dipole
moment of the electron. When coupled with a scanning platform, these nanoscale
SQUIDs become powerful tools for studying properties of magnetic systems, including
persistent currents in normal metal rings, local measurements of penetration depths in
high Tc
superconductors, vortex dynamics in type II superconductors, and imaging edge
currents in topological insulators. I will describe two such nanoscale SQUID sensors,
planar SQUID susceptometers with sensitivities of ~70 µB/√Hz and needle-like SQUID
magnetometers with sensitivities better than 1 µB/√Hz, concluding with recent results of,
and future plans for, studies using these sensors.
We present an analytical study of diamagnetism and transport in a film with superconducting phase fluctuations, formulated in terms of vortex dynamics within the Debye-Hückle approximation. We find that the diamagnetic and Nernst signals decay strongly with temperature in a manner which is dictated by the vortex core energy. Furthermore, we find that at high temperatures the ratio between the magnetization and the transverse thermoelectric conductivity tends to a constant, determined by the variation of the core energy near the edges of the system. We use the theory to interpret measurements of underdoped La2-xSrxCuO4 above the critical temperature regime and obtain a considerably better fit to the data than a fit based on Gaussian order-parameter fluctuations. Our results indicate that the core energy in this system scales roughly with the critical temperature. The onset of the Nernst effect lies in a regime which can not be described in terms of a liquid of well defined vortices and involves substantial amplitude fluctuations.
TBDWe construct a scattering matrix formulation for the topological classification of one-dimensional superconductors with effective time reversal symmetry in the presence of interactions. For a closed geometry, Fidkowski and Kitaev have shown that such systems have a $\mathbb{Z}_8$ topological classification. We show that in the weak coupling limit, these systems retain a unitary scattering matrix at zero
temperature, with a topological index given by the trace of the Andreev reflection matrix, $\mbox{tr}\, r_{\rm he}$. With interactions, $\mbox{tr}\, r_{\rm he}$ generically takes on the finite set of values $0$, $\pm 1$, $\pm 2$, $\pm 3$, and $\pm 4$. We show that the two topologically equivalent phases with $\mbox{tr}\, r_{\rm he} = \pm 4$ support emergent {\it many-body} end states, which we identify to be a topologically protected Kondo-like resonance. The path in phase space that connects these equivalent phases crosses a non-fermi liquid fixed point where a multiple channel Kondo effect develops. Our results connect the topological index to transport properties, thereby highlighting the experimental
signatures of interacting topological phases in one dimension.
The problem of dissipationless spin transport (spin superfluidity) has occupied the minds of condensed matter physicists for decades. The interest to this problem revived nowadays in connection with the emergence of spintronics. It was in the past, and still is, a matter of controversy. One source of controversy is that spin is not a conserved quantity. This leads to many complications and ambiguities in defining such concepts as spin current, or spin transport. Sometimes these complications are purely semantic, but they can be and were a serious obstacle for understanding physics and for deriving proper conclusions concerning observation and practical application of the phenomenon. The talk will address these problems, connection of spin superfluidity with magnon BEC among them.
A prominent example of anisotropic spin-orbital models is the Kitaev-Heisenberg (KH) model on
the honeycomb lattice [1,2]. This model was proposed as the minimal model to describe the low-
energy physics of the quasi two-dimensional compounds, Na2IrO3 and Li2IrO3. In these compounds,
Ir4+ ions are in a low spin 5d5 con guration and form weakly coupled hexagonal layers. Due to
strong SOC, the atomic ground state is a doublet where the spin and orbital angular momenta of Ir4+
ions are coupled into Je
= 1=2. The KH model describing the interactions between Je
moments
contains two competing nearest neighbor interactions: an isotropic antiferromagnetic Heisenberg
exchange interaction originated mainly from direct direct overlap of Ir t2g orbitals and a highly
anisotropic Kitaev exchange interaction [3] which originates from hopping between Ir t2g and O 2p
orbitals via the charge-transfer gap.
We study critical properties of the KH model on the honeycomb lattice at nite temperatures [4,5].
The model undergoes two phase transitions as a function of temperature. At low temperature,
thermal uctuations induce magnetic long-range order by order-by-disorder mechanism. This mag-
netically ordered state with a spontaneously broken Z6 symmetry persists up to a certain critical
temperature. We nd that there is an intermediate phase between the low-temperature, ordered
phase and the high-temperature, disordered phase. Finite-size scaling analysis suggests that the
intermediate phase is a critical Kosterlitz-Thouless phase with continuously variable exponents. We
argue that the intermediate phase has been likely observed above the magnetically ordered phase
in A2IrO3 compounds.
[1] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009).
[2] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. 105, 027204 (2010).
[3] A. Kitaev, Ann. Phys. 321, 2 (2006).
[4] C. Price and N. B. Perkins, Phys. Rev. Lett. 109, 187201 (2012).
[5] C. Price and N. B. Perkins, Phys. Rev. B 88, 024410 (2013).
I will present scanning tunneling spectroscopy measurements performed on half-metallic ferromagnetic La2/3Ca1/3MnO3 (LCMO) films of variable thicknesses epitaxially grown on two types of high temperature superconductor films: the hole-doped YBa2Cu3O7-d (YBCO) and the electron doped Pr1.85CeCuO4 (PCCO). Surprisingly, our tunneling spectra reveal long-ranged penetration of superconductor order parameter into the LCMO layer, to distances as long as 30 nm, an order of magnitude larger than the expected coherence length associated with singlet-pairing superconductivity in LCMO. This anomalous proximity effect manifests itself in the tunneling spectra as gaps and zero-bias conductance peaks (ZBCPs), and in some cases split-ZBCPs. Our observations are accounted for by the emergence of parallel-spin triplet-pairing superconductivity at the bilayer’s interfaces, promoting the long-ranged proximity effect. The appearance of ZBCPs indicates that the orbital symmetry of the induced order parameter is anisotropic and sign-changing, of either d-wave of p-wave character, corresponding to an odd or even dependence on the Matsubara frequency, respectively, and possibly also of a complex symmetry, e.g., px+ipy. The latter symmetry was also observed in our tunneling spectra measured on the topological insulator Bi2Se3 deposited on the s-wave superconductor NbN. Finally, I will discuss the effect of applied magnetic field on the proximity effect. Interestingly, a non-monotonic behavior was observed, where the proximity effect first increased and then suppressed with magnetic field. The consequence of this behavior regarding the possible mechanisms yielding induced triplet superconductivity, and the corresponding role of magnetic inhomogeneity, will be discussed.
A band with a nontrivial topology, as characterized by a nonzero Chern number, is called a Chern band. When such a band is filled, it displays a quantized integer Hall conductance. Recent numerics show that when such a band is partially full and the electrons in it are subject to strong repulsive interactions, ground states reminiscent of fractional quantum Hall states are found at particular fractional filling. I will show that this is not an accident by constructing an algebraically exact mapping from such bands to fractional quantum Hall problems in terms of operator-based Composite Fermions. On the lattice, one can realize not only the usual FQH states of the continuum, but also states whose existence depends on the lattice, and whose filling can be different from the Hall conductance.
In this talk I will describe the quench dynamics of isolated interacting systems in 1-d, governed by integrable Hamiltonians . I shall study the time evolution of a gas of interacting bosons moving on the continuous infinite line and interacting via a short range potential (the Lieb-Liniger model). For a system with a finite number of bosons we find that independently of the initial state the system asymptotes towards a strongly repulsive gas for any value of repulsive coupling, while for any value of attractive coupling, the system forms a maximal bound state that dominates at longer times. In either case the system equilibrates but does not thermalize, an effect that is consistent with prethermalization. For an infinite system we find that the system equilibrates to a generalized Gibbs ensemble. If time permits I shall discuss also the quench dynamics of the XXZ Heisenberg chain and of a mobile impurity in an interacting Bose gas (quantum Brownian motion).
The rise of graphene has marked the beginning of a renaissance for the almost forgotten class of layered materials. Layered materials are composed of two dimensional sheets of r strongly intralaye bonded compounds stacked together by weak interlayer VdW forces.
Similarly to graphene, the two-dimensional building block of graphite, quantum confined layers of such materials possess fascinating physical properties. Among such properties are the (pseudo)spin-valley coupling of Dirac fermions at the zone edges and giant Stark effect. In this talk we will review some of these properties with an emphasis on transition-metal dichalcogenide semiconductors. Finally, we will discuss the potential of these materials within the roadmap for development of nano-electronic devices.
The study of topology in condensed matter is a rapidly growing field.
From classifying the possible topological insulators and superconductors to ab-initio calculations of realizable materials.
In this talk I will use simple language to introduce the subject and survey some recent proposals. In the first part of my talk I will focus on ideas in which a topologically trivial system is driven into a topological phase. In the second part I will discuss the possibility of realizing topological superconductors in two dimensions which support Majorana Fermions in their vortex cores.
תקציר
ההרצאה תכלול את החלקים הבאים:
1. תאור המערכת, עקרונות פיזיקליים וטכנולוגיים, דוגמות של פערים קריטיים שפתרנו במשך הפרויקט
ושילוב המערכת בפלטפורמות שונות
2. תאור מטריצת ניסויי המערכת: ניסויי המפתח ודוגמאות ניסויים לאישור הביצועים
3. היסטורית פיתוח המערכת, חוויות וקשיים בתקופות שונות: תקופת בדיקת ההיתכנות, תקופת המדגים המערכתי,
תקופת פיתוח המערכת המבצעית תקופת ההספקות והוכחת יכולת המערכת בתרחישים מבצעים
In this talk I will discuss how non-abelian anyons that are richer than Majorana fermions may be engineered by coupling counter-propagating edge modes of abelian quantum Hall states. Furthermore, I will discuss whether and when such anyons may be created in one dimensional wires. I will NOT assume that the audience is fluent in the field of non-abelian anyons.
attached
Anyons play an important role in topological quantum computation. They appear as
the natural quasiparticle excitation of fractional quantum Hall states.
At the edge of the two dimensional electron liquid, the excitations
can be described by one dimensional anyons. In this talk I will
present a model for anyonic excitations which is exactly solvable by
Bethe Ansatz, and I will discuss the role of the statistic parameter
in the single component and multicomponent ground state energy.
The inner ear is a remarkable detector of sound waves, sensitive to signals that vary over 3 orders of magnitude in frequencies and 6 orders in pressure. This detection furthermore occurs in a noisy and highly viscous environment, as the sensory cells – the hair cells – are immersed in a fluid compartment, at room temperature or higher. It was proposed that this sensitivity is achieved by poising the system close to a critical point described by a Hopf bifurcation. I present a new model based on a different bifurcation that can likewise act as an amplifier, and compare the performance with previously suggested models as well as with experimental data, obtained both in vivo and in vitro. This new approach leads to predictions that are in agreement with experiments. In addition, I will demonstrate that ambient noise enhances the detection sensitivity of this mechanism.
*Please not the time change*
In two dimensions, as the microscopic disorder is increased, superconducting films evolve toward an insulating state. This change in ground state has commonly been described as a direct Superconductor–to–Insulator Transition (SIT) and results from the competition between disorder-induced Anderson localization and the formation of a macroscopic superconducting coherent state. a-NbxSi1-x thin metal–alloy films are a model system to study the influence of disorder on superconductivity through a modification of composition, thickness or annealing. I will first present low temperature DC transport measurements performed on this material. We have evidenced non-predicted dissipative states resulting from the disorder-induced destruction of the superconducting long range order. Second, I will focus on a broadband microwave experimental setup we have developed, and the first measurements we have performed at low temperature to probe the electrodynamicresponse of disordered thin films.
Flat lens concept proposed in 1968 by V.G. Veselago has been mostly investigated in the monochromatic regime. It was recently recognized that the time development of the superlensing effect is yet to be assessed and may spring surprises. Here we extend flat lens focusing to elastic waves in a thin plate. A 45°-tilted square lattice of circular holes drilled in a Duraluminium plate has been chosen to experimentally demonstrate focusing of flexural waves. Lamb wave pulse focusing is achieved below the first stop band. Time-resolved experiments reveal that the focused image shrinks with time below diffraction limit, with a lateral resolution increasing from 1.20 λ to 0.37 λ. Finite-difference time-domain simulations confirm the role in pulse reconstruction of lens resonances which repeatedly self-organize in amplitude and phase to provide with super-oscillations.
Magnetic refrigeration is a promising technology for energy efficient and environmentally friendly cooling. Magnetic refrigeration is based on a fundamental thermodynamic property of magnetic materials: the so-called magnetocaloric effect, which causes a temperature change if the material is subject to an applied magnetic field under adiabatic conditions. This changes of the temperature upon magnetization and demagnetization is used to generate cooling and the magnetocaloric effect is most pronounced in the vicinity of a magnetic phase transition of the material, e.g. from a non-ordered (paramagnetic) to a ferromagnetic state. In the Department of Energy Conversion and Storage, we have been working on magnetic refrigeration since 2001. This presentation focuses on the crucial challenges for the technology: development of magnetocaloric materials, high‐field permanent magnets, and the design and optimization of the entire system. Finally, recent results of our magnetic refrigeration prototype will be presented.
Electronic confinement at nanoscale dimensions remains a central means of science and technology. I will describe a novel method for producing electronic nanostructures at the interface between two normally insulating oxides, LaAlO3 and SrTiO3. These structures and devices are “written” by a conductive atomic force microscope probe in ambient conditions at room temperature, and can be erased and reconfigured. The spatial dimensions of these structures are comparable to the width of a single-wall carbon nanotube (~2 nm). A wide variety of devices can be created, including nanowires, tunnel junctions, diodes, field-effect transistors, single-electron transistors, superconducting nanowires, quantum dots and nanoscale THz emitters and detectors. This new, on-demand nanoelectronics platform has the potential for widespread scientific and technological exploitation.
Counter propagating (upstream) chiral neutral edge modes, which were predicted to be present in hole-conjugate states, were observed recently in a variety of fractional Quantum Hall States, by measuring charge noise that resulted after partitioning the neutral mode by a constriction. The observation of such modes raised new questions regarding their microscopical nature and the interplay between the charge and neutral edge modes. Moreover, it opened a new way to probe the wave function of the fractional Quantum Hall states at the first excited Landau level, where some states are predicted to be of non-abelian nature.
In this talk, I will present this emerging new field. First I will focus on measurements performed at the ubiquitous n=2/3, establishing our measurement techniques and revealing new details on the properties of the neutral modes. Secondly, I will present detailed investigation of the neutral modes at the first excited Landau level, demonstrating the existence of an upstream neutral mode in the n=8/3 and n=5/2 states and excluding its existence at the n=7/3 state. These results supports the non-abelian, anti-pfaffian to be the n=5/2 wave function.
Skyrmions are topologically protected states which are at the focus of considerable theoretical and experimental efforts in recent years. Being originally proposed in the context of particle physics, skyrmions now play a major role in almost every field in physics from condensed matter to cosmology. Particularly they are related to quantum Hall effect and fractional quantum Hall effect, liquid crystals, Bose-Einstein condensates, magnetism and even superconductivity.
We show that dipolar interaction in quasi two dimensional ferromagnetic materials breaks the symmetry between different topological excitations. Being sensitive to long range order the interaction prefers the skyrmion topology with positive winding number over the anti-skyrmion topology with negative winding number. We show that the broken symmetry leads to the occurrence of giant anomalous Hall effect (AHE) due to real-space Berry phase.
Title: "Collective Excitations, Stability of the Excitonic Phase, and Electronic Ferroelectricity in the Extended Falicov--Kimball Model"
Abstract:
We consider the excitonic insulator state (often associated with electronic ferroelectricity), which arises on the phase diagram of an extended spinless Falicov--Kimball model (FKM) at half-filling. Within the Hartree--Fock approach, we calculate the spectrum of low-energy collective excitations in this state up to second order in the narrow-band hopping and/or hybridisation.
This allows to probe the mean-field stability of the excitonic insulator.
The latter is found to be unstable when the case of the pure FKM (no hybridisation with a fully localised band) is approached. The excitonic phase, however, may be stabilised further away from the pure FKM limit. In this case, the low-energy excitation spectrum contains new information about the properties of the excitonic condensate (including the strongly suppressed critical temperature).
Our results on stability and degeneracies of the excitonic insulator phase imply that the presence of both hybridisation and narrow-band hopping is required for electronic ferroelectricity. When only one of these is present, the dielectric constant diverges, yet the electrostatic dipole interactions preclude the formation of spontaneous polarisation.
References: Phys Rev B vol 86, 155134 (2012)
Phys. Status Solidi B vol. 250, p. 557 (2013)
The Kagome Heisenberg antiferromagnet is the most likely candidate for a quantum disordered ground state in two dimensions.
State of the art variational methods on large cylinders found no broken spin or translational symmetries, which defines a "quantum spin liquid".
However little is known about its excitation spectrum, and possible point group symmetry breaking.
Using Contractor Renormalization (CORE) we have recently discovered spectral degeneracies in the large system limit.
These signal the onset of p6 - chirality symmetry breaking in the spin liquid phase.
Experimentally, the p6 chiral order parameter may be detected as a splitting of the optical phonons degeneracy near the zone center.
A scanning probe microscope based on a unique nanoSQUID which is fabricated on the apex of a quartz tip has been developed. The nanoSQUID-on-tip device is fabricated by pulling a quartz tube into a sharp pipette with diameters down to 50 nm followed by deposition of a thin superconducting film onto the sides and the apex of the pipette. The devices operate at 4 K in applied magnetic fields of up to 1T and display an extremely low flux noise. As a result a record spin sensitivity of better than 1 μB/Hz1/2 is achieved that is sufficient for detecting the magnetic moment of a single electron. Using a quartz tuning-fork based AFM technique the nanoSQUID can be scanned few nm above the surface of the sample. The combination of high sensitivity, high spatial resolution, wide bandwidth, and close proximity to the sample opens the pathway to direct investigation and imaging of static and dynamic magnetic phenomena on the nanoscale. Preliminary results of study of vortex dynamics in superconductors and magnetic structures in doped topological insulator will be presented.
We study the effect of an embedded weak scatterer (WS) or a weak link (WL) on the electronic transport in a Luttinger liquid (LL) with interacting electrons coupled to massless bosons (e.g., acoustic phonons). We find that a well-known (for the standard LL) duality relation between scaling dimensions of the electron backscattering in the WS and WL limits, D_ws D_wl = 1, holds in the presence of the additional coupling for an any fixed strength of boson scattering from the impurity. This means that at low temperatures such a system remains either an ideal insulator or an ideal metal, regardless of the scattering strength. On the other hand, when fermion and boson scatterings from the impurity are correlated, the system has a rich phase diagram that includes a metal-insulator transition at some intermediate values of the scattering. The results are translated to a 1D flow of fermion-boson mixture of cold atoms through a constriction.
We study the non-abelian statistics characterizing systems where counter-propagating gapless modes on the edges of fractional quantum Hall states are gapped by proximity-coupling to superconductors and ferromagnets. The most transparent example is that of a fractional quantum spin Hall state, in which electrons of one spin direction occupy a fractional quantum Hall state of $\nu= 1/m$, while electrons of the opposite spin occupy a similar state with $\nu = -1/m$. However, we also propose other examples of such systems, which are easier to realize experimentally. We find that each interface between a region on the edge coupled to a superconductor and a region coupled to a ferromagnet corresponds to a non-abelian anyon of quantum dimension $\sqrt{2m}$. We calculate the unitary transformations that are associated with braiding of these anyons, and show that they are able to realize a richer set of non-abelian representations of the braid group than the set realized by non-abelian anyons based on Majorana fermions. We carry out this calculation both explicitly and by applying general considerations. Finally, we show that topological manipulations with these anyons cannot realize universal quantum computation.
We study transport properties of the helical edge states of 2D integer and fractional
topological insulators (TIs/ FTIs), via one and two constrictions (quantum point
contacts). Such constrictions can be made by adding a gate to the systems where the
coupling between edge states on either side of 2D sample is electronically tuned by this
gate. We study the stability of both the conducting (weak backscattering limit) and
insulating fixed points (weak tunneling limit). Moreover, we explore interesting physics
when double impurity is on resonance, leading to perfect transmission (weak
backscattering limit) and Kondo physics (weak tunneling limit). Using renormalization
group and duality mapping, we analyze phase diagrams for the following cases: (i) single
constriction in FTI, which is a generalization of the single constriction in TIs studied by
J. Teo and C. Kane. (ii) two constrictions in TIs, and (iii) two constrictions in FTIs. We
find different behaviors depending on interaction strength and particularly a regime
where conductance is non-monotonic as a function of temperature in the experimentally
accessible parameter regime.
It is shown that a junction of three quantum Ising chains ($\Delta$-junction) can be described as the 2-channel Kondo model in a box which size is of the order of the Ising model correlation length with spin S=1/2 localized at the junction. The local spin is composed of the zero energy boundary Majorana modes of the Ising models.
Experiments on spins and momentum in quasi one-dimensional structures in the GaAs heterostructure will be described. It will be shown that when the momentum degeneracy is lifted by a source-drain voltage a spontaneous spin polarisation occurs. This polarisation is present when the confinement potential is weakened and electrons can relax and form two rows to minimise repulsion.
The spin polarisation can be further investigated by the focussing of electrons in a weak magnetic field. It is shown that inducing the spin polarisation by lifting the momentum degeneracy gives the same results as a large magnetic field which directly lifts the spin degeneracy.
Cold two-dimensional dipolar fluids are predicted to display a very rich phase diagram and intricate particle correlations in both the classical and quantum regimes, far beyond the well studied weakly interacting gases and are currently a major thrust in modern cold atoms and molecules research.
A dipolar exciton fluid in a semiconductor bilayer is a good system to study such physics directly. Furthermore, these fluids can be transported, controlled, and manipulated over macroscopic distances via their interactions with externally applied potentials, a property that can be utilized for new types of coherent exciton based circuitry on a chip.
I will give an overview of our recent work on dipolar exciton fluids, where we have observed interaction induced particle correlations and a transition from a classical to a quantum many-body fluid, as well as a possible evidence for a sharp macroscopic redistribution with the dark spin states.
I will also present a working multi-functional integrated exciton device, which implements static as well as complex moving external potentials to transport, gate, and route excitonic fluxes over macroscopic distances. This is a proof-of-principle for a building block of an excitonic circuitry.
Controlling the coupling between itinerant electrons and localized spins can lead to exotic magnetic states. A novel model system is the interface between LaAlO3 and SrTiO3, where local magnetic moments are believed to coexist with extended two-dimensional electrons. When the density of the itinerant electrons is tuned above a universal critical density, nc, sub-bands with new orbital symmetries are populated, likely leading to a change in the coupling to the localized spins. In this talk I will present our measurements showing that strong, symmetry-dependent, coupling between itinerant electrons and localized moments leads to an unconventional phase diagram for the LaAlO3/SrTiO3 system. Using anisotropic magnetoresistance and anomalous Hall effect measurements, we identify two phases in the space of carrier density and in-plane magnetic field. At densities n>nc and high fields the system is strongly polarized and shows large crystalline anisotropy. Surprisingly, below a density-dependent critical field which is divergent at nc, the polarization and anisotropy vanish whereas the resistivity sharply rises. This behavior, unobserved in other coupled magnetic systems, could be understood within a model involving a coupling that depends on the symmetry of the itinerant electrons. The interplay between the two phases enables gate-tunable magnetism at the LaAlO3/SrTiO3 interface.
Graphene supports a number of remarkable electronic properties, some
of which make it a candidate for certain microelectronic applications.
The challenge, however, of opening a gap in its electronic spectrum has limited its use for basic
circuit elements such as transistors. In this talk I will review recent work
in which an analog of such a gapped spectrum is induced by a time-dependent potential.
The resulting system turns out to have electronic structure with non-trivial
topology, and is an example of a "Floquet Topological Insulator." It supports
surprising fundamental behaviors -- including a quantized Hall effect with
no magnetic field -- but there are fundamental challenges to predicting its
electronic behavior in settings where it can be measured. I will present
results of numerical calculations in which we meet some of these challenges, and
show what should be found in the simplest possible measurement geometry, a
two-terminal conductor. I will discuss the features of the results that demonstrate
the unusual topology of the electronic structure, as well as surprising properties
that are unique to the time-dependent nature of the system.
Unusual magnetic response of As-deficient iron pnictide LaOFeAs is
discussed. It is shown that As vacancies being nominally non-magnetic
defects induce strong enhancement of paramagnetic response of
nonstoichiometric 1111 iron pnictides. This strong magnetic response
however is not detrimental for superconductivity. The microscopic
theory explaining unconventional properties
of defects in Fe-As planes is based on a generalized Anderson - Wolff
impurity model adapted for multivalley semimetals.
We find a connection between quasicrystals and topological matter, namely establishing that quasicrystals exhibit non-trivial topological phases attributed to dimensions higher than their own. Quasicrystals are materials which are neither ordered nor disordered, i.e. they exhibit long-range order only. Recently, the unrelated discovery of Topological Insulators defined a new type of materials classified by their topology. We show that the one-dimensional Harper and Fibonacci quasicrystals are topologically nontrivial. As a result they exhibit topologically-protected boundary states equivalent to the edge states of the two-dimensional Integer Quantum Hall Effect. Additionally, topological bulk phase transitions occur between topologically nonequivalent quasicrystals. We present experiments using photonic lattices, which harness the resultant boundary phenomena to adiabatically pump light across the quasicrystal, and study the bulk phase transitions using a smooth boundary between different quasicrystals.
In 1981 W.G. Unruh predicted that a thermal spectrum of sound waves would be emitted from the sonic horizon in transonic fluid flow, in analogy to black-hole evaporation. Based on this idea, extended to the realm of nonlinear optics, we explore an optical analogue of the Laval nozzle, in which light propagation through a suitably shaped waveguide, filled with a self-defocusing nonlinear medium, mimics the transonic acceleration of a real fluid expanding through a propulsive exhaust nozzle. Experimental demonstrations of transonic flow in prototype optical nozzles will be presented, and the prospect of observing fluctuations that are classical analogues of Hawking radiation will be discussed.
Thermal and thermoelectric conductivities are ideal probes of interaction effects in
correlated electron systems. This is because, in contrast to an electric current, a heat current can be transmitted also by neutral quasiparticles. For instance, energy can be carried by excitations that mediate interactions between other quasiparticles.
In my talk I will present two examples of the dramatic effect of interactions on thermal and thermoelectric transport phenomena. The first is the Nernst effect in the vicinity of the superconducting phase transition. I will demonstrate that the giant Nernst signal, experimentally observed in amorphous films far above Tc, is caused by the fluctuations of the superconducting order parameter. Moreover, I will discuss the anomalous behavior of the Nernst effect near the magnetic-field-induced quantum critical phase transition. The second example is thermal conductivity in spin liquids. Spin liquids can form in the vicinity of the Mott metal-insulator transition when the charge is gapped while the spin degrees of freedom strongly fluctuate. These low energy excitations, dubbed spinons, can conduct heat. The spinons also exhibit a magnetic interaction that leads to non-Fermi liquid behavior. I will show that even in the absence of disorder this strong interaction provides an efficient relaxation mechanism for heat and spin currents, keeping them finite at the lowest temperatures.
The analogue of friction in fluid dynamics is provided by viscosity, which causes dissipation of the energy of the flow. However, if time reversal symmetry is broken, the viscosity tensor may have non-dissipative components, termed “Hall viscosity”, similarly to the non-dissipative Hall conductivity. The Hall viscosity was shown recently to be topologically-protected in rotationally-invariant systems, and to be equal to half the particle density times the orbital angular momentum per particle. Its observation can therefore be of interest in elucidating the nature of the more exotic quantum Hall filling fractions and related systems (e.g., p+ip superfluids), including the possibility of non-abelian statistics and its use for topological quantum computation. However, no concrete measurement scheme has hitherto been proposed.
With this motivation in mind we developed a microscopic linear response theory of viscosity (dissipative as well as Hall), based on the equivalence between a strain rate and time-dependence of the spatial metric of the system. We applied this formalism to rederive and extend previous results on the Hall viscosity. Furthermore, we have established a general relation between the viscosity tensor and the wave-vector dependent conductivity tensor for Galilean-invariant quantum fluids. This relation enables one to extract the Hall viscosity, as well as other viscosity coefficients (shear and bulk) when relevant, from electromagnetic response measurements.
We show that in disordered ferromagnets, where magnetization reversal involves nucleation, domains’ expansion and annihilation, differences between the time dependencies of these processes are responsible for accumulation of the reversed nuclei, for the asymmetry of forward and backward magnetization reversals and for the respective cumulative growth of hysteresis loops. Presence of dilute enclaves of opposite magnetization within a magnetized ferromagnet can be detected by time and field dependent magnetization reversal. Such enclaves are stable due to dipolar fields generated by the surrounding material, and their presence at the onset of reversal can speed the process by orders of magnitude. The field required to suppress these nuclei and reach true magnetic saturation is found to be an order of magnitude higher than the observable macroscopic saturation field. Fatuzzo – Labrune model is extended to describe magnetization reversal starting with ready nucleation domains.
Majorana qubits in semiconducting wires with strong spin-orbit coupling
.and proximity induced superconductivity are new and promising quantum systems
We study the decoherence of Majorana qubits due the non-adiabatic processes as
.well as the ways to manipulate these qubits non-adiabatically
Topological superconducting wires are predicted to support a localized Majorana bound state at each end. These Majorana states are particle-hole symmetric with a zero excitation energy. Their non-local properties and non-abelian braiding statistics, makes them potentially useful in topological quantum computation schemes. The present wave of interest is driven by a number of proposals that suggest ways of realizing and manipulating Majorana states in solid state systems. One proposal that stands out is the use of semiconductor nanowires with strong spin-orbit interaction that are placed in proximity to s-wave superconductors. Here Majorana manipulation require a mere series of gate operations. In light of these proposals, the experimental observations of zero-bias peaks in normal-metal superconductor tunnel junctions, may indicate the presence of a Majorana bound state. However, for the unambiguous identification of this topologically non trivial state, it is crucial to rule out alternative mechanisms for the zero-bias conductance peak in this apparatus. In the experimental setup the wires are terminated by gate induced potentials which are typically smooth functions of position. We show that even in the topological trivial state such an adiabatic confinement can lead to a fermionic end state with an anomalously small energy. The possible existence of such near-zero-energy levels implies that the mere observation of a zero-bias peak in the tunneling conductance is not an exclusive signature of a topological superconducting phase, even in the ideal clean single channel limit.
In this talk I present a theory of the Coulomb drag effect between two closely positioned graphene monolayers. The interest to the Coulomb drag is motivated by recent experiments with by-layer graphene systems, which use either boron-nitride or aluminium oxide as a spacer. These experiments reveal a strongly non-monotonous dependence of drag resistivity on the electron concentration in graphene. I give a qualitative explanation of the experimental data and discuss the limits of applicability of the Fermi-liquid theory of the Coulomb drag. I also show how to extend the theory to the regime of non-degenerate Fermi liquid which is realized in clean graphene samples doped to a close vicinity of the Dirac point. A further increase in the drag resistivity is predicted in this case.
Recently there is a burst of interest in measurements of interference
patterns and noise of topologically-protected edges states in the quantum
Hall effect regime. One reason for this interest is the potential of these
experiments in demonstrating fractional and non-Abelian statistics between
particles, which may lead to a realization of quantum computation. Another
reason is that a new and diverse group of experiments revealed a non-trivial
influence of the coulomb interactions between electrons on the interference
patterns and on the noise, which we only now begin to understand.
In the talk I will give an overview of the various experiments done in this
research field and the connections between them, and I will outline the
theories which we developed in order to explain their results.
The classical dynamics in stationary potentials that are random both in space and time is studied. It can be intuitively understood with the help of Chirikov resonances that are central in the theory of Chaos, and explored quantitatively in the framework of the Fokker-Planck equation. In particular, a simple expression for the diffusion coefficient was obtained in terms of the average power density of the potential. The resulting anomalous diffusion in velocity is classified into universality classes. The general theory was applied and numerically tested for specific examples relevant for optics and atom optics.
Fractals define a new and interesting realm for a discussion of basic phenomena in QED and
quantum optics and their implementation. This interest results from specific properties of
fractals, e.g., their dilatation symmetry as opposed to the translation symmetry of Euclidean
space and the corresponding absence of Fourier mode decomposition. Moreover, the
existence of a set of distinct (usually non integer) dimensions characterizing the physical
properties (spatial or spectral) of fractals make them a useful testing ground for
dimensionality dependent physical problems.
We shall start by noting that the absence of Fourier transform on a fractal implies necessarily
different notions of volume in direct and reciprocal spaces and thus the need to modify the
Heisenberg uncertainty principle. Implications for field quantization and the definition of the
notion of photon on a fractal will be further addressed.
These ideas will find interesting applications in quantum optics of fractal cavities. More
specifically, we shall discuss the existence of a strong Purcell effect, the modification of
spontaneous emission, and the Casimir effect.
We shall then turn to the case of massive bosons and discuss the nature of Bose-Einstein
condensation and the onset of superfluidity in fractal structures. The existence of distinct
fractal dimensions characterizing spatial and spectral properties is instrumental in
understanding the dimensionality dependence of the BEC and the existence of a superfluid
order either through the existence of an “Off Diagonal Long Range Order” (ODLRO) or the
generalization of the Mermin-Wagner theorem.
We have studied the electron energy loss processes in individual single-walled cnts of very high quality. (1) We are currently conducting studies of Terahertz absorption to find Plasmon spatial resonances of an individual swcnt, to verify the predictions for a Luttinger liquid.(2) We report mechanisms of energy loss and far-infrared absorption. Analogous studies of electron-phonon scattering in graphene have only been reported at high temperatures, or at very high electron densities, ≥ 10-13 /cm2. We discuss the prospects for such studies of graphene at lower temperatures and densities.
1. "Energy loss of the electron system in individual single-walled carbon nanotubes," D.F. Santavicca, J.D. Chudow, D.E. Prober, M.S. Purewal and P. Kim, Nano Lett. 10, 4538 (2010); also Appl. Phys. Lett. 98, 223503 (2011), and APL to appear (see www.yale.edu/proberlab).
2. “Luttinger Liquid Theory as a model of the Gigahertz Electrical Properties of Carbon
nanotubes” P. J. Burke, IEEE Trans. Nanotech. 1, p.129 (2002)
The advent of high performance computing and the development of sophisticated numerical techniques have opened new vistas for researchers in condensed matter physics. Exciting predictions of new quantum phases of matter in model systems can often be substantiated or falsified by numerical methods. In this talk, I will present recent development in applying the celebrated density-matrix renormalization–group method to quasi-one dimensional systems. I will show the power of this technique by computing, with high accuracy, critical points in quantum phase transitions induced by small inter-chain interactions. I show that this technique can accurately compute the critical exponent n of the correlation length in the Mott transition. The computed value of n suggest, in agreement with recent slave-rotor speculation, that the Mott transition belongs to the universality class of the classical 3D XY model.
Field-theoretical approach to Anderson localization in 2D disordered fermionic systems of chiral
symmetry classes (BDI, AIII, CII) is developed. Important representatives of these symmetry
classes are random hopping models on bipartite lattices at the band center. As was found by Gade
and Wegner two decades ago within the sigma-model formalism, quantum interference e
ects in these classes are absent to all orders of perturbation theory. We demonstrate that the quantum
localization effects emerge when the theory is treated non-perturbatively. Specifically, they are controlled by topological vortex-like excitations of the sigma models. We derive renormalization group equations including these non-perturbative contributions. Analyzing them, we find that the 2D disordered systems of chiral classes undergo a metal-insulator transition driven by topologically
induced Anderson localization. We also show that the Wess-Zumino and Z2 theta terms on surfaces
of 3D topological insulators (in classes AIII and CII, respectively) overpower the vortex-induced localization.
The basic single file process is the diffusion of N (N → ∞) identical Brownian hard spheres in a quasi-one-dimensional channel of length L (L → ∞), such that the spheres do not jump one on top of the other, and the average particle's density is approximately fixed. The most known statistical properties in this process are that the mean square displacement (MSD) of a particle in the file follows, MSD~t1/2 and its probability density function (PDF) is a Gaussian in position with a variance, MSD.
I’LL focus in the talk on three new variants in file dynamics and address the following questions:
(*) First, the question about the origin of the unique scaling, MSD~t1/2, in simple files, is addressed using scaling law analysis and a new approach for full mathematical computations in normal files.
(*) The MSD is derived in normal files with particles’ density that is not fixed and with particles that are not identical, yet, the diffusion coefficients of the particles are distributed according to a probability density function.
(*) Files with anomalous basic dynamics, both renewal ones and those that are not renewal are solved.
We discuss the full counting statistics of charge transport through a nano device and introduce a general way to calculate it numerically at zero-temperature. We apply this to a strongly correlated model: the interacting resonant level model; where it turns out we can also calculate the full counting statistics exactly, to good agreement with the numerics. From the analytic properties of the full solution, we show that the system undergoes charge fractionalization when the bias voltage exceeds a certain threshold.
We study the topological classification of spin pumps consisting of a family of one-dimensional insulators with a time reversal restriction on the pumping cycle. We find that when adiabatically varied in time, certain band insulators allow for the quantized noiseless pumping of spin even in the presence of strong spin orbit scattering. These spin pumps are closely related to the quantum spin Hall system, and their properties are protected by a time-reversal restriction on the pumping cycle. Based on these findings, we study spin pumps with a bulk energy gap which arises due to electron-electron interactions. We find that the correlated gapped phase can lead to novel pumping properties. In particular, systems with $d$ different ground states can give rise to $d+1$ different classes of spin pumps, including a trivial class which does not pump quantized spin and $d$ non-trivial classes allowing for the pumping of quantized spin $\hbar/n $ on average per cycle, where $1\leq n\leq d$. We discuss an example of a spin pump that transfers on average spin $ \hbar/2$ without transferring charge.
