Nanoscale thermal and magnetic imaging: Observation of intricate current patterns and heat flow in WTe2 flakes

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.