Tuning and probing symmetry breaking in graphene
Andrea Young, MIT
04/12/2014 - 12:30 - 13:30
Resnick Building 209, room 210
In monolayer and bilayer graphene, the carbon sublattices endow the electron wavefunctions with a valley degree of freedom in addition to the usual electron spin. At high magnetic fields, this additional degeneracy leads to multicomponent Landau levels characterized by an approximate SU(4) symmetry in the combined spin-valley isospin space. Electornic interactions tend to break this symmetry, leading to a plethora of isospin quantum Hall ferromagnetic states with different spin and valley order. In this talk, I will discuss our recent efforts to both understand and manipulate these broken symmetry states.
First, I will describe how the fundamental difference between spin- and valley- ordered states allows us to realize an analog of the quantum Spin Hall effect in monolayer graphene. This state, which we induce with large in-plane magnetic fields, hosts counterpropagating, spin-filtered edge states protected from backscattering by an emergent spin-rotation symmetry. By balancing the applied field against an intrinsic antiferromagnetic instability, we can break this symmetry controllably to create a one dimensional system with tunable band gap and spin texture.
In the second part of the talk, I will descibe a recent experiment to directly probe isospin order in bilayer graphene. In bilayers, the sublattices giving rise to the valley degeneracy are on different layers; in the zero energy Landau level this makes valley polarization synonymous with layer polarization. I will show how capacitance measurements can be used to measure this layer polarization, revealing several previously unobserved gapped states as well features associated with phase transitions between states of different layer polarization.