Photocurrent measurements of thermoelectric and photogalvanic effects in magic angle twisted bilayer graphene

Twisting and encapsulating van der Waals materials breaks the inversion and mirror symmetries that are present in the parent monolayers, rendering the heterostructures chiral. The breaking of all mirror symmetries permits finite components of the Berry curvature and quantum geometry, which can manifest in nonlinear optical responses, such as the circular photo-galvanic (CPG) effect. In my talk, I will describe scanning photo-current measurements that can disentangle contributions from the photo-thermoelectric effect and the photo-galvanic effect in magic-angle twisted bilayer graphene. I will show evidence of an in-plane circular photo-galvanic effect, which point to a low symmetry monoclinic structure. The carrier density and temperature dependence of the photo-galvanic effect provide insight into its origin. Our results pave the way to understanding the interplay between structural and interaction-driven symmetry breaking in van der Waals materials, and their relation to optical responses.