Discovery of tunable excitons, giant valley orbital magnetic moment, and unconventional optical selection rules in bilayer graphene*

Multilayer graphene in the rhombohedral stacking order is a unique semiconductor where the bandgap can be continuously controlled by an external electric field. It provides an exciting platform to study conventional exciton physics in the context of valley pseudospin and quantum geometry in an in situ tunable semiconductor bandgap. We employed the photocurrent spectroscopy technique to study AB-stacked bilayer graphene, and observed tunable exciton states with unusual optical selection rules, strong optical resonances and extremely narrow line width. Upon the application of a perpendicular magnetic field, a large valley-dependent magnetic moment was observed which can be traced back to the fundamental Berry curvature effect. We also observed inter Landau level transitions with unconventional optical selection rules, and established the continuous evolution from Coulomb-interaction-dominated to Landau-quantization-dominated optical response. We further used photoluminescence and Raman spectroscopy to study the exciton evolution with tunable carrier density. I will also discuss exciton physics in rhombohedral stacked graphene with more than 3 layers and the effect of Moire superlattice from hBN substrate.