UC Davis

Bismuth selenide is a prototypical topological insulator, which is a remarkable classof materials due to their linear dispersion relation owing to their time-reversal symmetry- protected Dirac cones. Recently, antimony-doped bismuth selenide MOSFETs grown by chemical vapor deposition have displayed millimeter-long diffusion lengths at cryogenic tem- peratures with a tunable chemical potential. Motivated by the hypothesis that these highly extended and efficient photocurrents are reliant on the topological surface states, I will dis- cuss recent experiments to probe this observation through several avenues: first, by creating MOSFET architecture that allows us to locally gate the chemical potential; second, through ultrafast photocurrent studies achieved with a Ti:sapphire laser to dramatically modulate the carrier concentration; third, by studying illuminated magnetotransport studies to shed light on contributions to weak antilocalization; fourth, through a tandem approach of experimental and theoretical investigations into the excitations generated through helical radiation. Our results provide clues about the nature of the carriers responsible for these uniquely nonlocal photocurrents which may be due to the carrier states forming an exciton condensate. ii