Electric field effect on exciton transport in topological insulator nanoribbons

At cryogenic temperatures, photoexcited charge carriers in topological insulators can form an exciton condensate that exhibits dissipationless transport. Using ultrafast transient photovoltage microscopy, we previously observed exceptionally high carrier diffusion coefficients, up to 800 cm²/s, in Sb-doped Bi₂Se₃ nanoribbons at low temperatures, indicative of such a condensate. To further confirm its excitonic nature, we investigate how an external electric field influences the transport of photoexcited carriers, as the transport of charge-neutral excitons is insensitive to electric field, in contrast to free carriers.  Our measurements reveal that the charge carrier transport velocity is indeed independent of electric field at 12K and low excitation fluence, consistent with charge-neutral transport. At higher fluence, carrier drift emerges even at low fields, attributed to a Mott transition from bound excitons to an electron-hole plasma. Furthermore, we observe a temperature-dependent reduction in carrier diffusion length at a critical electric field, consistent with field-induced exciton dissociation.