Abstract
Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature. Here by using photocurrent imaging we report experimental evidence of formation and efficient transport of non-equilibrium excitons in Bi2-xSbxSe3 nanoribbons. The photocurrent distributions are independent of electric field, indicating that photoexcited electrons and holes form excitons. Remarkably, these excitons can transport over hundreds of micrometers along the topological insulator (TI) nanoribbons before recombination at up to 40 K. The macroscopic transport distance, combined with short carrier lifetime obtained from transient photocurrent measurements, indicates an exciton diffusion coefficient at least 36 m2 s−1, which corresponds to a mobility of 6 × 104 m2 V−1 s−1 at 7 K and is four order of magnitude higher than the value reported for free carriers in TIs. The observation of highly dissipationless exciton transport implies the formation of superfluid-like exciton condensate at the surface of TIs.
Highlights
Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature
Previous experimental evidence of exciton condensation in gapped semiconductors has been obtained from spatially resolved photoluminescence (PL) measurements, where PL images exhibit macroscopically ordered patterns[2], or PL peak intensity sharply increases with reduced peak widths at lower temperature[3,5]
As the laser beam is raster scanned on the device substrate, the photo-induced current is measured as a function of laser position and plotted into a 2D map (Fig. 1c)
Summary
Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature. A variety of systems, including double quantum wells[1,2,3,4], microcavities[5], graphene[6,7] and transition metal dichalcogenides[8], have shown signatures of exciton condensation Dirac materials such as graphene and topological insulators (TIs) with strong Coulomb attraction and vanishing effective mass emerge as a new promising platform for achieving exciton condensate potentially at room temperature[9,10,11]. Photoexcited electrons and holes in TIs relax to the surface Dirac cones on sub-picosecond (ps) timescales, while further carrier recombination can be much slower, ranging from a few ps to over 400 ps[14,15,16,17,18,19] This long-lived population inversion allows electrons and holes in the transient state to form pairs (Fig. 1a). Previous photocurrent studies of TIs have largely been on degenerately n-doped TIs, where photocurrent is weak with an external quantum efficiency (EQE) of
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