Millimetre-long transport of photogenerated carriers in topological insulators

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.

Dynamically Induced Excitonic Instability in Pumped Dirac Materials

“…A recent experimental study reported the evidence of excitonic superfluid phase on the surface of Bi 2−x Sb x Se 3 3D TI. [131] By using scanning photocurrent microscopy, it was shown that photoexcited electrons and holes form charge neutral bound states which propagate over distances of 1 mm at temperatures up to 40 K. There are indications that the observed effect could be explained by the theoretical model discussed in this work. For instance, in Bi 2−x Sb x Se 3 samples the Fermi level is close to the Dirac point which is an important condition for the formation of excitonic state.…”

“…[63] Recently, transport signatures of the excitonic condensate phase have been found on 3D TI surfaces, with critical tem-perature consistent with the values predicted by the theoretical model. [131] Although the existence of the condensate requires rigorous confirmation, this work gives an incentive to use 3D TIs as a model system for further investigation of the transient excitonic states. One interesting problem is the effect of spin-momentum locking on the spin texture of the excitonic states and their dynamics.…”

“…[63,64] There are no signatures of the excitonic phase in pure Bi 3 Se 3 , in which these conditions are not satisfied. [131] Although the existence of the transient condensate with extremely long decay time needs to be verified further, this is a promising step towards realization of dynamically induced excitonic phase.…”

Дефекты межслоевой поверхности и термоэлектрические свойства в слоистых пленках топологических изоляторов n-Bi-=SUB=-2-=/SUB=-Te-=SUB=-2.7-=/SUB=-Se-=SUB=-0.15-=/SUB=-S-=SUB=-0.15-=/SUB=-

“…В спинтронике операция преобразования между спином и зарядом при комнатной температуре реализуется в ТИ на несколько порядков эффективнее, чем тяжелых металлах [7,8]. В термоэлектричестве и электронике рассматривается возможность использования эффекта свертекучести экситонов [9,10].…”

“…В тонкопленочных двухслойных экситонных структурах на основе n-и p-типа Bi 2 Te 3 , разделенных изолирующим слоем, за счет кулоновского взаимодействия электроннодырочных пар на границе полупроводника и изолятора возникают межслоевые экситоны при оптимальных толщинах пленок и изолирующих слоев. По оценкам [10,19] сопротивление, связанное с эффектом кулоновского увлечения, становится существенно меньше, чем сопротивление каждой пленки, подвижность носителей заряда увеличивается на порядок, что позволяет использовать двухслойные экситонные структуры для увеличения термоэлектрической эффективности.…”