Helicity dependent photocurrent in electrically gated (Bi1\u2212xSbx)2Te3 thin films

Yu Pan,Thomas C Flanagan,David D Awschalom,Nitin Samarth,Timothy Pillsbury,Chao-Xing Liu,Haijun Zhang,Qing-Ze Wang,Andrew L Yeats,Anthony Richardella

doi:10.1038/s41467-017-00711-4

Abstract

Circularly polarized photons are known to generate a directional helicity-dependent photocurrent in three-dimensional topological insulators at room temperature. Surprisingly, the phenomenon is readily observed at photon energies that excite electrons to states far above the spin-momentum locked Dirac cone and the underlying mechanism for the helicity-dependent photocurrent is still not understood. Here we show a comprehensive study of the helicity-dependent photocurrent in (Bi1−xSbx)2Te3 thin films as a function of the incidence angle of the optical excitation, its wavelength and the gate-tuned chemical potential. Our observations allow us to unambiguously identify the circular photo-galvanic effect as the dominant mechanism for the helicity-dependent photocurrent. Additionally, we use an analytical model to relate the directional nature of the photocurrent to asymmetric optical transitions between the topological surface states and bulk bands. The insights we obtain are important for engineering opto-spintronic devices that rely on optical steering of spin and charge currents.

Highlights

Polarized photons are known to generate a directional helicity-dependent photocurrent in three-dimensional topological insulators at room temperature
Experiments have shown that circularly polarized light induces a directional helicity-dependent photocurrent (HDPC) in 3D topological insulators (TIs): in other words, light of opposite circular polarization yields a photocurrent propagating in opposite directions[15]
The natural impulse is to immediately attribute the HDPC in 3D TIs to the helical spin texture of the Dirac surface states[15]: circularly polarized photons couple to the spin-momentumlocked topological surface states, yielding a circular photogalvanic effect (CPGE), a phenomenon well-established in semiconductor quantum wells, where the inversion symmetry breaking is the cause[16, 17]

Summary

Introduction

Polarized photons are known to generate a directional helicity-dependent photocurrent in three-dimensional topological insulators at room temperature. Time-resolved measurements of the HDPC in 3D TIs seem to confirm the surface-related origin for the HDPC by showing that the group velocity of the induced photocurrent matches that expected for Dirac surface electrons[18] This interpretation is difficult to reconcile with the fact that the photon energy used in the experiments is about 5 times larger than the bulk energy gap. Another relevant question is the underlying microscopic mechanism for the HDPC While both the CPGE and the photon drag effect can be related to optical transitions involving the lowest energy surface states alone, there are several other possibilities.

Methods

Results

Conclusion