Role of Spin Hall Effect in the Topological Side Surface Conduction

Abstract

The nature of spin transport in the bulk and side surface of three-dimensional topological insulator thin film geometry is a relatively unexplored subject, compared to the extensively studied top and bottom surface states. Here we employ time- and space-resolved helicity-dependent photocurrent measurements to investigate the effect of optically excited bulk carriers on the spin-polarized topological side surface conduction. Time-resolved femtosecond double-pulse excitation reveals that the spin current toward the side surface arises from the bulk-originated spin Hall effect (SHE), whose microscopic origin is governed by an Elliott–Yafet-type spin relaxation mechanism via an extrinsic side jump process. Bias- and temperature-dependent measurements further confirm that the spin scattering in Bi2Se3 has multiple sources including impurity and electron–phonon scattering. The SHE-assisted side surface spin conduction shows an exceptionally high charge-to-spin conversion efficiency of 35% at 77 K, which may offer new spintronic applications of topological insulators such as spin–orbit torque or spin-flip controlled light-emitting devices.