Origin of the giant spin Hall effect in BiSb topological insulator

The giant spin Hall effect (SHE) at room temperature is one of the most attractive feature of topological insulators (TIs) for applications to nano-scale spin devices. Its origin, however, remains a controversial problem. Here, we identify the origin of the giant SHE in BiSb thin films by measuring the spin Hall angle ${\theta}_{SH}$ under controllable contribution of surface and bulk conduction. We found that ${\theta}_{SH}$ of a Bi$_{0.6}$Sb$_{0.4}$ TI thin film takes colossal values (450 - 530 at 8 K, and 38 at 300 K), and is almost governed by contribution from the topological surface states. Meanwhile, ${\theta}_{SH}$ in a Bi$_{0.2}$Sb$_{0.8}$ semi-metallic thin film without topological surface states drastically decreases. Our results provide a quantitative tool for analysing the origin of the giant SHE in TI thin films, as well as a strategy for designing spin current source utilizing the surface states of TI in high-performance nano-scale spin devices.