Bulk spin Hall effect in topological insulator thin films

Spin current generation in topological insulators(TI) is usually attributed to the spin-momentum locking of topological surface states, where the direction of current fixes the direction of net spin-polarization. However, this mechanism cannot explain the relatively large spin currents observed in experiment. Recently, it has been proposed that the insulating, but topologically non-trivial bulk of a TI may produce large transverse spin currents through the bulk-spin Hall effect. This effect was first predicted in 2004[1], but never experimentally observed. In this work, we study magneto-transport in Bi₂Se₃ thin films in perpendicular and parallel magnetic fields as a function of sample thickness, disorder strength and temperature and reveal direct evidence for a transverse bulk current that electrically connects the top and bottom topological surface states. From this, we estimate the bulk spin Hall conductivity in our samples that closely matches with theoretical predictions. Our work constitutes the first experimental observation of the bulk spin Hall effect in TIs and paves the way towards designing highly efficient charge-to-spin converters using TIs.([1]. Phys. Rev. Lett. 93, 156804(2004))