We report successful spin injection into the surface states of topological insulators by using a spin pumping technique. By measuring the voltage that shows up across the samples as a result of spin pumping, we demonstrate that a spin-electricity conversion effect takes place in the surface states of bulk-insulating topological insulators Bi(1.5)Sb(0.5)Te(1.7)Se(1.3) and Sn-doped Bi(2)Te(2)Se. In this process, the injected spins are converted into a charge current along the Hall direction due to the spin-momentum locking on the surface state.

We detected the spin polarization due to charge flow in the spin nondegenerate surface state of a three-dimensional topological insulator by means of an all-electrical method. The charge current in the bulk-insulating topological insulator Bi1.5Sb0.5Te1.7Se1.3 (BSTS) was injected/extracted through a ferromagnetic electrode made of Ni80Fe20, and an unusual current-direction-dependent magnetoresistance gave evidence for the appearance of spin polarization, which leads to a spin-dependent resistance at the BSTS/Ni80Fe20 interface. In contrast, our control experiment on Bi2Se3 gave null result. These observations demonstrate the importance of the Fermi-level control for the electrical detection of the spin polarization in topological insulators.

The mixed-chalcogenide compound TlBiSSe realizes a three-dimensional (3D) Dirac semimetal state. In clean, low-carrier-density single crystals of this material, we found Shubnikov-de Haas oscillations to signify its 3D Dirac nature. Moreover, we observed very large linear magnetoresistance (MR) approaching 10,000% in 14 T at 1.8 K, which diminishes rapidly above 30 K. Our analysis of the magnetotransport data points to the possibility that the linear MR is fundamentally governed by the Hall field; although such a situation has been predicted for highly-inhomogeneous systems, inhomogeneity does not seem to play an important role in TlBiSSe. Hence, the mechanism of large linear MR is an intriguing open question in a clean 3D Dirac system.