A topological insulator (TI) is a new phase of solids. In TIs, the surface (or edge) state has a Dirac-like linear band structure with 100% spin polarization due to spin-momentum locking, whereas the bulk state has band gap. The spin-polarized state is topologically protected and persistent spin current flows in the state, which greatly attracts researchers in spintronics. In 3-dimensional (3-D) TIs, the surface state exists in the whole surface and detection of the spin-momentum locking in the surface state has been awaited. Experimentally, APRES allows the surface states, however in a spintronics viewpoint, an electrical detection of them has been a significant milestone.
We selected BiSbTeSe (BSTS)  as a TI for realizing electrical detection of the surface spin polarization, since BSTS is bulk insulative, the Fermi level can be tuned at the surface state (in the conduction band side), i.e., in between the gapped bulk state. We fabricated a spin valve device using the BSTS thin film and a local 3-terminal method was used for the detection of the surface spin polarization . When the device temperature goes down, thermal activation of carriers from the Fermi level to the bulk states is efficiently suppressed, and a magnetoresistance effect due to the spin-momentum locking was observed. The effect was observed up to 120 K. A number of control experiments supported our claim. We have also investigated the other bulk insulative 3-D TI, TlBiSe [3,4], electrically. We have observed quantum oscillation due to the existence of the surface state. The effective mass in the state was estimated to be 0.03m0, and the Berry phase of the state is non-zero, which is evidence that the state is topological surface state . Furthermore, it was clarified that the dominant carrier is hole in TlBiSe. This is the first report on hole dominant TIs. The detail is discussed in the presentation.
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