Probing two topological surface bands of Sb2Te3 by spin-polarized photoemission spectroscopy

Using high-resolution spin-and angle-resolved photoemission spectroscopy, we map the electronic structure and spin texture of the surface states of the topological insulator Sb2Te3. In combination with density functional calculations (DFT), we directly show that Sb2Te3 exhibits a partially occupied, single spin-Dirac cone around the Fermi energy E-F, which is topologically protected. DFT obtains a spin polarization of the occupied Dirac cone states of 80-90%, which is in reasonable agreement with the experimental data after careful background subtraction. Furthermore, we observe a strongly spin-orbit split surface band at lower energy. This state is found at E - E-F similar or equal to -0.8 eV at the (Gamma) over bar point, disperses upward, and disappears at about E - E-F = -0.4 eV into two different bulk bands. Along the (Gamma) over bar-(K) over bar direction, the band is located within a spin-orbit gap. According to an argument given by Pendry and Gurman in 1975, such a gap must contain a surface state, if it is located away from the high-symmetry points of the Brillouin zone. Thus, the novel spin-split state is protected by symmetry, too.