Nonequilibrium spin texture within a thin layer below the surface of current-carrying topological insulator Bi2Se3: A first-principles quantum transport study
Abstract
We predict that unpolarized charge current injected into a ballistic thin film of prototypical topological insulator (TI) Bi2Se3 will generate a noncollinear spin texture S(r) on its surface. Furthermore, the nonequilibrium spin texture will extend into 2 nm thick layer below the TI surfaces due to penetration of evanescent wavefunctions from the metallic surfaces into the bulk of TI. Averaging S(r) over few along the longitudinal direction defined by the current flow reveals large component pointing in the transverse direction. In addition, we find an order of magnitude smaller out-of-plane component when the direction of injected current with respect to Bi and Se atoms probes the largest hexagonal warping of the Dirac-cone dispersion on TI surface. Our analysis is based on an extension of the nonequilibrium Green functions combined with density functional theory (NEGF+DFT) to situations involving noncollinear spins and spin-orbit coupling. We also demonstrate how DFT calculations with properly optimized local orbital basis set can precisely match putatively more accurate calculations with plane-wave basis set for the supercell of Bi2Se3.