Spin-Seebeck effect on the surface of a topological insulator due to nonequilibrium spin-polarization parallel to the direction of thermally driven electronic transport

We study the transverse spin-Seebeck effect (SSE) on the surface of a three-dimensional topological insulator (TI) thin film, such as Bi2Se3, which is sandwiched between two normal metal leads. The temperature bias Delta T applied between the leads generates surface charge current which becomes spin polarized due to strong spin-orbit coupling on the TI surface, with polarization vector acquiring a component P-x similar or equal to 60% parallel to the direction of transport. When the third nonmagnetic voltage probe is attached to the portion of the TI surface across its width Ly, pure spin current will be injected into the probe where the inverse spin Hall effect (ISHE) converts it into a voltage signal |V-ISHE|(max)/Delta T similar or equal to 2.5 mu V/K (assuming the SH angle of the Pt voltage probe and L-y = 1 mm). The existence of predicted nonequilibrium spin polarization parallel to the direction of electronic transport and the corresponding electron-driven SSE crucially relies on orienting quintuple layers (QLs) of Bi2Se3 orthogonal to the TI surface and tilted by 45 degrees. with respect to the direction of transport. Our analysis is based on the Landauer-Buttiker-type formula for spin currents in the leads of a multiterminal quantum-coherent junction, which is constructed by using nonequilibrium Green function formalism within which we show how to take into account arbitrary orientation of QLs via the self-energy describing coupling between semi-infinite normal metal leads and the TI sample.