Spin Transport and Accumulation in a 2D Weyl Fermion System.

In this work, we demonstrate a new route to realizing an experimentally controllable spin-Hall effect (SHE), on the surface of 3D topological insulators, which depends fundamentally on the spin-orbit-coupled (SOC) nature of the 2D Weyl fermions. Spin transport is solely due to the spin-torque current in this system, and the SHE arises from chiral spin-flip scattering off non-SOC scalar impurities, of potential strength V and size a. The resultant spin-Hall angle has a fixed sign, with θ^{SH}∼O{[V^{2}]/[v_{F}^{2}/a^{2}](k_{F}a)^{4}} being a strongly dependent function of the Fermi wave vector (k_{F}) and Fermi velocity (v_{F}). Hence, the SHE can be tuned by adjusting the Fermi energy or impurity size.