Spin filtering and spin separating effects in U-shaped topological insulator devices

Using non-equilibrium Green's function (NEGF), we investigate spin transport properties of a U-shaped, multi-terminal device based on two-dimensional topological insulator (2DTI), such as HgTe quantum well (HgTe QW). The results show that this 2DTI multi-terminal device can be implemented as a perfect spin filter operating in the absence of ferromagnetic electrodes and external magnetic field. The electrically tunable spin polarization can approach 100%. Moreover, the device can redistribute electrons with opposite spins from an unpolarized source to different leads, resulting in a bifunctional device, which can simultaneously operate as a spin filter and a spin separator. These interesting transport properties can be attributed to the unique helical edge states in a quantum spin Hall system. The performance of the U-shaped device is robust against the weak disorder and the variation in widths and bias voltages, but deteriorates in the presence of the strong disorder and vacancies. The capability of generating highly polarized and controllable spin currents in 2DTI indicates the application of topologically protected and low-powered spintronics. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4757411]