Quantum dot spin filter in resonant tunneling and Kondo regimes

A quantum dot with spin-orbit interaction can work as an efficient spin filter if it is connected to N (≥ 3) external leads via tunnel barriers. When an unpolarized current is injected to a quantum dot from a lead, polarized currents are ejected to other leads. A two-level quantum dot is examined as a minimal model. First, we show that the spin polarization is markedly enhanced by resonant tunneling when the level spacing in the dot is smaller than the level broadening. Next, we examine the many-body resonance induced by the Kondo effect in the Coulomb blockade regime. A large spin current is generated in the presence of the SU(4) Kondo effect when the level spacing is less than the Kondo temperature.