Generation of adjustable pure spin currents in negative-U systems

Abstract

Single-particle sequential tunneling is studied through a negative-U center hybridized with a superconducting, a ferromagnetic, and a normal metal electrodes. In stark contrast to the case of positive U, the single-particle tunneling in attractive charging energy is usually prohibited by ground states with electrons in pairs. We find a microscopic mechanism to induce single-particle sates from pair states. As a consequence, in the nonpolarized metal terminal a remarkable pure spin current with no charge currents survives over a wide range of gate- and bias-voltages, which is rather crucial for experimental observation and design of spintronic devices. In addition, a significant spin-filter effect is presented in certain bias regime.