Nonmagnetic spin current generation as nonequilibrium Kondo effect in a spin-orbit nano interferometer*

We investigate electric generation of spin-dependent transport through a single-level quantum dot embedded in a ring by help of the Rashba spin-orbit coupling\footnote{N. Taniguchi and K. Isozaki, arXiv:1210.6428 (2012).}. Although it is known for some time that applying finite bias to this type of the spin-orbit interferometer induces finite spin polarization on the dot\footnote{M. Crisan et al. Phys. Rev. B \textbf{79} 125319 (2009).}, the mechanism of driving such spin polarization to flow has not fully been understood. For instance, in spite of finite spin polarization on a noninteracting single-level dot, no spin current is found to appear. We show theoretically that it is possible to generate electrically large spin-dependent current through an interacting single-level dot, as a combined effect of the Kondo effect and finite bias as well as the Rashba spin-orbit interaction. In contrast to earlier work\footnote{H.-F. L\"{u} and Y. Guo, Phys. Rev. B \textbf{76}, 045120 (2007).}, we argue the emergent spin-dependent transport in the present model is viewed as a new type of nonequilibrium Kondo effect; it appears in the middle of the Kondo valley and is suppressed by bias voltage larger than the Kondo energy properly renormalized by the Rashba spin-orbit coupling.