Spin-polarized currents and noise in NS junctions with Yu-Shiba-Rusinov impurities

TL;DR

This paper investigates how magnetic impurities in superconductors create Yu-Shiba-Rusinov states that enable highly spin-polarized tunneling currents, with potential applications in superconducting spintronics.

Contribution

It demonstrates that YSR states can produce highly spin-polarized currents, controllable by bias voltage and impurity alignment, even with uncorrelated spins if the interface is spin-active.

Findings

YSR states form metallic bands enabling spin-polarized currents

Spin polarization can be switched by bias voltage

High spin polarization achieved even with uncorrelated spins

Abstract

Conventional superconductors disordered by magnetic impurities demonstrate physical properties drastically different from their pristine counterparts. In our previous work [Phys. Rev. B 92, 245430 (2015)] we explored spectral and thermodynamic properties of such systems for two extreme cases: completely random and ferromagnetically aligned impurity magnetic moments. Here we consider transport properties of these systems, and show that they have a potential to be used in superconducting spintronic devices. Each magnetic impurity contributes a Yu-Shiba-Rusinov (YSR) bound state to the spectrum, residing at sub-gap energies. Provided the YSR states form metallic bands, we demonstrate that the tunneling current carried by these states can be highly spin-polarized when the impurities are ferromagnetically ordered. The spin polarization can be switched by simply tuning the bias voltage. Moreover, even when the impurity spins are completely uncorrelated, one can still achieve almost 100% spin polarization of the current, if the tunnel interface is spin-active. We compute electric current and noise, varying parameters of the interface between tunneling and fully transparent regimes, and analyze the relative role of single-particle and Andreev reflection processes.