Spin imbalance in hybrid superconducting structures with spin-active interfaces

TL;DR

This paper investigates how spin-active interfaces in superconductor-normal metal heterostructures induce localized spin imbalances and how these effects can be used for spectroscopic purposes without external magnetic fields.

Contribution

It demonstrates the role of Andreev bound states in spin imbalance and proposes a new method for observing spin imbalance through bias voltage dependence.

Findings

In-gap Andreev bound states dominate spin imbalance near the interface.

Spin imbalance can be observed without external magnetic fields.

Bias voltage dependence enables bound-state spectroscopy.

Abstract

We consider a heterostructure consisting of a normal metal and a superconductor separated by a spin-active interface. At finite-bias voltages, spin-filtering and spin-mixing effects at the interface allow for an induced magnetization (spin imbalance) on the superconducting side of the junction, which relaxes to zero in the bulk. Such interfaces are also known to host a pair of in-gap Andreev bound states which were recently observed experimentally. We show that these states are responsible for the dominant contribution to the induced spin imbalance close to the interface. Motivated by recent experiments on spin-charge density separation in superconducting aluminum wires, we propose an alternative way to observe spin imbalance without applying an external magnetic field. We also suggest that the peculiar dependence of the spin imbalance on the applied bias voltage permits an indirect bound-state spectroscopy.