Spin-galvanic response to non-equilibrium spin injection in superconductors with spin-orbit coupling

TL;DR

This paper demonstrates that nonequilibrium spin injection can induce supercurrents and phase gradients in superconductors with spin-orbit coupling without breaking time-reversal symmetry, explaining experimental observations and predicting new effects.

Contribution

It reveals a novel mechanism for spin injection effects in superconductors relying on nonequilibrium conditions, independent of time-reversal symmetry breaking.

Findings

Spin injection enhances superconductivity near the gap.

Superconductors exhibit anomalous supercurrents due to spin injection.

Predicted electrical control of phase gradients in spin-orbit coupled superconductors.

Abstract

We show that nonequilibrium spin injection into a superconductor can generate an anomalous supercurrent or induce a phase gradient, even for spin voltages below the superconducting gap. Our mechanism does not require breaking time-reversal symmetry in the effective superconducting Hamiltonian, but instead relies on nonequilibrium spin injection. We further demonstrate that superconductivity enhances spin injection due to the large quasiparticle density of states near the pairing gap, an effect that persists well below the gap. This contrasts with earlier works predicting the absence of spin injection at zero temperature and small spin voltages. Our results provide a natural explanation for long-standing experimental observations of spin injection in superconductors and predict novel effects arising from spin-charge coupling, including the electrical control of anomalous phase gradients in superconducting systems with spin-orbit coupling. These effects are broadly testable in a variety of materials and hybrid superconducting structures.