Theory of the spin-galvanic effect and the anomalous phase-shift $\varphi_{0}$ in superconductors and Josephson junctions with intrinsic spin-orbit coupling

TL;DR

This paper develops a quasiclassical theory linking spin-orbit coupling to magneto-electric effects in superconductors, revealing how an equilibrium spin current induces an anomalous phase shift in Josephson junctions, with potential applications in spintronics.

Contribution

The paper introduces a unified theoretical framework connecting the spin-galvanic effect and the anomalous phase shift $oldsymbol{ ext{ϕ}_0}$ in superconducting Josephson junctions with spin-orbit coupling.

Findings

$oldsymbol{ ext{ϕ}_0}$ is proportional to the equilibrium spin current.

The theory generalizes previous results to broader SOC types.

Magneto-electric effects can be harnessed in superconducting spintronics.

Abstract

Due to the spin-orbit coupling (SOC) an electric current flowing in a normal metal or semiconductor can induce a bulk magnetic moment. This effect is known as the Edelstein (EE) or magneto-electric effect. Similarly, in a bulk superconductor a phase gradient may create a finite spin density. The inverse effect, also known as the spin-galvanic effect, corresponds to the creation of a supercurrent by an equilibrium spin polarization. Here, by exploiting the analogy between a linear-in-momentum SOC and a background SU(2) gauge field, we develop a quasiclassical transport theory to deal with magneto-electric effects in superconducting structures. For bulk superconductors this approach allows us to easily reproduce and generalize a number of previously known results. For Josephson junctions we establish a direct connection between the inverse EE and the appearance of an anomalous phase-shift $\varphi_{0}$ in the current-phase relation. In particular we show that $\varphi_{0}$ is proportional to the equilibrium spin-current in the weak link. We also argue that our results are valid generically, beyond the particular case of linear-in-momentum SOC. The magneto-electric effects discussed in this study may find applications in the emerging field of coherent spintronics with superconductors.