Spin-Orbit-Induced Spin, Charge, and Energy Transport in Diffusive Superconductors

TL;DR

This paper investigates how spin-orbit interactions influence spin, charge, and energy transport in diffusive superconductors, revealing renormalization effects and enhanced spin swapping signals below the transition temperature.

Contribution

It derives coupled transport equations and analyzes the renormalization of spin Hall and spin swapping effects in superconductors using the Keldysh Green's function technique.

Findings

All contributions to spin Hall conductivity are renormalized in the superconducting state.

Spin swapping signal increases significantly below the transition temperature.

Moderate changes observed in the (inverse) spin Hall effect below Tc.

Abstract

We consider the spin-orbit-induced spin Hall effect and spin swapping in diffusive superconductors. By employing the non-equilibrium Keldysh Green's function technique in the quasiclassical approximation, we derive coupled transport equations for the spectral spin and particle distributions and for the energy density in the elastic scattering regime. We compute four contributions to the spin Hall conductivity, namely, skew scattering, side-jump, anomalous velocity, and the Yafet contribution. The reduced density of states in the superconductor causes a renormalization of the spin Hall angle. We demonstrate that all four of these contributions to the spin Hall conductivity are renormalized in the same way in the superconducting state. In its simplest manifestation, spin swapping transforms a primary spin current into a secondary spin current with swapped current and polarization directions. We find that the spin-swapping coefficient is not explicitly but only implicitly affected by superconducting correlations through the renormalized diffusion coefficients. We discuss experimental consequences for measurements of the (inverse) spin Hall effect and spin swapping in four-terminal geometries. In our geometry, below the superconducting transition temperature, the spin-swapping signal is increased an order of magnitude while changes in the (inverse) spin Hall signal are moderate.