Injection of nonequilibrium quasiparticles into Zeeman-split superconductors: a way to create long-range spin imbalance

TL;DR

This paper develops a theory explaining how injecting unpolarized quasiparticles into Zeeman-split superconductors can produce long-range spin imbalances, accounting for experimental observations of extended spin relaxation lengths.

Contribution

It introduces a novel theoretical framework for spin transport in Zeeman-split superconductors, highlighting the role of nonequilibrium quasiparticles and distinguishing between thermalized and nonthermalized signals.

Findings

Long spin relaxation lengths explained by energy relaxation mechanisms.

Different decay lengths for thermalized and nonthermalized signals.

Growth of relaxation lengths with magnetic field and voltage observed experimentally.

Abstract

A theory of spin transport and spin detection in Zeeman-split superconducting films at low temperatures is developed. It is shown that an injection of spin-unpolarized quasiparticles into a Zeeman-split superconductor gives rise to a spin imbalance. The relaxation length of such a spin signal is determined by the energy relaxation length and can be extremely large as compared to the relaxation length of spin-polarized quasiparticles. There can exist two types of signals: due to nonthermalized quasiparticle distribution and due to thermalized overheated electron distribution. They have different decay lengths and can be distinguished by their different dependencies on the applied voltage. The decay length of the nonthermalized signal is determined by the electron-electron scattering rate, renormalized due to superconductivity. The decay length of the thermalized signal is determined by the length on which energy leaves the electronic subsystem and can be very large under special conditions. Applications of the theory to recent experimental data on spin relaxation in Zeeman-split and exchange-split superconductors are discussed. In particular, it can explain the extremely high spin relaxation lengths, experimentally observed in Zeeman-split superconductors, and their growth with the magnetic field and with the applied voltage.