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

摘要

A theory of spin transport and spin detection in Zeeman-split superconductingfilms at low temperatures is developed. It is shown that an injection ofspin-unpolarized quasiparticles into a Zeeman-split superconductor gives riseto a spin imbalance. The relaxation length of such a spin signal is determinedby the energy relaxation length and can be extremely large as compared to therelaxation length of spin-polarized quasiparticles. There can exist two typesof signals: due to nonthermalized quasiparticle distribution and due tothermalized overheated electron distribution. They have different decay lengthsand can be distinguished by their different dependencies on the appliedvoltage. The decay length of the nonthermalized signal is determined by theelectron-electron scattering rate, renormalized due to superconductivity. Thedecay length of the thermalized signal is determined by the length on whichenergy leaves the electronic subsystem and can be very large under specialconditions. Applications of the theory to recent experimental data on spinrelaxation in Zeeman-split and exchange-split superconductors are discussed. Inparticular, it can explain the extremely high spin relaxation lengths,experimentally observed in Zeeman-split superconductors, and their growth withthe magnetic field and with the applied voltage.