Frequency-Domain Measurement of the Spin Imbalance Lifetime in Superconductors

TL;DR

This study measures the spin imbalance lifetime in superconductors using frequency domain techniques, revealing a cutoff frequency related to the lifetime and aligning with theoretical models of quasiparticle tunneling.

Contribution

It introduces a frequency domain measurement method for spin imbalance lifetime in superconductors, providing new insights into quasiparticle dynamics and tunneling processes.

Findings

Spin imbalance lifetime is approximately 1/(2πf_RF) at the cutoff frequency.

Lifetime remains relatively constant across temperature ranges, slightly increasing with magnetic field.

Results align with theoretical models considering quasiparticle tunneling processes.

Abstract

We have measured the lifetime of spin imbalances in the quasiparticle population of a superconductor ($\tau_s$) in the frequency domain. A time-dependent spin imbalance is created by injecting spin-polarised electrons at finite excitation frequencies into a thin-film mesoscopic superconductor (Al) in an in-plane magnetic field (in the Pauli limit). The time-averaged value of the spin imbalance signal as a function of excitation frequency, $f_{RF}$ shows a cut-off at $f_{RF} \approx 1/(2\pi\tau_s)$. The spin imbalance lifetime is relatively constant in the accessible ranges of temperatures, with perhaps a slight increase with increasing magnetic field. Taking into account sample thickness effects, $\tau_s$ is consistent with previous measurements and of the order of the electron-electron scattering time $\tau_{ee}$. Our data are qualitatively well-described by a theoretical model taking into account all quasiparticle tunnelling processes from a normal metal into a superconductor.