Quasiparticle scattering time in superconducting films: from dirty to clean limit

TL;DR

This study investigates quasiparticle energy relaxation in superconducting Nb films across dirty to clean limits, revealing temperature-dependent relaxation times linked to electron-phonon interactions, supported by experimental and theoretical analysis.

Contribution

It provides the first detailed comparison of quasiparticle relaxation times in Nb films across different purity regimes using flux-flow instability measurements.

Findings

Constant relaxation time in dirty samples

Strong temperature dependence in clean samples

Electron-phonon scattering contribution increases towards the clean limit

Abstract

We study the quasiparticle energy relaxation processes in superconducting Nb films of different thicknesses corresponding to different electron mean free paths in a state far from equilibrium, that is the highly dissipative flux-flow state driven up to the instability point. From the measured current-voltage curves we derive the vortex critical velocity $v^{*}$ for several temperatures. From the $v^{*}(T)$ values, the quasiparticle energy relaxation time $\tau_{\epsilon}$ is evaluated within the Larkin-Ovchinnikov model and numerical calculations of the quasiparticle energy relaxation rates are carried out to support the experimental findings. Besides the expected constant behavior of $\tau_{\epsilon}(T)$ for the dirty samples, we observe a strong temperature dependence of the quasiparticle energy relaxation time in the clean samples. This feature is associated with the increasing contribution from the electron-phonon scattering process as the dirty limit is approached from the clean regime.