Evidence of a nonequilibrium distribution of quasiparticles in the microwave response of a superconducting aluminium resonator

TL;DR

This study investigates how microwave power affects the quasiparticle distribution in a superconducting aluminium resonator, revealing non-equilibrium effects that influence its electrodynamic response.

Contribution

It provides experimental evidence of a non-thermal quasiparticle distribution affecting the microwave response of superconducting aluminium resonators.

Findings

Below 200 mK, increased microwave power reduces quality factor and frequency.

Above 200 mK, increased power enhances these properties.

The observed effects are explained by a non-thermal quasiparticle distribution.

Abstract

In a superconductor absorption of photons with an energy below the superconducting gap leads to redistribution of quasiparticles over energy and thus induces a strong non-equilibrium quasiparticle energy distribution. We have measured the electrodynamic response, quality factor and resonant frequency, of a superconducting aluminium microwave resonator as a function of microwave power and temperature. Below 200 mK, both the quality factor and resonant frequency decrease with increasing microwave power, consistent with the creation of excess quasiparticles due to microwave absorption. Counterintuitively, above 200 mK, the quality factor and resonant frequency increase with increasing power. We demonstrate that the effect can only be understood by a non-thermal quasiparticle distribution.