Strong Reduction of Quasiparticle Fluctuations in a Superconductor due to Decoupling of the Quasiparticle Number and Lifetime

TL;DR

This study investigates quasiparticle fluctuations in a superconductor, revealing that quasiparticle number remains thermal at low temperatures due to trapping effects, challenging existing theoretical relations.

Contribution

It demonstrates that quasiparticle trapping affects the relationship between quasiparticle number and lifetime, providing new insights into superconductor quasiparticle dynamics.

Findings

Quasiparticle fluctuation level reduces by a factor of ~100 at low temperatures

Recombination time saturates and is limited by trapping effects

Quasiparticle number remains thermal despite reduced fluctuations

Abstract

We measure temperature dependent quasiparticle fluctuations in a small Al volume, embedded in a NbTiN superconducting microwave resonator. The resonator design allows for read-out close to equilibrium. By placing the Al film on a membrane, we enhance the fluctuation level and separate quasiparticle from phonon effects. When lowering the temperature, the recombination time saturates and the fluctuation level reduces a factor $\sim$100. From this we deduce that the number of free quasiparticles is still thermal. Therefore, the theoretical, inverse relation between quasiparticle number and recombination time is invalid in this experiment. This is consistent with quasiparticle trapping, where on-trap recombination limits the observed quasiparticle lifetime.