Experimentally and mysteriously, the concentration of quasiparticles in a gapped superconductor at low temperatures always by far exceeds its equilibrium value. We study the dynamics of localized quasiparticles in superconductors with a spatially fluctuating gap edge. The competition between phonon-induced quasiparticle recombination and generation by a weak non-equilibrium agent results in an upper bound for the concentration that explains the mystery.
@article{arxiv.1603.04273,
title = {How many quasiparticles can be in a superconductor?},
author = {Anton Bespalov and Manuel Houzet and Julia S. Meyer and Yuli V. Nazarov},
journal= {arXiv preprint arXiv:1603.04273},
year = {2017}
}