The response of superconducting aluminum to electromagnetic radiation is investigated in a broad frequency (45 MHz to 40 GHz) and temperature range (T>Tc/2), by measuring the complex conductivity. While the imaginary part probes the superfluid density (Cooper-pairs), the real part monitors the opening of the superconducting energy gap and -- most important here -- the zero-frequency quasi-particle response. Varying the mean free path gives some insight into the dynamics, scattering and coherence effects of the quasi-particles in the superconducting state.
@article{arxiv.0712.3391,
title = {Quasi-Particle Dynamics in Superconducting Aluminum},
author = {Katrin Steinberg and Marc Scheffler and Martin Dressel},
journal= {arXiv preprint arXiv:0712.3391},
year = {2008}
}