Quasiparticle response of superconducting aluminum to electromagnetic radiation

The response of superconducting aluminum to electromagnetic radiation is investigated in broad frequency (45 MHz--40 GHz) and temperature ranges (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 quasiparticle response. Here we observe the full temperature and frequency dependences of the coherence peak. Varying the mean-free path gives insight into the dynamics, scattering, and coherence effects of the quasiparticles in the superconducting state.