Quasiparticle and phonon propagation in bulk, superconducting lead

We present a detailed account of our work on the propagation of quasiparticles and phonons in bulk, superconducting lead. Time-of-flight techniques combined with tunnel junction detection (for quasiparticles) and bolometer detection (for phonons) are used. A transition from quasiparticle diffusion (and ballistic phonon propagation) to diffusion in the combined gas of quasiparticles and phonons is observed as the temperature is increased. We give a theoretical description of the pulse propagation characteristics in the diffusive regimes. The theoretical line-shape fits to the data yield a measure of the quasiparticle recombination time and their number decay time as a function of temperature.