Dynamics and energy distribution of nonequilibrium quasiparticles in superconducting tunnel junctions

We present a full theoretical and experimental study of the dynamics and energy distribution of nonequilibrium quasiparticles in superconducting tunnel junctions (STJ's). STJ's are often used for single-photon spectrometers, where the numbers of quasiparticles excited by a photon provide a measure of the photon energy. The magnitude and fluctuations of the signal current in STJ detectors are in large part determined by the quasiparticle dynamics and energy distribution during the detection process. We use this as motivation to study the transport and energy distribution of nonequilibrium quasiparticles excited by x-ray photons in a lateral, imaging junction configuration. We present a full numerical model for the tunneling current of the major physical processes which determine the signal. We find that a diffusion framework models the quasiparticle dynamics well and that excited quasiparticles do not equilibrate to the lattice temperature during the time scales for tunneling. We extract physical time scales from the measured data, make comparisons with existing theories, and comment on implications for superconducting mesoscopic systems and single-photon detectors.