Measurements of Effective Quasiparticle Recombination Times and of Densities of Electronic States at the Fermi Level in Superconducting Al- and Pb-Films

Temperature-dependent quasiparticle recombination lifetimes-Cexp(r) and densities N o of electronic states at the Fermi level have been measured from time decay experiments of excess quasiparticle concentrations in evaporated, superconducting A1- and Pb-tunnel junctions. Current pulses were used to inject excess, nonthermal quasiparticles in a single junction acting simultaneously as generator and detector. The experimental lifetimes in "unperturbed " A1 show satisfactory agreement with calculations based on the 2A-phonon trapping lifetime model. Zexp decreases with increasing perturbations of the A1 film structure by oxygen background evaporation. In Pb the measured times indicate 2A-phonon volume losses. The densities N o in Pb-films and "unperturbed " as well as oxygen-perturbed Al-films differ by less than 5 % from the corresponding bulk material data. Therefore, in trying to explain the enhancement of the transition temperature from 1.23 K to 1.85 K in perturbed, granular Al-films a change of N o can be ruled out. I