Electron energy down-conversion in thin superconducting films

Abstract

We have developed a theory of photoelectron energy down-conversion in superconductors in the vicinity of interfaces. Significant differences from the situation in bulk materials arise when the process takes place closer to an interface than the mean free path of pair-breaking phonons. Then some of the energetic phonons generated in the down-conversion cascade can escape from the superconducting film, giving rise to a decrease in the mean number of quasiparticles generated and to statistical fluctuations in that quantity. An additional source of variability is the spatial distribution of the initial photoabsorption sites, giving rise to vertical inhomogeneity. Both effects can be observed in photoabsorption experiments on thin film superconducting tunnel junctions, the former at optical energies and the latter primarily with x rays.