Anomalous Density of States in Thick Superconducting Lead Films

Aluminum is used as the reference superconductor in a detailed study of the fine structure in the current vs voltage characteristics of superconductor- barrier-lead tunneling junctions. The fine structure shows a marked dependence on the thickness of the lead film and it is concluded that this anomalous behavior can best be explained in terms of an effective density of states. The effective density of states is thought to arise from two parts of the Fermi surface contributing different energy gaps and having different normal densities of states associated with them. The characteristics, measured as a function of temperature and lead film thickness, are presented. (C.E.S.)