Electrical characterization of Nb/Nb oxide/PbAuIn Josephson tunnel junctions

We have studied the low temperature electrical characteristics of high quality, high current density (∼500 A/cm²) Nb-Nb oxide-PbAuIn Josephson junctions with the aims of understanding the deviations of these characteristics from the ideal and of correlating the deviations with information about the physical characteristics of the barrier region of the junction. We find we can understand the knee structure at the sum gap in terms of a proximity effect with a normal metal-like NbOxCytransition layer. Excess currents in the subgap region in the highest quality junctions are mostly consistent with a multiparticle tunneling picture. In the lower quality junctions, the excess subgap currents appear to be due to tunneling between the superconducting Pb alloy and some low energy states in the Nb electrode. We are unable to distinguish between several possible sources of such states.