Electron and hole transmission through superconductor — Normal metal interfaces

We have investigated the transmission of electrons and holes through interfaces between superconducting aluminum (Tc = 1.2K) and various normal non-magnetic metals (copper, gold, palladium, platinum, and silver) using Andreev-reflection spectroscopy at T = 0.1K. We analysed the point contacts with the modified BTK theory that includes Dynes’ lifetime as a fitting parameter Γ in addition to superconducting energy gap 2Δ and normal reflection described by Z. For contact areas from 1 nm2 to 10000nm2 the BTK Z parameter was 0.5, corresponding to transmission coefficients of about 80%, independent of the normal metal. The very small variation of Z indicates that the interfaces have a negligible dielectric tunneling barrier. Fermi surface mismatch does not account for the observed transmission coefficient.