We have investigated the transmission of electrons and holes through interfaces between superconducting aluminum (T c = 1.2 K) and various normal non-magnetic metals (copper, gold, palladium, platinum, and silver) using Andreev-reflection spectroscopy at T = 0.1 K. 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 2∆ and normal reflection described by Z. For contact areas from 1 nm 2 to 10000 nm 2 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.

We have investigated point contacts between a superconductor (Nb, AuIn 2 ) and a normal metal (ferromagnetic Co, non-magnetic Cu). The observed Andreev-reflection spectra were analyzed using the modified BTK theory including spin-polarization effects. This resulted in a polarization of Co that agrees with observations by others, but lifetime effects describe the spectra equally well. On the other hand, the spectra with non-magnetic Cu can be well described using the spin-polarization model. The ambiguity between polarization and lifetime interpretation poses a dilemma which can be resolved by considering the normal reflection at those interfaces due to Fermi surface mismatch. Our data suggest that Andreev reflection at Nb -Co contacts does deliver the true magnetic polarization of Co only when lifetime effects and the above mentioned intrinsic normal reflection are included.

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