Andreev reflections at metal/superconductor point contacts: Measurement and analysis - art. no. 104510

We present point-contact Andreev reflection measurements of X/Nb contacts, where X = Ni, Co, Fe, and Cu. Experimental conductance-voltage curves were analyzed with the Blonder-Tinkham-Klapwijk theory [Phys. Rev. B 25,4515 (19 82)], extended to include the polarization P of the metal and proximity eff ects. For Ni, Co, and Fe the conductance-voltage curves can be well describ ed by the model with P and Z as the fitting parameters, where Z is a dimens ionless barrier strength included in the model to describe elastic scatteri ng at a nonideal metal/superconductor interface. The polarization for Fe, C o, and Ni depends on the magnitude of Z. The Value of the intrinsic P can b e obtained by extrapolation to Z=0 (perfectly transparent inter-Face). For Cu, the conductance-voltage curves show a clip at the position of the Nb su perconducting gap, due to proximity effects, which reduce the effective gap value for the normal to supercurrent conversion at the Cu/Nb interface, wh ile leaving the gap for quasiparticle transport essentially unchanged. In a ddition, an overall decrease of the gap is observed when the size of the po int contact approaches the superconducting coherence length in Nb. We have included these effects in our model and obtained very good agreement betwee n experimental data and model calculations.