Andreev reflections at metal/superconductor point contacts: Measurement and analysis

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 (1982)], extended to include the polarization P of the metal and proximity effects. For Ni, Co, and Fe the conductance-voltage curves can be well described by the model with P and Z as the fitting parameters, where Z is a dimensionless barrier strength included in the model to describe elastic scattering at a nonideal metal/superconductor interface. The polarization for Fe, Co, and Ni depends on the magnitude of Z. The Value of the intrinsic P can be 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 superconducting gap, due to proximity effects, which reduce the effective gap value for the normal to supercurrent conversion at the Cu/Nb interface, while leaving the gap for quasiparticle transport essentially unchanged. In addition, an overall decrease of the gap is observed when the size of the point contact approaches the superconducting coherence length in Nb. We have included these effects in our model and obtained very good agreement between experimental data and model calculations.