ASYMMETRIC DIFFERENTIAL RESISTANCE OF POINT CONTACTS ON NORMAL-METAL-SUPERCONDUCTOR BILAYERS

Point-contact junctions on normal-metal-superconductor bilayers show asymmetries of different magnitudes and signs in the differential resistance versus voltage curves for opposite-bias voltages. In the absence of Andreev reflection (i.e. for energies outside the energy gap) no asymmetry is found. The asymmetries are investigated with Ag, Cu, Pd, Pt, Ni, and Co as point-contact material on Ag-Pb bilayers. For all materials the authors provide the statistics of the asymmetries. They discuss effects that may be responsible for this new phenomenon.