Microshort-to-tunneling transition in Au-YBa₂Cu₃O_7-δ single-crystal point contacts

We report on the evolution of dynamic conductance curves [(dI/dV)-V] as a function of junction resistance in Au-YBa₂Cu₃O_7-δ single-crystal point-contact junctions. We have demonstrated that it is possible to evolve continuously from a microshort (N-S) to a tunnel junction (N-I-S) by adjusting the contact pressure in the point-contact junction. At the lowest junction resistances, we see a peak in dI/dV at zero bias, which we explain as due to Andreev reflection. As the junction resistance is gradually increased, the zero-bias peak diminishes and gaplike features appear in the (dI/dV)-V curves. At even higher resistance, the zero-bias peak vanishes, all features get smeared out, and a linear dI/dV variation with the bias voltage is seen. Based on the similarity of (dI/dV)-V curves obtained in high-resistance Au-YBa₂Cu₃O_7-δ junctions with those obtained in a nonsuperconducting perovskite oxide metal (Au-Na_xWO₃ junctions, x≃0.45), we suggest that the linear (dI/dV)-V curves arise due to inelastic scattering of electrons at a ‘‘metallic’’ layer on the surface of the superconductor.