Proximity effect in the SN interface of high-Tc superconductors: Tunnel spectroscopy and surface impedance

The authors describe a theoretical investigation of superconducting properties when a thin nonsuperconducting layer exists upon a high-T_c superconductor surface. The critical temperature, the density of states, the quasi-particle tunneling characteristics, and the surface impedance are calculated, taking into account the proximity effect. The steep order parameter variation in the SN (superconductor-normal metal) interface is shown to cause a large pair breaking perturbation and to affect the surface superconducting properties. It is noted that even the very thin normal layer causes serious degeneration of surface properties in the case of the high-T_c superconductor because of its short coherence length