Proximity Effect in Gold Coated $YBa_2Ca_3O_{7-δ}$ Films Studied by Scanning Tunneling Spectroscopy

Abstract

Scanning tunneling spectroscopy on gold layers over-coating \textit{c}-axis $YBa_2Ca_3O_{7-δ}$ (YBCO) films reveals proximity induced gap structures. The gap size reduced exponentially with distance from \textit{a}-axis facets, indicating that the proximity effect is primarily due to the (100) YBCO facets. The penetration depth of superconductivity into the gold is $\sim 30$ nm, in good agreement with estimations for the dirty limit. The extrapolated gap at the interface is $\sim 15$ meV, consistent with recent point-contact experiments. The proximity-induced order parameter appears to have predominant \textit{s}-wave symmetry.