Proximity Effect in Gold Coated $YB{a}_{2}C{a}_3{O}_{7-\delta }$ Films Studied by Scanning Tunneling Spectroscopy

Scanning tunneling spectroscopy on gold layers over-coating $\mathit{\text{c}}$-axis $YB{a}_{2}C{a}_3{O}_{7-\delta }$ (YBCO) films reveals proximity induced gap structures. The gap size reduced exponentially with distance from $\mathit{\text{a}}$-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 $\mathit{\text{s}}$-wave symmetry.