The superconductor proximity effect in Au-YBa₂Cu ₃O_7-δ bilayer films: The role of order parameter anisotropy

Scanning tunneling spectroscopy of Au-YBa₂Cu₃O _7-δ bilayers reveals proximity effect features that reflect the anisotropic dx2-y2-wave order parameter in YBa₂Cu₃O _7-δ (YBCO). No proximity effect takes place at the (001)YBCO-Au interface, manifesting the quasi two-dimensional nature of the order parameter. Proximity gaps are induced in the gold primarily via (100)YBCO facets, and their size decays exponentially on a scale of ∼30 nm, in good agreement with estimations for the dirty limit. In the nodal (110)YBCO-Au interface, the proximity effect manifests itself by the penetration of Andreev bound states into the gold. The corresponding zero bias conductance peaks are nearly constant in magnitude for Au layers thinner than 7 nm, and then decay abruptly with Au thickness, vanishing above 10 nm, on the scale of the (ballistic) mean free path.