Anomalous proximity effect in gold coated (110) $YBa_2Cu_3O_{7-delta}$ films: Penetration of the Andreev bound states

Scanning tunneling spectroscopy of (110) $YBa_2Cu_3O_{7-delta}/Au$ bi-layers reveal a proximity effect markedly different from the conventional one. While proximity-induced mini-gaps rarely appear in the Au layer, the Andreev bound states clearly penetrate into the metal. Zero bias conductance peaks are measured on Au layers thinner than 7 nm with magnitude similar to those detected on the bare superconductor films. The peaks then decay abruptly with Au thickness and disappear above 10 nm. This length is shorter than the normal coherence length and corresponds to the (ballistic) mean free path.