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

TL;DR

This study reveals that in gold-coated (110) YBa2Cu3O7−δ films, Andreev bound states penetrate the gold layer up to about 10 nm, showing a proximity effect that differs from conventional expectations, with zero bias peaks decaying rapidly.

Contribution

The paper demonstrates that Andreev bound states can penetrate into a normal metal layer in high-temperature superconductor heterostructures, challenging traditional proximity effect models.

Findings

Andreev bound states penetrate gold layers up to 10 nm.

Zero bias conductance peaks are observed on thin gold layers.

Proximity-induced mini-gaps are rarely seen in the gold layer.

Abstract

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.