Transparency-controlled multiple charge transfer in superconducting junctions with local shot-noise scanning tunneling spectroscopy

TL;DR

This study uses advanced shot noise scanning tunneling microscopy to control and analyze charge transfer processes in superconducting junctions, revealing how transparency influences multiple Andreev reflections.

Contribution

It introduces a new experimental approach to systematically control junction transparency and directly observe its effect on charge transfer in superconducting junctions.

Findings

Shot noise evolves from single electron to multiple charge transfer regimes with increasing transparency.

Results align with theoretical models of Andreev reflections.

Demonstrates noise-STM as a tool for atomic-scale charge transport studies.

Abstract

Charge transport in superconducting junctions at finite voltages is governed by Andreev reflections, including multiple Andreev reflections, which are processes that enable multiple charge transfer, a hallmark that shot noise can directly quantify. Since the effective charge extracted from shot noise measurements varies with the transparency of the junction, systematic control of transparency is essential but experimentally challenging. Here, we present shot noise scanning tunneling microscopy measurements enabled by a newly developed amplifier, allowing access to different transparency regimes. We perform shot noise measurements on Pb(111) with tunable transparency at 2.2 K and observe that the shot noise evolves from a single electron tunneling regime to multiple charge transfer regime as transparency increases. Our results are quantitatively consistent with theoretical simulations of Andreev reflections and multiple Andreev reflections for a single-channel system. These results establish junction transparency as the key parameter governing the evolution of charge transport and demonstrate that noise-STM is a powerful platform for investigating microscopic charge transport mechanisms with controlled junction transparency at the atomic scale.