Control of bulk superconductivity in a BCS superconductor by surface charge doping via electrochemical gating

TL;DR

This study demonstrates that electrochemical gating can reversibly modify the bulk superconducting transition temperature in NbN thin films, revealing a dependence on film thickness and an extended screening length at high electric fields.

Contribution

It shows that electrochemical gating can control bulk superconductivity in a BCS superconductor, challenging standard screening theory and highlighting the role of proximity effects.

Findings

Reversible tuning of superconducting transition temperature in NbN films.

Effective screening length increases with induced charge density.

Surface charge doping influences bulk properties via proximity effects.

Abstract

The electrochemical gating technique is a powerful tool to tune the \textit{surface} electronic conduction properties of various materials by means of pure charge doping, but its efficiency is thought to be hampered in materials with a good electronic screening. We show that, if applied to a metallic superconductor (NbN thin films), this approach allows observing reversible enhancements or suppressions of the \emph{bulk} superconducting transition temperature, which vary with the thickness of the films. These results are interpreted in terms of proximity effect, and indicate that the effective screening length depends on the induced charge density, becoming much larger than that predicted by standard screening theory at very high electric fields.