Ionic gating in metallic superconductors: A brief review

TL;DR

This review discusses how ionic gating can modulate the superconducting properties of metallic thin films, revealing long-range effects and modeling approaches that connect surface charge doping to bulk superconductivity changes.

Contribution

It summarizes experimental findings, models the proximity effect, and introduces a first-principles framework for predicting gated superconductor behavior.

Findings

Gate-induced charge doping affects bulk superconductivity.

Proximity effect explains long-range modulation.

First-principles calculations confirm screening length anomalies.

Abstract

Ionic gating is a very popular tool to investigate and control the electric charge transport and electronic ground state in a wide variety of different materials. This is due to its capability to induce large modulations of the surface charge density by means of the electric-double-layer field-effect transistor (EDL-FET) architecture, and has been proven to be capable of tuning even the properties of metallic systems. In this short review, I summarize the main results which have been achieved so far in controlling the superconducting (SC) properties of thin films of conventional metallic superconductors by means of the ionic gating technique. I discuss how the gate-induced charge doping, despite being confined to a thin surface layer by electrostatic screening, results in a long-range "bulk" modulation of the SC properties by the coherent nature of the SC condensate, as evidenced by the observation of suppressions in the critical temperature of films much thicker than the electrostatic screening length, and by the pronounced thickness-dependence of their magnitude. I review how this behavior can be modelled in terms of proximity effect between the charge-doped surface layer and the unperturbed bulk with different degrees of approximation, and how first-principles calculations have been employed to determine the origin of an anomalous increase in the electrostatic screening length at ultrahigh electric fields, thus fully confirming the validity of the proximity effect model. Finally, I discuss a general framework - based on the combination of ab-initio Density Functional Theory and the Migdal-Eliashberg theory of superconductivity - by which the properties of any gated thin film of a conventional metallic superconductor can be determined purely from first principles.