Can the noble metals (Au, Ag and Cu) be superconductors?

TL;DR

This paper extends BCS theory within the Eliashberg framework to predict that ultra-thin films of noble metals like Au, Ag, and Cu could become superconducting at low temperatures if their thickness is finely tuned near half a nanometer.

Contribution

It introduces a parameter-free Eliashberg-based model predicting superconductivity in noble metal thin films based on precise thickness control.

Findings

Maximum critical temperature depends on film thickness and carrier density.

Ultra-thin gold, silver, and copper films could be superconducting at accessible low temperatures.

Superconductivity emerges only at very specific, finely tuned film thicknesses.

Abstract

It is common knowledge that noble metals are excellent conductors but do not exhibit superconductivity. On the other hand, quantum confinement in thin films has been consistently shown to induce a significant enhancement of the superconducting critical temperature in several superconductors. It is therefore an important fundamental question whether ultra-thin film confinement may induce observable superconductivity in non-superconducting metals. We present a generalization, in the Eliashberg framework, of a BCS theory of superconductivity in good metals under thin-film confinement. By numerically solving these new Eliashberg-type equations, we find the dependence of the superconducting critical temperature on the film thickness $L$. This parameter-free theory predicts a maximum increase in the critical temperature for a specific value of the film thickness, which is a function of the number of free carriers in the material. Exploiting this fact, we predict that ultra-thin films of gold, silver and copper of suitable thickness could be superconductors at low but experimentally accessible temperatures. We demonstrate that this is a fine-tuning problem where the thickness must assume a very precise value, close to half a nanometer.