Understanding and enhancing superconductivity in FeSe/STO by quantum size effects

TL;DR

This paper develops a theoretical model to explain the size-dependent enhancement of superconductivity in FeSe/STO nano-islands, suggesting quantum size effects can further increase the critical temperature.

Contribution

The authors introduce an analytical formalism combining Eliashberg theory, forward scattering, and spectral fluctuations to accurately describe size-dependent superconductivity in FeSe/STO.

Findings

Quantitative agreement with experimental size dependence of the superconducting gap

Evidence supporting phonon-mediated pairing in FeSe/STO

Proposal to use quantum size effects to enhance critical temperature

Abstract

Superconductivity in one-atom-layer iron selenide (FeSe) on a strontium titanate (STO) substrate is enhanced by almost an order of magnitude with respect to bulk FeSe. There is recent experimental evidence suggesting that this enhancement persists in FeSe/STO nano-islands. More specifically, for sizes $L \sim 10$ nm, the superconducting gap is a highly non-monotonic function of $L$ with peaks well above the bulk gap value. This is the expected behavior only for weakly-coupled metallic superconductors such as Al or Sn. Here we develop a theoretical formalism to describe these experiments based on three ingredients: Eliashberg theory of superconductivity in the weak coupling limit, pairing dominated by forward scattering and periodic orbit theory to model spectral fluctuations. We obtain an explicit analytical expression for the size dependence of the gap that describes quantitatively the experimental results with no free parameters. This is a strong suggestion that superconductivity in FeSe/STO is mediated by STO phonons. We propose that, since FeSe/STO is still a weakly coupled superconductor, quantum size effects can be used to further enhance the bulk critical temperature in this interface.