Superconductivity dichotomy in K-coated single and double unit cell FeSe films on SrTiO3

TL;DR

This study investigates how potassium adsorption affects superconductivity in single and double unit cell FeSe films on SrTiO3, revealing a dichotomy in superconductivity evolution linked to interface electron-phonon coupling.

Contribution

It demonstrates the contrasting effects of K coverage on 1-UC and 2-UC FeSe films and highlights the role of interface electron-phonon coupling in superconductivity enhancement.

Findings

Superconductivity in 1-UC FeSe is suppressed with K adsorption.

2-UC FeSe becomes superconducting with a gap of ~17 meV or ~11 meV.

Interface electron-phonon coupling is crucial for Cooper pairing.

Abstract

We report the superconductivity evolution of one unit cell (1-UC) and 2-UC FeSe films on SrTiO3(001) substrates with potassium (K) adsorption. By in situ scanning tunneling spectroscopy measurement, we find that the superconductivity in 1-UC FeSe films is continuously suppressed with increasing K coverage, whereas non-superconducting 2-UC FeSe films become superconducting with a gap of ~17 meV or ~11 meV depending on whether the underlying 1-UC films are superconducting or not. This work explicitly reveals that the interface electron-phonon coupling is strongly related to the charge transfer at FeSe/STO interface and plays vital role in enhancing Cooper pairing in both 1-UC and 2-UC FeSe films.