Single-layer FeSe films grown on (001) SrTiO3 substrates have shown a significant increase in superconducting transition temperature compared to bulk FeSe. Several mechanisms have been proposed to explain such enhancement, including electron doping, interfacial electron-phonon coupling, and strong electron correlations. To pinpoint the primary driver, we grew FeSe films on SrTiO3 substrates with coexisting TiO2 and SrO surface terminations. Scanning tunneling spectroscopy revealed a larger superconducting gap of 17 meV for FeSe on TiO2 compared to 11 meV on SrO. Tunneling spectroscopy also showed a larger work function on SrO, leading to reduced charge transfer, as confirmed by angle-resolved photoemission spectroscopy. Scanning transmission electron microscopy revealed distinctive interfacial atomic-scale structures, with the Se-Fe-Se tetrahedral angle changing from 109.9{\deg} on SrO to 105.1{\deg} on TiO2. Compared to dynamical mean field theory calculations, these results suggest optimal electron correlations in FeSe/TiO2 for enhancing high-temperature superconductivity.
@article{arxiv.2502.16784,
title = {Unraveling Enhanced Superconductivity in Single-layer FeSe through Substrate Surface Terminations},
author = {Qiang Zou and Gi-Yeop Kim and Jong-Hoon Kang and Basu Dev Oli and Zhuozhi Ge and Michael Weinert and Subhasish Mandal and Chang-Beom Eom and Si-Young Choi and Lian Li},
journal= {arXiv preprint arXiv:2502.16784},
year = {2025}
}