Unraveling Enhanced Superconductivity in Single-layer FeSe through Substrate Surface Terminations

TL;DR

This study investigates how different substrate surface terminations influence superconductivity in single-layer FeSe films, revealing that TiO2 termination enhances superconducting properties through atomic-scale structural and electronic modifications.

Contribution

It demonstrates that substrate surface termination significantly affects superconductivity in FeSe, highlighting the role of atomic structure and charge transfer in enhancing Tc.

Findings

Larger superconducting gap on TiO2-terminated surfaces

Distinct interfacial atomic structures observed via STEM

Optimal electron correlations linked to TiO2 termination

Abstract

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.