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

Single-layer FeSe on SrTiO₃(001) substrates shows a superconducting transition temperature much higher than that of bulk FeSe, which has been attributed to factors such as electron doping, interfacial electron-phonon coupling, and electron correlations. To pinpoint the primary driver, we grew single-layer FeSe films on SrTiO₃(001) substrates with coexisting TiO₂ and SrO surface terminations. Scanning tunneling spectroscopy revealed a larger superconducting gap (17.0 meV) on the TiO₂-termination than on the SrO-termination (10.5 meV). Tunneling spectroscopy also showed a larger work function on the SrO surface, resulting in reduced charge transfer to FeSe, as confirmed by angle-resolved photoemission spectroscopy. Scanning transmission electron microscopy further revealed distinctive interfacial atomic-scale structures, with the Se-Fe-Se tetrahedral angle changing from 109.5° on the SrO-termination to 104.9° on the TiO₂-termination. Compared to dynamical mean field theory calculations, our results indicate that enhanced superconductivity in single-layer FeSe/TiO₂ arises from optimal electron correlations, in addition to sufficient charge transfer from the substrate.