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.