Superconductivity at a Magic X-Fe-X Bond Angle: Correlation Enhanced Electron-Phonon Coupling in FeSe/SrTiO₃

We combine low-temperature STM/STS and embedded DMFT calculations to study superconductivity in single-layer FeX (X = Se, S, Te) films epitaxially grown on SrTiO₃(100) by MBE. Growing all FeX alloys on the same substrate allows a direct comparison of how the local X–Fe–X tetrahedral geometry governs superconducting behavior. At 4.3 K, tunneling spectroscopy reveals no superconducting gap in FeTe; it emerges above ~25% Se, reaches fully gapped, U-shaped spectra with coherence peaks at 12 and 17 meV in FeSe, and decreases to ~3 meV in FeS with V-shaped spectra, forming a distinct superconducting dome across the series. Embedded DMFT calculations of electron-phonon coupling reproduce this dome and link the maximum gap to a “magic” X–Fe–X bond angle where A₁g phonon coupling is strongest. These results reveal the cooperative interplay of lattice geometry, correlations, and phonons in driving unconventional superconductivity.