Revealing the Microscopic Origin of Electron-Phonon Coupling at the FeSe/SrTiO₃ Interface by Atomically Resolved Vibrational Spectroscopy

The significant increase in superconducting transition temperature (Tc) at the interface of a one-unit-cell thick FeSe layer on a SrTiO₃ substrate (1uc FeSe/STO) has drawn substantial research interest. While this high Tc is believed to be linked to electron-phonon coupling (EPC), the microscopic mechanisms involved and their role in superconductivity remain unclear. In this study, we utilize q-selective high-resolution electron energy loss spectroscopy (EELS) to atomically resolve the phonons across the FeSe/STO interface. We identify new optical phonon modes, strongly coupled with electrons, within the energy range of 75-99 meV. These modes are attributed to the out-of-plane vibrations of oxygen atoms in the unique double-TiOx layer at the interface and the apical oxygen in the STO substrate. Additionally, we discover that both the EPC strength and the superconducting gap of 1uc FeSe/STO are closely tied to the interlayer spacing between FeSe and the TiOx-terminated STO. Our results highlight the critical role of spatial overlap between electron and phonon wavefunctions in modulating the interfacial EPC. These findings elucidate the microscopic origin of the interfacial EPC and provide insights for achieving substantial and consistent Tc enhancement in FeSe/STO and potentially other superconducting systems.