Phonon modes and electron-phonon coupling at the FeSe/SrTiO₃ interface

The remarkable increase in superconducting transition temperature (T_c) observed at the interface of one-unit-cell FeSe films on SrTiO₃ substrates (1 uc FeSe/STO)¹ has attracted considerable research into the interface effects^2-6. Although this high T_c is thought to be associated with electron-phonon coupling (EPC)², the microscopic coupling mechanism and its role in the superconductivity remain elusive. Here we use momentum-selective high-resolution electron energy loss spectroscopy to atomically resolve the phonons at the FeSe/STO interface. We uncover new optical phonon modes, coupling strongly with electrons, in the energy range of 75-99 meV. These modes are characterized by out-of-plane vibrations of oxygen atoms in the interfacial double-TiO_x layer and the apical oxygens in STO. Our results also demonstrate that the EPC strength and superconducting gap of 1 uc FeSe/STO are closely related to the interlayer spacing between FeSe and the TiO_x terminated STO. These findings shed light on the microscopic origin of the interfacial EPC and provide insights into achieving large and consistent T_c enhancement in FeSe/STO and potentially other superconducting systems.