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) has attracted considerable research into the interface effects. Although this high Tc is thought to be associated with electron-phonon coupling (EPC), the microscopic coupling mechanism and its role in the superconductivity remain elusive. Here 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 oxygen in 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.