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

The remarkable increase in superconducting transition temperature (T) observed at the interface of one-unit-cell FeSe films on SrTiO substrates (1 uc FeSe/STO) has attracted considerable research into the interface effects. Although this high T is thought to be associated with electron-phonon coupling (EPC), 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 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 terminated STO. These findings shed light on the microscopic origin of the interfacial EPC and provide insights into achieving large and consistent T enhancement in FeSe/STO and potentially other superconducting systems.