The Study of Electron-Phonon Coupling at the FeSe/SrTiO₃ Interface.

The significant role of interfacial coupling in the superconductivity enhancement in FeSe films on SrTiO₃ (STO) has been widely recognized. But the explicit origination of this coupling is yet to be identified. Using high resolution electron energy loss spectroscopy, both phonons and plasmons of FeSe/STO were measured and studied. We show that, the Fe- and Se-derived phonons do not exhibit differences between films with different thicknesses. Theoretical calculations indicate the antiferromagnetic correlation is uniform in both FeSe films and bulk. However, the growth of FeSe films leads to a significant change of Fuchs-Kliewer (F-K) phonons and plasmons in STO. Thus, the interfacial coupling between the electrons in FeSe film and the F-K phonons from substrate is essential for the superconductivity enhancement. Combining the electronic structures and polaron excitations observed in experiments, we find that this electron-phonon coupling at FeSe/STO interface belongs to the non-retarded regime, resulting the formation of interfacial polarons. We propose that the formation of interfacial polaron directly relates to the superconductivity enhancement and the corresponding interaction locates the superconductivity inside the BSC-BEC crossover regime.