Existence of interfacial polaronic plasmon: A comparative study between FeSe/SrTiO3 and CoSe/SrTiO3

The discovery of monolayer FeSe on ${\mathrm{SrTiO}}_3$ (STO) with the boosting superconducting transition temperature has prompted an intense increase in experimental and theoretical researches on interfacial superconductivity enhancement. One of the most concerning research subjects is to explore the nature of the interfacial coupling. Here, we performed a comparative study between the isostructural CoSe/STO and FeSe/STO by measuring the phonons and plasmons via high-resolution electron energy loss spectroscopy. With the increasing thickness of the epitaxial film, the measured intensity of the Fuchs-Kliewer (F-K) phonons and the plasmon from STO decay much faster in the CoSe film than those in FeSe, owing to the higher carrier concentration in CoSe. By comparing the temperature-dependent behaviors of the F-K phonons and plasmons, we discovered that the interfacial coupling in CoSe/STO is adiabatic because of its large Fermi energy. In contrast, the interfacial electron-phonon coupling in FeSe/STO is nonadiabatic, resulting in the formation of interfacial polarons that plays an important role in the superconductivity enhancement.