A Comparative Study Of Different Types Of FeSe/SrTiO₃ Interfaces

Single-layer FeSe film grown on SrTiO₃（001）（1 uc-FeSe/STO）has attracted much attention due to the significant enhancement of the superconductivity by the interface.The discovery of FeSe/STO provides an ideal platform for the mechanism of high-temperature superconductors and opens up new ideas in the field of interfacial superconductivity.Up to now,however,the understanding of the FeSe/STO interface is limited,and the microscopic picture of its superconductivity enhancement is still missing.In this context,taking FeSe/STO as a reference,we carried out our comparative study by changing the film or the substrate.We designed several kinds of heterointerfaces similar to FeSe/STO and investigated the factors affecting the superconductivity of FeSe with high-resolution electron energy loss spectroscopy（HREELS）and scanning tunneling microscopy/spectroscopy（STM/STS）.The main contents of this thesis include the following:（1）We perform a comparative study on the elementary excitations between the isostructural FeSe/STO and CoSe/STO via HREELS.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 discover that the interfacial coupling in CoSe/STO is adiabatic because of its large Fermi energy.In contrast,the interfacial electron-phonon coupling（EPC）in FeSe/STO is nonadiabatic,resulting in the formation of interfacial polarons.Our study further verifies the essential role of the non-adiabatic EPC in FeSe/STO and provides a reliable reference for the exploration of more systems with pronounced interfacial superconductivity enhancement.（2）We successfully grow CoSe film on monolayer FeSe/STO and investigate its electronic properties with STM/STS.FeSe exhibits a high metallicity due to the raised Fermi energy level after the coverage of the CoSe film.This suggests that charge transfer occurs from CoSe to FeSe.However,there is no sign of superconductivity for the FeSe film in our experiment,since the electron-doped FeSe may lead the EPC of FeSe/STO to be adiabatic.（3）We conduct HREELS measurements to comparatively investigate the elementary excitations of FeSe/6 uc-LaFeO₃/STO（FeSe/LFO/STO）and 6 uc-LaFeO₃/STO（LFO/STO）.In addition to the F-K phonons and polaronic plasmons from the STO substrate,the optical phonons of LFO can also penetrate into the FeSe film and interact with electrons therein,providing an additional channel for electron-phonon interaction in FeSe.By analyzing the temperature dependence of the energy of the LFO phonon,the F-K phonons,and the polaronic plasmons before and after the growth of FeSe film,the interfacial polarons involving F-K phonons also exist in FeSe/LFO/STO,whereas the LFO phonon has no obvious contribution to them.Our study indicates that the EPC of FeSe/STO,as well as FeSe/LFO and the magnetic fluctuation of LFO,plays a significant role in the superconductivity enhancement for FeSe/LFO/STO.