The Study Of Low Energy Excitations At The System Of FeSe/SrTiO₃

The discovery of high temperature superconductors brings new challenge to study the many-body interactions in condensed matter physics.To construct the microscopic picture of high temperature superconductors,the understanding of superconduting properties is a crucial issue.In iron-based superconductors,FeSe is a prototypical system with the simplest crystal structure.Although the critical temperature（T_C）of FeSe bulk is only 8 K,electron doping can enhance it to⁴0 K.More intriguingly,the T_C of single layer FeSe films grown on SrTiO₃（STO）substrate（1uc-FeSe/STO）is significantly enhanced to～60–70 K.This discovery serves as a prototypical example of interfacial T_C enhancement,which has drawn the attention of the community.Up to date,however,the microscopic picture of the outstanding interfacial Tc enhancement is still unclear.We use Electron Energy Loss Spectroscopy（EELS）and Angle Resolved Photoemission Spectroscopy（ARPES）to study the elementary excitations and electronic structures in the system of FeSe/STO,which can clarify the possible underlying interactions at the interface to unveil the Tc enhancement picture.Our research includes four parts:（1）elementary excitations on pure STO;（2）lattice dynamics of FeSe films;（3）interfacial electron-phonon interactions at FeSe/STO;（4）The microscopic picture of the T_C enhancement at FeSe/STO interface.First,previous researches suggest that the STO substrate is of determinative significance to the T_C enhancement at FeSe/STO interface.Therefore,understanding of substrate properties is a fundamental issue to study the interfacial interactions.Via EELS measurements on Nb-doped STO surface,two Fuchs-Kliewer（F-K）phonons and one plasmon are observed.F-K phonons are optical vibration modes on the surface of ionic crystals which accompany with strong dipole oscillations of oxygen ions.In the ionic crystals with large dielectric constant,electrons in STO can be dressed by the localized polarized field of lattice,and thereby the qusiparticles called polarons are formed.Consequently,the observed plasmon in STO is a collective oscillation of polarons.These information provides the basis to study of elementary interactions in FeSe/STO.Second,we study the lattice dynamics of FeSe films to uncover the substrate effect on FeSe films.We show that,despite the significant effect from the substrate on the electronic structure and superconductivity of the system,the FeSe phonons in the films are unaffected.The energy dispersion and linewidth associated with the FeSe vibrational modes are thickness and temperature independent.Therefore,phonons of FeSe do not participate into the superconductivity at FeSe/STO interface.Theoretical calculations indicate the crucial role of antiferromagnetic correlation in FeSe to reproduce the experimental phonon dispersions,and the antiferromagnetic correlation also unchanging with films thickness.Thus the antiferromagnetic correlation is also crucial in single layer FeSe.Third,on 1uc-FeSe/STO surface,the F-K phonons of STO substrate are observed indicating the electric field generated by F-K phonons can penetrate into FeSe films and strongly interact with the electrons therein.The electron-phonon coupling strength is determined from the phonon and electronic structures.With increasing FeSe thickness,the penetrating field intensity decays exponentially,which matches well the observed exponential decay of the superconducting gap.It is unambiguously shown that the SrTiO₃ F-K phonon penetrating into FeSe is essential for interfacial superconductivity enhancement.Fourth,with FeSe capping,electrons in FeSe/STO can also form the polaronic plasmon at surface,exhibiting an unusual polaronic feature across the interface.The experimentally determination of electron-phonon interaction also demonstrate that the electrons in FeSe/SrTiO₃ systems are instantaneously dressed by the strongly polarized local lattice distortions of the SrTiO₃ substrates.Such non-adiabatic electron-phonon interactions in the non-adiabatic regime naturally lead to the formation of interfacial polarons.Our calculations show that the interfacial polaron-polaron interaction can induce additional attraction between electrons for the FeSe systems,leading to enhanced electron pairing and,accordingly,superconductivity.The microscopic picture of the interfacial T_C enhancement provides new insights in understanding the microscopic mechanisms of high-T_C superconductivity,potentially leading to new routes towards materials realization of higher-T_C superconductors.