Synchrotron Radiation Based ARPES Studies On Electron And Lattice Coupling Of Charge Density Wave Instability In Transition Metal Compounds

Charge density wave(CDW)in low dimensional solid systems is a classical and important research topic.Up to now,there are still many mechanisms that need to be further studied,involving the cooperation or competition of various quantum degrees of freedom,which is helpful to reveal the many-body interaction in the strongly correlated electron systems.During the past 50 years of research on two-dimensional transition metal compounds,the mechanism of CDW varies greatly in different materials,from quasi-one-dimensional Peierls instability to electron-phonon coupling and electron-electron correlation effect.It is very important to reveal the physical origin of CDW from the perspective of electronic structure because of the complex coupling between electron and lattice system involved.Synchrotron radiation light source can provide a variety of advantages such as good purity,high monochromaticity,small divergence,small light spot,high flux,high brightness,adjustable energy range,adjustable polarization,good time resolution,etc.Therefore,the angle-resolved photoemission spectroscopy(ARPES)based on synchrotron radiation light source provides a powerful technical tool to study the electron structure related to charge density wave phase transition.In this dissertation,the coupling between electron system and lattice system in charge density wave phase transition of transition metal compounds was studied by angle-resolved photoemission spectroscopy on beamline 13U endstation of large scientific facility of Hefei Light Source(HLS,BL-13U):This dissertation is divided into six chapters.The first chapter reviews the lattice structure of transition metal compounds and the research development and status of CDW.The second chapter introduces the experimental techniques of single crystal growth,ARPES principle and synchrotron radiation light source endstation vacuum equipment,device for fixing sample holder in ultra-high vacuum,etc.The third chapter to the fifth chapter shows the evolution of band structure in three systems among Sn intercalated 1T-TiSe2,kagome metal CsV3Sb5,and S substituted Ta2NiSe5 by means of surface alkali metal deposition,atomic intercalation,and element substitution doping.The coupling between the electronic system and the lattice system is revealed.The sixth chapter summarizes main work of the dissertation and gives the outlook.The main studies of this dissertation is divided into the following parts:1.Study on the band structure of CDW phase transition of Sn intercalated 1T-TiSe2 based on synchrotron radiation.The electronic structure evolution of parent 1T-TiSe2 and intercalated SnxTiSe2 single crystal samples before and after CDW transition was studied by ARPES based on synchrotron radiation.The Sn intercalation induces charge transfer,providing additional electrons and completely suppressing the CDW state at x=0.1.The Jahn-Teller lattice distortion shortened the length of Ti-Se bond and split the band at L point,which was conducive to the overlap of p-d electron clouds.In this section,the key role of electron and lattice are analyzed by analyzing the Se 4p valence band at Γ and the Ti 3d conduction band and folded band at L point,which provides more information for understanding the mechanism of 2× 2 × 2 CDW in 1T-TiSe2.2.Study on the band structure of CDW phase transition of kagome metal CsV3Sb5 based on synchrotron radiation.The splitting behavior of van Hove singularities after the 2×2×2 CDW phase transition was observed in CsV3Sb5 by ARPES.In the absence of folding bands on the low-Cs terminated surface,the splitting behavior can be attributed to the shift to deeper binding energies of the electron states at kz=0 and kz=π/c.At the same time,there is ISD+ISD(with a π phase shift)lattice distortion between adjacent layers.In addition,alkali metal deposition can inhibit CDW by electron doping.Our analysis from the perspectives of electron and lattice provides important information for understanding the mechanism of CDW formation.3.Study on the band structure of S Substituted excitonic insulator Ta2NiSe5 based on synchrotron radiation.Ta2Ni(Se1-xSx)5 single crystals have been synthesized by substitution of S element in excitonic insulator candidate Ta2NiSe5.X-ray diffraction shows that S substitution reduces the lattice constant.The evolution of band structure has been directly observed by temperature and polarization dependent ARPES experiments,and excitonic insulator state is gradually inhibited.On the other hand,the alkali metal deposition experiment found that the absence of the spectral weight of valence-band maximum in monoclinic phase gradually recovered with electron filling,then the lattice changed into orthorhombic phase.Therefore,the coupling of electron many-body interaction and lattice distortion should be considered in the formation of excitonic insulator.