Angle Resolved Photoemission Spectroscopy Study On The Electronic Structure Of Kagome Superconductors And Two-Dimentional Materials

The study of quantum materials is one of the most active and important research fields in contemporary physics.Angle-resolved photoemission spectroscopy(ARPES)is an important technique to study the electronic structures of various quantum materials,such as superconductors,two-dimensional materials,topological materials and so on.This thesis mainly introduces ARPES studies on Kagome superconductors and singlelayer two-dimensional materials.In terms of kagome superconductors,I carried out ARPES measurements on AV3Sb5(A=K,Rb,Cs),including the study of the electronic structures of charge density wave state and the study of multiple flat bands.In terms of single-layer two-dimensional materials,I carried out ARPES measurements on airsensitive two-dimensional materials.As for instruments,I developed vacuum ultraviolet laser micro-ARPES technique.Finally,the photoluminescence and Raman studies of transition metal chalcogenides MoS2 with bubble structure are also introduced.This thesis is summarized in the following chapters:1.A brief introduction of quantum materials that are usually studied by ARPES,mainly including unconventional superconductors and two-dimensional materials.2.An introduction of the principle of ARPES and its main components.In addition,four vacuum deep ultraviolet laser ARPES systems with different characteristics and advantages developed and built in our laboratory are introduced.3.ARPES study of the electronic structures in charge density wave phase of Kagome superconductor KV3Sb5.First,the Fermi surface sheets of KV3Sb5 is clearly distinguished by using helium lamp as light source.Then,the influences of charge density waves on the electronic structures of KV3Sb5 are observed.They include the reconstruction of Fermi surface sheets and the folding of band structures caused by 2×2 charge density wave;the gap opening and the band splitting at the boundaries of original and reconstructed Brillouin zone;the measurements of Fermi surface-and momentumdependent charge density wave gaps.Finally,it is observed that the vanadium-orbital band structures have the signatures of electron-phonon coupling,which may be related to the formation of charge density wave in KV3Sb5.These results provide key insights in the origin of charge density wave and its interaction with other physical properties.4.Further ARPES investigation on AV3Sb5(A=K,Rb,Cs)and Ti-doped CsV3Sb5.There are multiple flat bands not coming from the band structure of Kagome lattice.Multiple flat bands are ubiquitous and has similar energy positions in AV3Sb5(A=K,Rb,Cs).As the increase of Ti-doping,the charge density wave phase transition are suppressed and the high energy flat band changes significantly.Furthermore,temperaturedependent measurements on RbV3Sb5 found that the merging of high-energy flat bands occurs near the transition temperature of charge density wave.This shows that the evolution of these flat bands is related to the transition of charge density waves.Finally,by quantitatively comparison,it is found that there is a highly consistency between the binding energies of each flat band and the van Hove singularities at M point of the Brillouin zone.This indicates that the origin of the flat bands is strongly related to the van Hove singularities,which may involve new physical mechanism.5.ARPES with micro-focused 7eV-laser was developed,including measuring the vibration of ARPES system,designing the laser path and realizing the spot size of～4.5μm×8.5μm.This technique is suitable for studying the electronic structures of two-dimensional materials.A large number of monolayer and few-layer twodimensional materials are obtained by mechanical exfoliation technique.The problem of air-sensitivity of most monolayer materials is overcome.The electronic structures of haif-layer Bi2Sr2CaCu2O8+δ and monolayer black phosphorene ware observed by ARPES.6.The photoluminescence phenomena of multilayer MoS2 bubbles is studied.The multilayer MoS2 bubbles with tensile strain possess properties belonging to its monolayer counterpart.Through the temperature-dependent photoluminescence measurements,it is identified that one photoluminescence peak comes from the direct-gap transition and the other two photoluminescence peaks come from the indirect-gap transitions.It is confirmed by density functional calculation and Raman experiment that the bubble structure makes the multilayer MoS2 has larger interlayer spacing and weaker interlayer coupling.Consequently,multilayer MoS2 is similar to monolayer MoS2 to some extent.Monolayer devices are difficult to prepare,which widens the way for the application of optoelectronic devices.