Synchrotron ARPES Studies Of Low-dimensional Layer Materials

Currently the largest branch of physics is condensed matter physics,which encompasses a number of fields including semiconductor physics,magnetism,surface physics,strong correlations,topology,and others.These fields concern both the microscopic and macroscopic laws governing single-particle and collective excitations in matter.A series of novel quantum phenomena have been discovered with the continuous progress of research.Examples of these include the quantum Hall effect,high-temperature superconductivity,magnetoresistance,heavy fermions,and giant thermoelectric effects.These physical phenomena have their nature mostly due to the competition and synergy between the microscopic degrees of freedom present in the system.These microscopic degrees include electron charge,electron spin,orbitals,and lattices.External regulation can cause abrupt changes in physical properties at the macroscopic level by facilitating transitions between different quantum phases.Angular resolved photoelectron spectroscopy(ARPES)is a crucial tool employed in the study of novel physical properties of materials.It allows researchers to explore the band dispersion of electronic states and visualize the band structure.ARPES also enables the study of microscopic effects,such as reactive electron-electron interaction,electronphonon interaction,and spin splitting.With its unique capabilities,ARPES plays an irreplaceable role in advancing our understanding of materials science.By using angle-resolved photoelectron spectroscopy based on a synchrotron radiation source,as well as first-principles calculation and electrical transport,this thesis investigates low-dimensional materials exhibiting quantum phenomena such as topology and charge density wave(CDW).The study focused on understanding the novel electronic structure of these materials and uncovered the coexistence and synergy between different quantum phases.The main finding of this research is as follows.1.Study on the topological band structure of two-dimensional layered NbsSiTe6.The chemical vapour transport method was used to synthesise the layered Nb3SiTe6 single crystal.The high quality of the Nb3SiTe6 single crystal was demonstrated by X-ray diffraction and high-angle annular dark-field scanning transmission electron microscopy.Nb3SiTe6 was also found to have an anisotropic crystal structure by Raman scattering.The band structure of Nb3SiTe6 was comprehensively detected by ARPES.The details of the band structure along the S-R high symmetry direction were revealed by density function theory calculations,supporting the existence of hourglass-type dispersion protected by nonsymmorphic group symmetry.In addition,two nodal lines are found along the S-X and S-R-U high symmetry directions.This work highlights the fact that Nb3SiTe6 is a new topological semi-metal with a variety of topological electronic states.2.Study on the charge density wave band structure of doped two-dimensional TiSe2.By chemical vapour transport method,gradient doped VxTi1-xSe2(x=0～0.2)single crystals were synthesised.We found the V atoms were substituted Ti atoms and occupy Ti atomic sites by X-ray diffraction combined with high-angle annular darkfield scanning transmission electron microscopy.Electrical transport characterisation shows that low V doping concentration induces metal-insulator phase transition.To analyse the robustness of the CDW phase,the band structure evolution of VxTi1-xSe2(x=0～0.2)has been studied by angular resolution photoelectron spectroscopy.The folded band in the ARPES data is still present when the doping amount of V reaches 10%,although the electronic system has undergone dramatic changes after doping.The band structure is obviously different from the parent compound TiSe2.This indicates that the electron-driven situation cannot explain the persistence of CDW in VxTi1-xSe2(x=0.05,0.1).Lattice distortion at low temperature plays a important role in the stabilisation of the CDW phase.This work has implications for the understanding of the CDW mechanism in TiSe2.3.The topological band and charge density wave band structures in quasione-dimensional NbTe4 are studied.The crystal structure of NbTe4 at room temperature and 100 K has been obtained by means of single crystal diffraction.Using the density function theory method,the topological band structure of NbTe4 in the normal phases is revealed,and the phonon spectrum is calculated to obtain the CDW wave vector through the position of the imaginary frequency,and the CDW twisted supercell is constructed.The evolution of the band during the charge density wave transition at different temperatures was directly observed using ARPES.The band evolution in the process of CDW suppression is studied by electron doping.It is found that the spectral intensity near the Fermi level is strongly suppressed,which may be related to the energy gap opened by the CDW.In the region of this energy gap,there is a robust electron-coherent state crossing the Fermi level.The underlying topological properties in quasi-1D compounds make NbTe4 an excellent platform for studying the relationship between topology and charge density waves.