Physical Property And Angle Resolved Photoemission Spectroscopy Studies On Topological Materials

Topological materials,owing to their unique electronic structure,exotic physical properties and potential applications,have become the frontier research field in con-densed matter physics.Searching for novel topological materials and exploration of new topological phases and topological properties are the main subject of topological material research.In this thesis,by using angle resolved photoemission spectroscopy(ARPES)combined with physical property measurements,the physical properties and electronic structure of topological materials LaSbTe and EuCd2As2 have been studied systematically.The thesis mainly includes the following parts:1.The history of topological materials research is briefly reviewed starting from the topological insulator to the topological semimetals,the topological superconductors and some magnetic topological materials.In particular,the significant results from ARPES measurements on topological materials are highlighted.2.The basic principles and the experimental instruments of ARPES are introduced in detail.3.The Bi2Se3 nanoplate is grown by chemical vapor deposition(CVD)method and its basic physical properties are characterized.4.The thermal electric properties of the Weyl semimetal TaAs are studied by using PPMS combined with the lock-in technique.An anisotropic quantum oscillation of the thermopower is observed for the first time which suggests a liner dispersion,a highly anisotropic Fermi surface and non-zero Berry phase in TaAs.5.By carrying out ARPES measurements combined with band structure calcu-lations,it is found that LaSbTe is a genuine nodal-line semimetal.Taking spin-orbit coupling into account,the band structure calculations predict that a nodal line is formed in the boundary surface of the Brillouin zone,which is robust and lies close to the Fermi level.The Dirac nodes along the X-R line in momentum space are directly observed in our ARPES measurements,and the energies of these Dirac nodes are all close to the Fermi level.These results constitute clear evidence that LaSbTe is a genuine nodal-line semimetal,providing a platform to explore for novel phenomena and possible applications associated with the nodal-line semimetals.6.By carrying out thorough magnetic,electrical and thermodynamic property measurements,we discover for the first time a long time relaxation of the magnetic susceptibility in EuCd2As2.The(001)in-plane magnetic susceptibility at 5 K is found to continuously increase up to?10%over the time of?14 hours.It is anisotropic that the(001)in-plane relaxation is stronger than the out-of-plane case.The effect gets stronger with decreasing temperature in the magnetic state.It is prominent under low magnetic field and gets suppressed when the applied magnetic field is large.These observations will stimulate further theoretical and experimental studies to understand the origin of the relaxation process and its effect on the electronic structure and physical properties of the magnetic topological materials.7.By carrying out high-resolution laser-based ARPES measurements on the elec-tronic structure of EuCd2As2,the remarkable electronic structure relaxation at low tem-perature over several days are observed for the first time in EuCd2As2.The Fermi surface evolves with time from the initial a point to multiple large Fermi surface sheets and the corresponding band structure exhibits significant shift and splitting.The electronic structure of the fully relaxed EuCd2As2 sample shows a strong temperature dependence and the temperature evolution can be divided into four regimes.The observed electronic structure relaxation is not due to surface reconstruction or lattice structure change;it is likely related to the variation of the magnetic structure in EuCd2As2.