Investigation Of Three-dimensional Topological Quantum Material By Angle-resolved Photoemsission Spectroscopy

Topological materials are a kind of material with topological non-trivial structures in the energy band.The special quasi-particle excited states of topological materials that are different from traditional near-free electron metals or semiconductors,such as Dirac fermions and Weir fermions.These special electronic states can bring abundant novel properties,for instance,extremely high carrier mobility,high spin polarizability,giant magneto resistance,and quantum anomalous Hall effect.Topological materials have become a research highlight in recent years.Early researches about topological materials concentrated on two-dimensional and quasi-two-dimensional transition metal chalcogenides.The crystal structure of 2D materials are not complicated,so the growth process and characterizes are easy to achieve,2D topological materials have been widely studied.However,the simple structural of the two-dimensional system also leads to the lake of physical mechanism,such as symmetry,magnetism,and dimensional changes.So the potential for further exploration of topological physics is limit.Recently,researchers have turned their attention to three-dimensional topological materials,for the additional dimensions brings more freedom to the material,So complex and novel topological phases canl be found in 3D materials.Although there have been some researches on three-dimensional topological materials,most of three-dimensional materials are hard and difficult to cleave,and the crystal structure is complex.So the surface-based electronic structure characterizations on 3D materials are very difficult,especially the angle-resolved photoemsission spectroscopy(ARPES).In this paper,we used ARPES and other methods to study the electronic structure,topological phase,and evolution of topological states of the three-dimensional Weyl semimetal WP2 and Kagome metal FeSn,YCr6Ge6.At the same time,we have also explored the band shift of Cu-intercalated TaSe2.This paper includes the following research results:1.Band structure and special Fermi arc surface state study of three-dimensional Weyl semimetal WP2.Using special single crystal cleavage technique,we cleaved the(021)high-index crystal plane of WP2,and a long and straight Fermi arc which crossing the Brillouin zone was observed.Different kind of cleaved surfaces will lead to different surface states.This kind of long and straight Fermi arc was first discovered in Weyl materials,which may helps to understand the mechanism behind the superior transport performance of WP2.2.Research on the band structure of Kagome lattice material FeSn.In Kagome metal FeSn,though ARPES and DFT calculation,we verified the existence of massless Dirac fermions in the anti-ferromagnetic system under the protection of special symmetry.Through our theoretical calculations and experimental results,we also proved that the surface Dirac cone is split into a Weyl-like cone under the surface Stark effect.A flat band was observed near the Fermi level.After carefully analysis,we believed that the flat band was not derived from the magneto resistive setback,but the drumhead surface state.3.Study on the band structure of Kagome lattice material YCr6G6.We characterized the electronic structure of Kagome metal YCr6G6.By comparing with the DFT calculation,we found the electronic correlation effect in YCr6G6 was significant,and the kz dispersion along c axis was remarkable,indicating that there is a strong layer to layer coupling of the two Kagome layers.We also found that the composition of the energy band were complex.So the band model cannot be explained by the simple ideal band structure of Kagome model.By comparing the difference between the effective mass obtained from our experiments near EF with the specific heat data,we indirectly prove that there is a flat band near the Fermi level.4.Research on the regulation of energy band of materials by intercalation of metal atoms.Using X-ray near-edge absorption and ARPES,we studied the crystal structure and electronic state of Cu intercalated TaSe2.By comparing with the crystal constants of 2H-TaSe2,we found that the crystal structure of TaSe2 was not changed significantly after intercalation.The inserted Cu atoms form a chemical bond with Se and provide charge to the TaSe2 layer.Through the comparison of the energy band structure,we found that the band shift of Cu intercalated TaSe2 is very complicated and cannot be explained by a simple rigid band model with only the effect of charge doping.Our research shows that when the intercalated ions form a chemical bond with the host layer,the band dispersion in the host layer will be regulated by the intercalated atoms with a non-rigid shift.