First Principles Study On Magnetic Topological Materials（W,Mo）Se₂ And V₂（Cu,Ag）O₄

Magnetic topological materials are a new type of quantum materials that integrate the characteristics of crystallography,magnetism,and topology,including magnetic topological insulators,magnetic topological semimetals,and so on.They have unique physical phenomena,such as low dissipation electron transport.Therefore,they also have important application potential in the future realization of high-speed and low-energy spintronic devices.Magnetic topological insulators are a type of topological quantum state,which are different from ordinary insulators and topological insulators.They are characterized by nonzero Chern number and helical edge states within the bulk band gap.As a new member of topological quantum materials,magnetic topological semimetals have attracted extensive attention due to their combination of topology and spintronics properties.So far,only some of magnetic topological materials have been verified experimentally,so it is worth studying to predict more such materials with good properties theoretically.Based on first-principles calculations,this paper mainly focus on two new types of magnetic topological materials:two-dimensional（W,Mo）Se₂ and three-dimensional V₂（Cu,Ag）O₄.The main research contents are as follows:1.This paper proposed two novel two-dimensional quantum anomalous Hall materials:WSe₂ and Mo Se₂.The results of phonon spectra in the reciprocal space and ab initio molecular dynamics of these two materials show that they are both stable in dynamics and thermodynamics.When the spin-orbit coupling（SOC）is not considered,the band structures of WSe₂ and Mo Se₂ happening spin splitting,there is a Dirac cone protected by C_2v symmetry in the spin-up channel,showing typical Dirac half-metal properties.When SOC is considered,the crossing point of two-dimensional WSe₂ and Mo Se₂ open the band gaps of 17 me V and 30 me V,respectively.Further calculations show that these two materials have non-zero Chern number（C=-2）,which indicates that they are ferromagnetic Chern insulators,and they have quantum Hall conductance platform and gapless chiral edge states.In addition,the Curie temperature of WSe₂ and Mo Se₂ are two-dimensional candidate materials for realizing quantum anomalous Hall effect at higher experimental temperatures.By discussed the substrate requirements and the electronic properties on the substrate for the growth of these two materials,the results show that it is possible to fabricate these two materials by experiments.Therefore,this work provides a theoretical basis for application of Chern insulators in spintronics devices.2.This paper found two kinds of three-dimensional magnetic nodal line semimetal materials:V₂Cu O₄ and V₂Ag O₄.The results of phonon spectra in the reciprocal space and binding energy show that they have dynamic stability and can be synthesized in future experiments.Their band structures are fully spin-polarized near the Fermi level,and there are symmetrically protected crossing points along high symmetry linesΓ-X,Γ-K,andΓ-W in the spin-up channel.The results of the three-dimensional energy band diagram and band gap diagram show that the crossing points of V₂Cu O₄ and V₂Ag O₄ form the closed nodal lines,which further confirm the characteristics of nodal lines.In addition,the clear surface states of V₂Cu O₄ and V₂Ag O₄ on the（001）plane confirm the topological non-trivial properties of V₂Cu O₄and V₂Ag O₄.When SOC is considered,the crossing points formed by these single spin channels only open negligible band gap.This work provides new alternative material for the experimental research and exploration of magnetic nodal line semimetal,and it also provides a new scheme for the experimental study of spintronic materials in the future.