First-principles Study On Topological Electronic Materials And Their External Field Regulation

In recent years,the family of topological electronic materials has received widespread attention due to its novel physical properties and huge potential applications in the field of spintronic devices.Topological electronic materials are a class of materials with nontrivial topological properties,and can be further classified according to different physical states,electronic structures,symmetry and other characteristics,such as Z2 topological insulators,topological crystal insulators,Weyl semimetal,Dirac semimetal,topological superconductor etc.In the field of topological materials,computational physics has played a vital role.Through first-principles calculations,physicists have theoretically predicted and guided experiments to discover rich and diverse topological materials.More importantly,in the context of machine learning and high-throughput calculations,based on the symmetry index of materials,effective screening of nonmagnetic topological materials can be achieved.However,topological electronic materials still have more or less defects in practical applications,so we need to make reasonable regulation design and explore the physical laws.In this paper,first-principles calculation methods are used to systematically study the magnetically doped topological insulator Sb1.67Cr0.33Te3,topological insulator Bi2Te3-x Sex(x=0,1,2,3)thin films,and the rare earth monopnictide compounds family.Our results provide some theoretical guidance and design ideas for experimental research,and also provides some directions for the exploration of magnetic topological materials.In the first part,we studied the evolution of the topological insulator Sb1.67Cr0.33Te3band structure under mechanical strain.The calculation results show that Cr atoms doped with Sb2Te3 tend to replace Sb atoms and produce strong hybridization with surrounding Te atoms.The band gap of Sb1.67Cr0.33Te3 is 0.031e V,and the ground state magnetic moment is[0 0 1]direction,3.05?_B/Cr.When compressive strain is applied to Sb1.67Cr0.33Te3,the hybridization between Cr-d and Te-p orbitals increases,the band gap value of the system will gradually increase,and it will increase to a maximum at about=-2%.In addition,we also found that there is a certain correlation between the SOC strength of Sb1.67Cr0.33Te3 and the Van der Waals interaction in the system.In the second part,we calculated the electronic properties of the Bi2Te3-x Sex(x=0,1,2,3)thin films,and the results show that the increase of the content of Se atoms will reduce the energy gap of the thin films and change the charge distribution of the system.Besides,through further research,we found that the experimentally stable Bi2Te2Se and Bi2Se2Te thin films have strong robustness in the external electric field,the atomic position and electronic structure have almost no change in the electric field up to 0.2V/?,and the charge transfer properties are also not affected by the external electric field,which are closely related to the strong covalent bond between Te and Bi atoms in the thin films.In the third part,we systematically study the magnetoresistive mechanism,electronic structure,topological properties,and high-pressure phase transitions of several materials in the rare earth monopnictide compounds family.Firstly,through computational analysis,we find that the external magnetoresistance in YBi is probably due to the combined effect of the p-d mixed special orbital texture formed by the topological electronic structure and the compensation effect of electrons and holes in the system.Secondly,we find that Ho Bi has nontrivial topological properties,and Weyl fermions may exist in the ferromagnetic state.In addition,Ho Sb's different magnetic structures exhibit completely different topological properties,which are topologically trivial in nonmagnetic and ferromagnetic states,but have topologically nontrivial properties in antiferromagnetic structures.Finally,we studied the crystal structure and electronic properties of Lu Bi under pressure.The calculation results show that the crystal structure of Lu Bi will change from Na Cl structure to Cs Cl structure at high pressure.And with the increase of pressure,the topological properties of Lu Bi will also change from trivial to nontrivial,and fourfold degenerate Dirac band crossings occur in certain high-pressure phases.