First-Principles Study On Transition Metal Pnictides And Symmetry Analysis Using Wannier Orbitals

The study on topological properties of materials becomes one of hot research areas in condensed matter physics in recent decades,and the classification of band topology of electronic structure is strongly related to its symmetry.We develop a symmetry analysis code for Wannier orbitals,which is named WannSymm.When the basis of a real-space Hamiltonian consists of atomic orbital like Wannier orbitals,WannSymm can be used to symmetrize the Hamiltonian,to generate symmetry operators of the little group at a given k point and to perform symmetry analysis for the band structure.Generally,symmetrized Hamiltonian gives optimized calculation results compared with the original Hamiltonian in many cases,such as nodal structure calculation,surface state calculation and band structure unfolding calculation.Furthermore,WannSymm can be applied not only to none magnetic cases,but also to magnetic cases such as FM and AFM cases.We demonstrate the validity of WannSymm in many typical materials,including K2Cr3As3,CrO2,MnF2 and Ce3Pd3Bi4.With WannSymm,we perform symmetry analysis on Ce3Pd3Bi4.We symmetrize the original Hamiltonian with different symmetry groups.If the symmetry group contains no gliding mirror symmetry,the nodal structure derived from the symmetrized Hamiltonian is not complete.Only when all 6 gliding mirror symmetries are considered in the symmetry group,the symmetrized Hamiltonian gives clear and complete nodal-lines.These results show that the topological nodal-lines in Ce3Pd3Bi4 is protected by the gliding mirror symmetries.With the result of Berry phase calculation,we show that Ce3Pd3Bi4 is a topological non-trivial nodal-line semimetal.With DFT based first principles calculations,we employ PBE exchange and correlation functional and HSE06 hybrid functional to calculate the electronic structure of n-type diluted magnetic semiconductor Ba(Zn1-xCox)2As2.For the mother compound BaZn2As2,the band structure calculated with PBE functional does not give reasonable result,while the HSE06 calculation gives an energy gap close to the experiment results.The reason of the failure of PBE and the success of HSE06 in band gap calculation is that compared with PBE,HSE06 counts the exchange energy of electrons more accurately.In our calculations,many cases are considered,including only 1 Co atom each super cell,2 Co atoms at adjacent Zn sites each super cell and 3 Co atoms at adjacent Zn sites each super cell.Our calculation results show that,when there are 2 Co atoms at adjacent Zn sites in every super cell and their magnetic moments are in the same direction,there exist～0.16 n-type carriers and～0.16 p-type carriers provided by each Co.This thesis also contains some work related to experimental data analyzing.Based on the existing results of experiments,we re-process and analyze the data,correct certain fit errors in previous work and clarify the fit results of certain physical quantities.To summarize,our research improves the understanding of i)symmetry in electronic structure,ii)topological properties of band structure and iii)electronic structure of diluted magnetic semiconductors,and it contributes to the searching of novel topological materials and novel diluted magnetic semiconductors.