First-Principle Studies Of Half-Metal And Topological Materials

The study of half metal and topological materials is a very active research field in condensed matter physics in the early twenty-first Century.They play an important role in the field of spintronics.Since 1983 de Groot research group predicted the half metallicity of Ni Mn Sb using first-principles,more and more researchers have focused on the study of the half metal.The study of topological materials is more active.Since the discovery of the first topological insulator,the concept of topology have become the most exotic feature in different fields of physics,which promoted the development of physics,such as 2D topological insulator,3D topological insulator,topological photonics,topological semimetal,topological superconductor and so on.In the second chapter,we introduce the density functional theory and topological band theory.Density functional theory is a widely used method,which is mainly used to calculating the electronic structure of materials.All results of this thesis are based on the first-principles calculation.Finally,we simply introduce the topological band theory.In the third chapter,The half metallic properties of Rb S and KS compounds in Cs Cl structures were studied by first-principles method.Calculating the relation between the total energies and lattice parameters for Rb S and KS,we find out that the equilibrium lattice parameters are4.02 ??? and 3.84 ??? for Rb S and KS,respectively.Both Rb S and KS are half-metallic ferromagnets with the magnetic moments of 1 ?Bper formula unit,and band gap of 4.287 e V for Rb S and 4.395 e V for KS.We also have studied the electronic and magnetic properties of?111?surfaces of Rb S and KS,and have found out that the half-metallicity of their bulk is preserved in all of those surfaces.In the fourth chapter,We present a study of the electronic and thermodynamic properties of Sr₂ XMo O₆?X = Ge and Si?based on the first-principles calculations.It is found that Sr₂ Ge Mo O₆ is a half-metallic ferromagnet and Sr₂ Si Mo O₆ is a metallic ferromagnet within LSDA.But using LSDA+U,the results indicate that both compounds are half-metallic ferromagnets with a total spin magnetic moment per formula unit of 2.00 ?B.Finally,by using a quasi-harmonic Debye model,the bulk modulus,heat capacity,Debye temperature and the coefficient of thermal expansion have also been obtained.In the fifth chapter,a new type 3D topological insulator,Bi₂OSe₂,is studied by using firstprinciples calculation.Through our calculation,we find there exists a band inversion caused by the spin-orbit coupling,and the bulk energy gap is 0.126 e V.Finally,in order to further verify the topological properties of the compound,we also calculate the density of the surface states.We find a Dirac cone at the ??.In the sixth chapter,Based on first-principles band structure calculations,we predict that an inversionbreaking,stoichiometric single crystal Nb Ir Te4 is a Weyl semimetal candidate.Without SOC,we find that there are only eight Weyl points in the kz = 0 plane.The separation between Weyl points of opposite chrialities is determined by the magnitude of the band inversion.When considering the SOC,Nb Ir Te4 hosts sixteen Weyl points in the whole brillouin zone.Actually,there are only three irreducible Weyl points and two of them are Type-II Weyl points.Finally,the surface state and Fermi arc structures on?001?surface are also obtained.In the final chapter,the present work is summarized and the future work is prospected.