First-Principle Studies Of Half-Metal And Topological Metal

Half-metallic ferromagnets are predicted to be the most promising next-generation spintronic devices because of their unique properties:they have only one spin channel for conduction.Since de Groot's team theoretically predicted the semimetality of Ni Mn Sb in 1983,more and more researchers have focused on half metal.The study of topological materials is more active,since the discovery of the first topological insulator,and the concept of topology have became 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 calculate 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 third chapter,we first investigate the structural,electronic and magnetic properties of half-Heusler compounds Li XGe(X = Ca,Sr and Ba),by using the first-principles calculations within full-potential local-orbital minimum-basis method.The calculations show that all compounds are half-metallic ferromagnets with the same magnetic moment of 1.00 ?Bper formula unit and all compounds follow the Slater-Pauling rule: Mtot=(8-Ztot)?B.The half-metallicities of Li Ca Ge,Li Sr Ge and Li Ba Ge can be preserved when lattice constants are changed respectively from-3.6% to 21.4%,-3.8% to 16.1% and-1.2% to 14.7% relative to the equilibrium lattice constant.The compressed critical lattice constants with half-metallicity are equivalent to the added pressures of 3.55,3.22 and 0.63 GPa,respectively.For all alloys the cubic phase with half metallicity has a better stability than tetragonal phase.Furthermore,Li Ca Ge,Li Sr Ge and Li Ba Ge can preserve their half-metallic character when c/a is changed from 0.90 to 1.07,0.87 to 1.14 and 0.96 to 1.04,respectively.And by observing the calculated elastic parameters(C11,C12 and C44)and elastic properties(bulk modulus B,shear modulus G,Young's modulus E and anisotropy factor A),we can find that all the compounds are mechanically stable in nature,and the three compounds present a stronger resistance to the unidirectional than resistance to the pure shear deformation,and displays an excellent ductile behavior in nature.In fourth chapter,we investiage a new type of topological metal Mo C.The energy band structure and surface states of Mo C are calculated.The k · p model of the special thrible degenerate points(Nexus Fermions)in the system is deduced and discussed.It is found that the system is on the surface of k = ?.The mirrors Chern number is 1,which means that there is a surface state passing through the Fermi level.And the model also explained that surface state in the system is protected by the symmetry of the system.At the final chapter,the present work is summarized and the future work is prospected.