The Study Of The Heusler Compounds As Half-metal And Topological Insulating Materials

In this thesis, we study the application of Hesuler compound in the field of function materi-als by first-principle calculation. The works include that the full-Heusler compound Mn2CdMg as half-metal material and the half-Heusler compound as topological insulating materials such as LiAgSe etc..In the early1980s, Rob de Groot and collaborators discovered a new type of magnetic material:half-metal. The feature of this new type material is two spin channels shows differ-ent conductive characteristic, for which one of the two spin channels shows a typical metallic behavior while the other has semiconductor or insulator properties. At this material with100%spin-polarization, it has great potential application in random access memory and spintronic. Therefore, the topological insulating materials is that the bady band structure exhibit insulator characteristic but the surface states has Dirac electronic states at Fermi level exhibition matel characteristic. It is a new class of function materials.At first we gived a short introduction to the development and the basic concept of DFT. Only simple H atom can be calculated by quantum mechanics at the very beginning of its foundation. Several years later, E. A. Hylleraas and D. R. Hartree calculated the ground state of Helium atom by variational and self-consistent field method, respectively:V. Fork improved the self-consistent field method forming the well-know Hartree-Fork method. This method is the prototype of many modern methods for electronic structure calculation, The DFT was founded in1960’s, it is widely used in materials simulations and computational chemistry because of its high precision and moderate computational consumption.In the third chapter, The electronic and magnetic properties of Mn2CdMg were investigated using ab initio electronic structure calculations, that the CuHg2Ti-type structure is energetically more preferable compared with the AlCu2Mn-type structure in antiferromagnetism state and presents half-metallic property. Calculations exhibit that the CuHg2Ti-type Mn2CdMg alloy keeps100%spin polarization of states at the Fermi level. The total spin magnetic moment in the unit cell (Mt) follows a rule Zt=Zt-28where Zt represents the number of the valence electrons. However, with a pressure effect on it, its magnetic property changed linearly from0to809.03GPa. We also find that the alloy keeps in half-metallic property from0to421.57GPa and appears a magnetic phase transition at the pressure of809.03GPa. Thus, Mn2CdMg may be a promising material for future spintronic device applications.In the fourth chapter,,we investigate a series of ternary materials XYZ (X=Li, Na, K, R-b, Y=Cu, Ag, Au and Z=Se) in the Heusler structure using first-principles calculations. The results show that, out of these total twelve compounds considered, the LiAgSe, LiAuSe, Na-CuSe, NaAgSe, NaAuSe, KAuSe and RbAuSe compounds in cubic crystal structure have band inversion at the time reversal invariant momentum Γ point. The negative inversion energy△i, between Γ6and Γ8levels indicates that these compounds are topologically nontrivial while they are not naturally insulating. Therefore it is worthy further to investigate the characteristic band structures of these seven compounds under strains. Straining the crystals under different angles, we find that the reduction of the degree of crystal symmetry does not change the band inversion, but a band gap appears at the Γ point for each of these compounds. Our research shows that these compounds in topologically nontrivial phase may be tuned into new3D TI via proper strain engineering.In the final chapter, we made a conclusion for this thesis, and made a plan for the following work.