First-principles Study Of Half-heusler Compounds For Half-metallic Materials And Topological Insulators

In the past decades, the growing technology in spintronics promotes the rapiddevelopment of information technology industry. The emergence of various new type devicescan not be done without searching and developing of high-performance materials, such asternary half-Heusler compounds as half-metallic materials and topological insulators. Thisdissertation is devoted to the electronic structure of half-Heusler compounds fromfirst-principles. The main contents are as following with four chapters:In chapter1, we show a brief introduction to the significance and research background ofthe ternary half-Heusler compounds as half-metallic materials and topological insulators.Mainly, different kinds of half-metallic materials and the development of topologicalinsulators are discussed, followed by an outline of the dissertation.In chapter2, we introduce the first-principles method based on Density FunctionalTheory (DFT) and the code (VASP) adopted in our work.In chapter3, Te-based half-Heusler systems XYTe (X=Sc-Zn, and Y=Sc-Zn) are studiedby first-principles calculations. We found that CoMnTe and FeMnTe are robust half-metallic(HM) ferromagnetic alloys with large HM gaps of0.42and0.61eV, and band gaps inminority spin channel of1.13and1.24eV, respectively, larger than that of any Heusler orhalf-Heusler alloys reported in the literature. In addition, the half-metallicity of CoMnTe andFeMnTe can be maintained even under in-plane strains of-11.3%to6.1%and-11.7%to10.0%, respectively, implying their practical applications in spintronic device. The influenceof different chemical compositions to the band gaps and HM gaps are further discussed. Wefound that a proper choice of s-p element could increase the HM gaps of half-Heusler alloysby adjusting the location of Fermi level in the spin minority band gap, in addition to theprerequisite of strong d-d hybridization of transition metals.Ternary half-Heusler systems MM’X are studied by first-principles calculations inchapter4. Here, M and M’ represent elements from group IA to group IIB except H, Cs, thelanthanide, and those in period7. X is chosen from group IIIA to group VIIA except F, At, Poand those in period7. After the exploration of2295candidates, we demonstrated that LiAuSand NaAuS are excellent topological insulators with the bulk band gaps of0.20and0.19eVunder s-p band inversions. Then we have extracted maximally localized Wannier functions tostudying the surface state of LiAuS and show a strong evidence of the topologically nontrivialfeature. Moreover, LiAuS and NaAuS are found to be robust topological insulators underlarge in-plane strains, which make them suitable for epitaxial growth of films. Finally, we found that most of the half-Heusler compounds with band inversion are corresponding to therelative large difference of valence electrons between M and M’.