The Half-metallicity Of Co₂VZ（Z=Ga,Al） Thin Film And The Topological Insulating Phase Of Three-dimensional HgTe

Spintronics, developed by using the spin nature of electrons based on considering the electron charge, has attracted immense attentions of many scientists in recent years from the fields of physics and materials science. Compared to the traditional semiconductor devices, the spintronics devices have the advantages of lower energy consumption, quick working speed, smaller size and nonvolatility, and the potential application is immeasurable. Importantly, the coming of the half-metallic magnets and topological insulators with the peculiar properties separately, makes possible for the practical application of the spintronics devices in the information technology industry. Therefore, it is currently a research hotspot in condensed matter physics and materials physics to further explore the physical characteristics of the half-metals and topological insulators and make them applied to the spintronics devices.In this thesis, using the materials design codes of WIEN2K and CASTEP based on density functional theory, we performed the studies of the structural, electronic and magnetic properties of surface and interface for half-metallic full-Heusler alloys Co2VZ(Z=Ga, Al), and the lattice distortion-induced topological insulating phase in three-dimensional HgTe. Concretely, the main research contents and results are as follows:(1) We have investigated the structural, electronic and magnetic properties of four possible surfaces of the full-Heusler alloy Co2VGa in (111) direction. Both the structural relaxation and the calculated surface energy show that the Ga-terminated surface is the most stable among the four possible surfaces. It is found that the two surfaces terminated by atoms Ga and V preserve the half-metallic properties of the bulk system while the other two surfaces terminated by atom Co lose the half-metallicity.(2) We have studied the doping-induced half-metallicity at the (001) surface of full-Heusler alloy Co2VGa. We find that both Co-and VGa-terminated surfaces have lost the bulk half-metallicity while the pure V atom-terminated surface with V-doping at VGa-terminated (001) surfaces, has recovered the half-metallic character due to the rehybridization between the surface V atoms.(3) We have constructed the (111) heterostructures of full-Heusler alloy Co2VGa with semiconductor PbS and explored the interface effect on the electronic structure. Both the structural relaxation and the calculated interfacial adhesion energy indicate that V-S interface is the most favorable structure. We predict that the two interfaces of V-S and V-Pb exhibit half-metallicity and nearly half-metallicity respectively while some interfacial states exist in the other two interfaces of Ga-S and Ga-Pb.(4) We have studied the structural, electronic and magnetic properties of the bulk Co2VAl and its (111) surfaces and confirmed that full-Heusler alloy Co2VAl with L21structure exhibits half-metallic ferromagnetism with a ground state. It is predicted that V-and Al-terminated surfaces retain the bulk half-metallicity, but the half-metallicity is lost at the two surfaces terminated by atom Co. The structural relaxation shows that the two surfaces with the half-metallicity are more stable due to the smaller relaxation.(5) We have explored the topological insulating phase induced by the lattice distortion in three-dimensional HgTe. Our results show that a topological insulating phase is obtained by the lattice distortion in bulk HgTe with the assumption of the relaxed-volume and the constant-volume, which accords well with the recent theoretical and experimental results. Importantly, the topological insulating band gap exceeds0.3eV under proper lattice distortion.