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 theelectron charge, has attracted immense attentions of many scientists in recent years fromthe fields of physics and materials science. Compared to the traditional semiconductordevices, the spintronics devices have the advantages of lower energy consumption, quickworking speed, smaller size and nonvolatility, and the potential application isimmeasurable. Importantly, the coming of the half-metallic magnets and topologicalinsulators with the peculiar properties separately, makes possible for the practicalapplication of the spintronics devices in the information technology industry. Therefore, itis currently a research hotspot in condensed matter physics and materials physics tofurther explore the physical characteristics of the half-metals and topological insulatorsand make them applied to the spintronics devices.In this thesis, using the materials design codes of WIEN2K and CASTEP based ondensity functional theory, we performed the studies of the structural, electronic andmagnetic properties of surface and interface for half-metallic full-Heusler alloysCo₂VZ（Z=Ga, Al）, and the lattice distortion-induced topological insulating phase inthree-dimensional HgTe. Concretely, the main research contents and results are as follows:（1） We have investigated the structural, electronic and magnetic properties of fourpossible surfaces of the full-Heusler alloy Co₂VGa in （111） direction. Both the structuralrelaxation and the calculated surface energy show that the Ga-terminated surface is themost stable among the four possible surfaces. It is found that the two surfaces terminatedby atoms Ga and V preserve the half-metallic properties of the bulk system while the othertwo 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 lostthe bulk half-metallicity while the pure V atom-terminated surface with V-doping atVGa-terminated （001） surfaces, has recovered the half-metallic character due to therehybridization between the surface V atoms.（3） We have constructed the （111） heterostructures of full-Heusler alloy Co₂VGa withsemiconductor PbS and explored the interface effect on the electronic structure. Both thestructural relaxation and the calculated interfacial adhesion energy indicate that V-Sinterface is the most favorable structure. We predict that the two interfaces of V-S andV-Pb exhibit half-metallicity and nearly half-metallicity respectively while someinterfacial 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 bulkCo₂VAl 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 lostat the two surfaces terminated by atom Co. The structural relaxation shows that the twosurfaces with the half-metallicity are more stable due to the smaller relaxation.（5） We have explored the topological insulating phase induced by the latticedistortion in three-dimensional HgTe. Our results show that a topological insulating phaseis obtained by the lattice distortion in bulk HgTe with the assumption of therelaxed-volume and the constant-volume, which accords well with the recent theoreticaland experimental results. Importantly, the topological insulating band gap exceeds0.3eVunder proper lattice distortion.