The Studies On The Half-metallicity And Thermoelectric Transport Properties Of Low Dimension Quantum Systems

Half-metallic ferromagnets have been known as the effective spin injection sources for the future spintronic devices due to their complete spin polarization. They are important materials used in spintronic devices. The study on the thermoelectric transport properties of quantum-dots (QDs) is one of the hot areas in the field of thermoelectricity due to the possible high thermoelectronic conversion efficiency, It has been expected that Quantum-dots have the potential applications in the coming thermoelectronic conversion devices. In this thesis, by using the first-principles method and the non-equilibrium Green’s function, we perform calculations for the half-metallicity of low dimension multi-alloys surface and the thermoelectric transport properties of couple double-quantum dots (DQDs) array. The main results are as follows:Firstly, the studies on the surfaces of the binary zinc-blende half-metallic alloy CrSe and the superlattice of CrSe/ZnSe indicate that the bulk half-metallicity of CrSe is retained at the (110) surface and the Cr-and Se-terminated (001) surfaces. Compared to the bulk case, the atomic magnetic moments at the (001) surface are increased greatly, but the change for the case of the (110) surface is very slight. The reason is that the Cr-Se bond is broken at the (001) surface but kept at the (110) surface. We also calculate the surface energyies for the three surfaces studied, and find that the Se-terminated (001) surface is more stable than the (110) and the Cr-terminated (001) surfaces, which is helpful for the experimental synthesis of half-metallic thin films of CrSe. Moreover, CrSe/ZnSe (001) superlattice has good half-metallicity, and its half-metalic gap is a little smaller than the bulk CrSe.Then, calculating the electronic structure and magnetism for the twelve quaternary Heusler alloys of CoFeTiZ and CoFeVZ (Z=Al, Ga, Si, Ge, As, Sb), we find that only CoFeTiSi, CoFeTiAs and CoFeVSb are half-metallic ferro-magnets with considerable half-metallic gaps of0.31,0.18and0.17eV, respec-tively. CoFeTiAl and CoFeTiGa are conventional semiconductors, and other alloys are nearly half-metals or the half-metallic gaps are almost zero eV. Moreover, CoFeTiSi, CoFeTiAs and CoFeVSb are still half-metallic under appropriate uniform and in-plane strains. It is found that the bulk half-metallicity of CoFeVSb is retained at the VSb-terminated (001) surface but destroyed at the CoFe-terminated (001) surface.Finally, we investigate the thermoelectric effect in a laterally couple double-QDs array by using the nonequilibrium Green’s function method. The thermoelectric effects includ Seebeck coefficient (S), thermal conductance (k), and figure of merit (ZT). The theoretical results show that the non-uniform in energies in the side-attached QDs and the fluctration in the structure parameters much suppress the thermoelectric efficiency. Moreover, we also find that the Coulomb interactions result in the splitting of the asymmetric double-peak structure of the Seebeck coefficient S. From the thermal conduction spectrum, it is clearly seen that a strong violation of the Wiedemann-Franz law occurs, leading to a colossal enhancement of the figure of merit.