| Many interesting physical phenomena in solids result from the spin-orbit (SO) coupling, for example, abnormal Hall effect, spin Hall effect, magnetocrystalline anisotropy energy and magneto-optic effect. In spintronics, SO coupling provides a fully electrical strategy to control and manipulate the spin of the electon. Therefore, SO couping in solid materials has long been considered to be one of the important topics in spintronics. Graphene has many excellent physical and chemical properties, such as Dirac fermion carrier, very high carrier mobility at room temperature, very high thermal conductivity and mechanical strength, which enable graphene to hold great promises in many applications. Rather small intrinsic SO interaction for carbon atoms results in a very weak SO coupling in graphene. Consequently, graphene is considered to be an ideal material for spin transport due to the weak SO coupling and near absence of nuclear magnetic moments in carbon. However, this weak SO coupling in graphene in turn limits its other possible applications. Therefore, the focus of this dissertation is to explore how to enhance the SO coupling in graphene.With the rapid progress in graphene research, the other two dimension materials have also been receving ever-increasing attention. One of such materials is the single-layer hexagonal boron nitride (BN sheet). BN sheet is an insulator with a band gap of about6.0eV. It has profound chemical and thermal stabilies and simultaneously is equally as thermally conductive and mechanically robust as graphene. These unique properties make BN sheet an extremely attractive material for use in a harsh condition, especially in oxidative environments at high temperatures. In this dissertation the modulation of electronic and magnetic properties of BN sheet has been also reported by the transition metal adsorption.In Chapter1, the lattice structure, electronic structure and syntheis method of graphene are first briefly introduced.Then, the concept of SO and the relevant researches about SO splitting in graphene will be presented. Finally, some descriptions about BN sheet will be presented.In Chapter2, the many-body problem in solids is briefly discussed. Some basic concepts on density functional theory (DFT), such as Hohenberg-Kohn theorems and Kohn-Sham equation, have been given. The pseudopotential method and projector augmented wave (PAW) method significantly simplify the electronic structure calculations in DFT.In Chapter3, the SO splitting in graphene with adsorbed Au is presented. Using the first-principles method, we study the SO effects in π bands of graphene with adsorbed Au atoms. Strong Rashba-type SO splitting (up to200meV) can be obtained in graphene π bands when the Au adatom is located above a C atom or a C-C bond. When the Au atom is adsorbed above a C-C bond, Dresselhaus-type SO splitting is found to appear, maybe due to the contribution from Au5dx(?) state. When supported on substates, graphene is always subjected to some strain. Therefore, we consider the effect of strain (-10%to10%) on the adsorption stability and SO spitting. The strain can not change the type of SO splitting. A slight strain in graphene (-5%to5%) usually does not change much the SO splitting. The varation of SO spiting with strain is found to be closely related with the structural relaxation and the hybridization of graphene p=state with certain Au5d state. Our work may be helpful to the development of graphene spintronics based on its SO effect.In Chapter4, the electronic structures and SO coupling in Pb doped graphene are presented. Using the first-principles method, the stability, electronic structure and SO splitting of graphene containing single vacancy with adsorbed Pb single atom or dimer are investigated. It is found that both Pb single atom and dimer can chemically bind to defected graphene and turn the systems into semiconductors with moderate energy gaps. The SO splitting of the bands near the Fermi level is significantly enhanced by Pb doping, ascribed to the large intrinsic atomic SO interaction of the dopants. Not only the out-of-plane potential gradient but the in-plane potential gradient is found to contribute the SO splitting in doped graphene. It is possible to observe the Rashba spitting using photoelectron spectra following the injection of electrons into the system. We hope our results can intrigue more experimental and theoretical works in this direction.In Chapter5, the adsorption of the transition metal (TM) on the single-layer hexagonal boron nitride (BN sheet) is presented. Using the first-principles method, the electronic structures and magnetic properties of BN sheet modified by the TM (Fe, Co and Ni) single atoms and dimers are investigated. It is found that all of the TM atoms studied are chemically adsorbed on the BN sheet. According to the energies, Fe and Co atoms will have high mobility and isolated particles can be easily formed on the BN sheet; while Ni atoms may adopt a layer-by-layer growth mode. Fe, Co, and Ni dimers tend to lie (nearly) perpendicular to the BN sheet plane, similar to the cases of graphene. We can obtain very interesting electronic structures in the stable configurations, such as spin gapless semiconductors, half-metals and narrow-gap semiconductors. The magnetisms of TM adatoms are hardly changed on the BN sheet, very close to those of the corresponding free ones. For the adsorbed TM dimers, the distribution of local magnetic moments is similar to the cases of those on graphene, where the top-atom magnetic moment is larger than that of the lower one. These results suggest that BN nanostructures adsorbed with TM atoms have the potential application in fields such as spintronics and magnetic data storage.In Chapter6, a brief summary of the thesis is presented. |
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