The Theory Of Nanosystems Spin Polarization And Transport Properties The Theory Of Nanosystems Spin Polarization And Transport Properties

Spintronics is a multi-disciplinary field combining nano-electronics with magnet-ics; its central theme is about the spin-interaction and the active manipulation of spin in solid-state systems. Due to the possible applications in the electronic industry, spin-tronics has become an active research field. One of the research interest in spintronics is about the spin-orbit coupling. The spin-orbit coupling connects the momentum of an electron with its spin, so that one can possibly use the external electronic field to ma-nipulate the spin of electron. It may allow for purely electric manipulation of spin, i.e. magnetic material or external magnetic field is not required, and results in considerable interest of study. The discovery of the spin Hall effect induced by spin-orbit coupling further arouses the study of the spin-orbit effect and the development of spintronics. Another active related research field is molecular electronics, which may also lead to a revolutionary development of the present electronic industry. The central goal of mole-cular electronics is to construct the electronic device in the scale of molecule. Both the experimental improvement in instruments (such as STM, self-assembly and break junction) and the progress in theoretical methods (such as the non-equilibrium Green function method) lead to the great development of molecular electronics. In molecular junctions, electron transport through molecules may be controlled electrically, mag-netically, optically, mechanically, chemically or electrochemically, leading to possible applications of molecular devices. In this dissertation, we investigate the spin-orbit ef-fect, the spin polarization and the spin transport of nano-systems in the relative fields of spintronics and molecular electronics.In chapterⅠ, spintronics and molecular electronics are briefly introduced. The spin-orbit coupling and the spin Hall effect are especially presented in detail.In chapterⅡ, the theoretical methods used in the thesisit are presented. There are plane wave expanding, density function theory and non-equilibrium Green function method.In chapterⅢ, the spin polarization induced by Rashba spin-orbit coupling is in-vestigated. In equilibrium, spin-orbit coupling would not result in any spin polarization due to the time-reversal invariance of systems. However, in steady states, the spin polar-ization would have some symmetrical properties because of the symmetry in geometry and hamiltonian. We focus on the influence of evanescent waves on the spin polariza-tion in a ballistic Rashba bar. The consideration of the evanescent waves can improve the calculation results and be in favor of the symmetry of the spin polarization. The evanescent waves can also lead to obvious changes of the pattern of spin polarization not only in the regions near the interfaces but also in the middle region of a long sam-ple due to different mechanisms. The contribution of pure evanescent waves to spin polarization is found to depend sensitively on the incident energy. We also discuss its influence on spin current.In chapterⅣ, we investigate the effects of extended and localized states on spin Hall polarization in ballistic Rashba structures. The contribution from pure evanescent waves to the total spin Hall polarization is negligible while the contribution of the mixing terms from the evanescent and extended waves is comparable to that of the pure extended waves. In a narrow Rashba strip, the mixing terms are found to contribute dominantly to the total polarization. It is, however, the extended states that determine the behavior of total polarization in a wide sample. Due to the competition of the two kinds of contributions, spin flipping of the spin Hall polarization is obtained with the variation of the sample width or Rashba strength. The width of spin flipping is found to be closely related to the characteristic Rashba length of the system. No spin flipping is found in the semi-infinite system.In chapterⅤ, we give a brief introduction to graphene first, and then investigate the spin-orbit splitting of graphene on Ni(111), Au(111) and Ag(111) metals. The monolayer graphene does have Rashba splitting when it is deposited on Ni(111) sur-face. However, the Rashba splitting is just about 10 meV. The spin-orbit splitting of graphene on metals is just from the interaction between graphene and a few layer met-als near the interface. For the practical G/Au(111) structure, the spin-orbit splitting of graphene can be up to about 25 meV. We don't find an obvious relationship between the spin-orbit splitting and charge transfer. Thus, the effective electric field model is not suitable to be used to explain the Rashba splitting of graphene.In chapterⅥ, we investigate the magnetic properties of pure and nitrogen-doped carbon atomic wires using ab initio methods. The isolated and short carbon atomic wires may have very interesting magnetic properties, although the magnetism is sig-nificantly diminished when the wires are contacted by non-magnetic metal leads. In particular, for C2n atomic wires a total magnetic moment of 2μB was obtained. The magnetism is mostly localized at the terminals of the wires due to unpaired states. For nitrogen-doped wires, NC2n+1N and CnNCn, the ground state magnetic structure mim-ics a (spin density wave)SDW-like state. No magnetism is found for the wires of C2n+1 and NC2nN. The magnetic behaviors in the wires can be understood by the bonding patterns and the existence of unpaired states. Perfect spin filtering effect was obtained in the nitrogen-doped carbon wires sandwiched between Au leads by stretching the wires slightly. For the wires with or without intrinsic magnetism, different conducting mechanisms were found. The even-odd oscillatory behavior of the total conductance is seen for these atomic wires even after spin polarized transport occurs in the systems.In chapterⅦ, a brief summary of the thesis is presented.