The Property Of Electron Spin Transport In Graphene Nanostructure

In the past several years, a honeycomb geometry such as graphene and silicene have become hot materials for the research in condensed matter physics. In these materials, the energy dispersion of the electrons near Fermi level is linear, and the electrons are described by the relativistic Dirac equation. The graphene nanoribbons (GNRs) are very promising for the next-generation nano-electrical and spintronics devices because of their quasi-one dimensional geometrical structures and unique electrical and magnetic proper-ties. So, it is very important to study their electronic structures and transport properties for designing next-generation spintronics devices.In this thesis, combined with the current theory and experiment, we design a transverse-biased magnetic zigzag-edge graphene nanoribbon and a local-gated magnetic graphene transistor, we study the electron spin transport in this two various graphene nanoribbon-based structure. We also propose a silicene nanoribbon with a local exchange field on the bottom edge, considering the effect of the electric field, discuss the electron transport in this nanostructure. In these different structures, electron spin transport ex-hibits many interesting transport properties different from that in the structures made up of the traditional materials. In detail, the dissertation is organized as follows:In chapter one, we give a brief introduction to the discovery of graphene, the phys-ical and electron transport properties. Considering the influence of the boundary, we introduce the characters of graphene nanoribbon. Next, describe the preparation and the wide application of graphene and its nanoribbon. In chapter two, the close-path time-ordered Green’s function theory and Landauer-Buttiker formalism are introduced. Then two recursion algorithms, one for calculation of the electrical leads’ self-energy and the other for computing the Green’s function in the conductor between two leads are reviewed, we have used this method to study the transport properties in our nanodevices.In chapter three, we introduce the magnetism in zigzag graphene nanoribbons firstly. Next, in the framework of the Landauer-Buttiker formalism, we investigate the coherent spin transport through a transverse-biased magnetic zigzag-edge graphene nanoribbon, with a temperature difference applied between the source and drain. It is shown that a crit-ical source temperature is needed to generate a spin-polarized current due to the presence of a forbidden transport gap. The magnitude of the obtained spin polarization exceeds90%in a wide range of the source temperature, and its polarization direction could be changed by reversing the transverse electric field. We also find that, at fixed temperature difference, the spin-polarized current undergoes a transition from increasing to decreasing as the source temperature rises, which attributes to the competition between the excited energy of electrons and the relative temperature difference. Moreover, by modulating the transverse electric field, the source temperature and the width of the ribbon, we can control the device to work well for generating the highly spin-polarized current.In chapter four, at first, we introduce the parity in zigzag graphene nanoribbons. In the following, based on the mean-field Hubbard model, we study the thermally-driven spin-polarized transport through a local-gated magnetic zigzag graphene nanoribbon by using the nonequilibrium Green’s function method. The spin currents are tuned by the source temperature, the temperature bias, and the gate voltage as well. We find this tran-sistor exhibits a transition from the bipolar to unipolar spin transport under associated modulations of thermal bias and gate voltage. It is argued that the result originates from the band selective rule related to parity conservation of wave functions in quantum tunnel-ing. We also find the thermal magnetoresistance of the ribbon between the ferromagnetic excited state and antiferromagnetic ground state could reach up to105%under a small local gate voltage. This proposed device provides possibility for bettering control of the spin freedom of electrons in graphene materials.In chapter five, we firstly introduce the relative of graphene-silicene, mainly focus on their lattice structure, secondly, by applying nonequilibrium Green’s function technique, we investigate the thermally induced spin transport through a silicene nanoribbon with a local exchange field on the bottom edge. It is shown that the common effect of the local exchange field and the electric field perpendicular to silicene sheet can lead to asymmetry in the band structure and the transmission function, a temperature difference between the source and drain can induce the pure spin current, which can be tuned by altering the electric field and the strength of exchange field. The results indicate the potential application in future silicene-based spintronics devices.In the last chapter, we make a summary and give some outlook for the future inves-tigation.