Electronic And Magnetic Transport For Graphene And Graphene Nanoribbons

Graphene is found to be exhibit many unusual and intriguing physical prop-erties due to its linear dispersion relation near the Dirac point. Since successful preparation at the end of 2004, graphene has been becoming one of the hot sys-tems in condensed matter physics, material science, chemistry, information and biology technologies. In this thesis, electronic and magnetic transport proper-ties have been theoretically investigated based on the Dirac equation method, the tight-binding method, the non-equilibrium Green function technique, and the first principles method which could provide theoretical guidancd for their applications in nano-electronci devices and spintronics devices.The thesis is divided into six chapters. In the first chapter, we give a brief introduction about the discovery and fabrication techniques of graphene, the ap-plication background of graphene and its nanoribbons. In the second chapter, we give a detailed introduction about the electronic structure of graphene and its nanoribbons, and the unique physical properties in graphene.Chapter three, four and five are our works. In the third chapter, based on the Dirac equation method, we theoretcally investigate the mode-dependent and mode-independent transmission probability through a rectanglar potential barrier embedded in armchair-edge graphene nanoribbons (AGNRs) of various widths. It is demonstrated that, the transmission probability for the lowest mode (mode crossing the Fermi level) of the metallic AGNRs is always equal to unity, and the other modes transmission probability for metallic AGNRs is similar to that for semiconducting AGNRs, which depends sensitively on the widths of AGNRs, the barrier height and its range. Besides, the both metallic and semiconducting AGNRs in the vicinity of barrier own a minimum conductance associated with the maximum Fano factor which is larger than universal value 1/3 for a short and wide graphene strip at zero carrier concentration.Chapter four is our major works. Firstly, a momentum-space expression for the retard Green's function of graphene and AGNRs is derived. By using nonequilibrium Green's function techniques, we theoretically investigate the spin-dependent transport through a graphene sheet between two ferromagnetic leads with arbitrary polarization directions. A magnetic insulator is deposited on the graphene to induce an exchange splitting. It is noted that the density of states (DOS) decreases for spin-up and increases for spin-down when the polarization strength of the two leads in parallel alignment increases, while the conductance increases for spin-up but decreases for spin-down with an increase of the polar-ization. The currents for both spin-up and spin-down channels are found to be linearly proportional to the applied voltage at small bias, then grow nonlincarly as the bias increases. Interestingly, a pronounced cusp-like feature and the max-imum tunneling magnetoresistanec (TMR) value appear at zero bias without the exchange splitting, and the magnitude of TMR is dramatically suppressed with an increase of the exchange splitting. Furthermore, the current-induced spin transfer torque (STT) dependence on the relative angle between the magnetic moments of the two leads shows a sine-like behavior. The behavior of the bias dependent STT is similar to that of the current because the STT is proportional to the spin cur-rent for the two leads in parallel alignment, but it is not sensitive to the exchange splitting in graphene. In the same way, we also investigate the spin-dependent transport for the system of 7-and 8-AGNRs with a magnetic insulator deposited on the AGNRs. It is demonstrated that, a zero (nonzero) value plateau in DOS for 7-AGNR (8-AGNR) appears symmetrically with respect to the Fermi level. For larger electron incident energy, the DOS for the system of both 7-and 8-AGNRs shows an oscillation behavior with sharp peaks. Due to quantum size effect, the linear conductance and the differential conductance are demonstrated to an oscil-lation behavior with sharp peaks, the positions of the peaks for the system shift with the exchange field strength. Interestingly, a pronounced plateau for 7-and 8-AGNR systems appears at lower bias, the increase of the exchange spliitingΔsuppresses the amplitude of this structure for 7-AGNR system. However, the TMR is enhanced within bias range from-ΔtoΔfor 8-AGNR system. Similarly, the STT versus the angle shows a sine-like behavior for 7-and 8-AGNR systems, but it for 7-AGNR system is quantitatively larger than that for 8-AGNR system. In contrast to the STT of the FM/graphene/FM system, the STT with the exchange splitting displays a oscillation behavior for the 7-and 8-AGNR system.By the first principle (Atomistic Toolkit software packet) calculations, chap-ter five investigates the electronic transport property for a crossed junction of graphene nanoribbons with and without impurity doping. It is demonstrated that the transport property of the junctions very sensitively depends on their dopant positions and gcometic features, including the width and height of the branches. For example, the current is about zero for junction with N(B)-doped shoulder under small bias voltage, but increases notably with N(B)-doped stems.In chapter six, a summary of the work and a outlook of this topic are given.