Study Of Spin-dependent Electric Transport In Graphene-based Micro-nano Systems

Graphene is carbon-based with SP2orbital hybridization six angle type planar film honeycomb lattice. In the past few years, graphene which has caused researchers’extensive attention is becoming the international and hot topic in various relative fields. In this paper, electronic properties of some graphene nano-structures have been simulated and computed, which could provide theoretical guidance for their applications in nano-electronic devices. The major contents and important results are given as follows:(1) Electronic spin precession in graphene-based superlattice with periodical gate and magnetic modulation. It is found that the efficient exchange field can induce a spin precession, which is different from the case of the Rashba spin-orbit interaction. The spin precession is complete isoamplitude for normal incidence. For inclined incidence, the precession disappears when the effective exchange field is set into a certain range. It is also found periodical electrostatic field can make the disappeared precession revive, but electronic transport is suppressed, which leads to some dips in the conductance spectrum.(2) Magnetoresistance and shot noise in graphene-based nanostructure with effective exchange field. It is found that the effective exchange field induces a spin-dependent Klein tunneling. The magnetoresistance becomes a number of times larger than that in the case of the Rashba spin-orbit interaction. With increasing the effective exchange field strength, the magnetoresistance and the Fano factor exhibit periodic oscillation features. In graphene superlattice, when the effective exchange field satisfies a certain condition, the Fano factor can be tuned from nearly zero to1/3by applying an appropriate periodic gate voltage.(3) The effect of Fermi velocity on the magnetoresistance of graphene-based nanostructure modulated by effective exchange field. We investigate the spin-dependent transport properties of graphene nanostructures modulated by effective exchange field and Fermi velocity. The Brewster-like angle of spin transport becomes large and the spin-precession length becomes short with a decrease of the Fermi velocity in effective exchange field region. As a consequence, the magnetoresistance are enhanced remarkably and the number of dipss of the magnetoresistance increases. In the graphene-based periodic velocity barrier with the modulations of the electrostatic potential and the effective exchange field, the maximum of the magnetoresistance dips is a number of times larger than that of zero electrostatic potential.(4) Electronic energy band and transport properties in graphene with periodically modulated magnetic vector potential and electrostatic potential. It is found that both parallel magnetic vector potential and electrostatic potential can decisively shift Dirac point in a different way, which may be an efficient way to achieve electron or hole filter. We also find that applying modulated parallel and anti-parallel magnetic vector potential to the electrons can efficiently change electronic states between pass and stop states, which can be useful in designing electron or hole switches and lead to large magnetoresistance.(5) Effect of Bloch states of armchair graphene ribbons on transport in single molecular device. We study the electronic transport properties of single molecular device with the armchair graphene-ribbons (AGNRs) electrodes by using non-equilibrium Green’s functions in combination with the density-functional theory. We find that the electronic transport properties of the molecular device are greatly influenced by its position relative to the AGNRs electrodes, which are attributed to the especial Bloch states of AGNRs. And the unidirectional electronic transport can appear as the molecule is placed diagonally between the AGNRs electrodes.