Study On Electron Transport Through Graphene-based Systems

As a two-dimensional material,graphene has many unusual properties,such as high carrier mobility,quantum hall effect,fractional quantum hall effect,etc.These special properties stem from the linear dispersion relation of graphene around the zero energy points called Dirac points.In order to modify the properties of graphene,it is required to change the dispersion relation of the quasiparticles in graphene.It has been known that the ac field has become a powerful tool extensively utilized for the exploration and modification of graphene properties.In this thesis,we study the magnetotransport through ac driven ferromagnetic graphene devices.The main results are summarized as follows:1.Using Floquet theorem and non-equilibrium green's function method,we present a theoretical study on the spin-dependent transport through the ferromagnetic graphene nanoribbons in the presence of a magnetic and an in-plane ac electric field.We find that for the two-terminal ferromagnetic graphene device,the width of the even-number conductance plateaus decrease,and the new conductance plateaus appear at the odd-number positions.When the ac field strength becomes sufficiently strong,the even-number conductance plateaus at the high energy are quenched for the parallel configurations of the electrodes' magnetizations.In contrast,for the antiparallel configuration of magnetizations of both electrodes,the odd-plateaus of the conductance shrink,while the new plateaus developed at the even-number positions when the ac field is applied.The magnetic resistance exhibits a series of the rectangular oscillation structure close to the band edge,whereas experiences an alternative transition between the sharp peak and dip near the zero energy with increasing the ac field strength.Additionally,we calculated the longitudinal and Hall resistances numerically in the six-terminal ferromagnetic graphene device,and show the longitudinal resistance remains robust against the ac field,whereas the Hall resistance plateaus shrink as the ac field strength increases,simultaneously accompanied with the addition of the new quantized plateaus due to the lifting of the valley degeneracy2.Based on the first principle method,we study electron transport through zigzag edged graphene constriction sandwiched between the two ferromagnetic electrodes.The transmission coefficient is calculated numerically,which show that when the constriction is partly saturated by oxygen atoms,near the Fermi energy,the spin down electrons can travel from the left to right electrodes via the central constriction for the parallel configuration of two electrodes,whereas the spin up electrons are strongly suppressed.As the bias voltage is applied,the current as a function of the bias exhibits a salient peak,which indicated the negative differential resistance effect induced by the constriction.We also calculated the transmission coefficient as well as the current in the graphene constriction with the zigzag edge completely saturated by oxygen atoms.