Study On Electronic Transport Through Ac Driven Graphene Devices

Since the discovery of graphene,due to its special honeycomb lattice structure,many novel properties,such as half-integer quantum Hall effect,high carries mobility,etc,are uncovered rapidly.Thus,graphene is viewed as a promising candidate for a new revolution in the field of the traditional electronics.The special properties of graphene stem from a linear dispersion relation around the zero energy points,called Dirac points,which are very robust against disorder because they are protected by time-reversal and inversion symmetry.Thus in order to modify the properties of graphene,it's very necessary to find some useful techniques to precisely manipulate the behaviors near Dirac points.Recently,a number of methods,including strain,special cutting diraction and chemical modification(doping or absorb),have been reported.Among them,an external radiation field is a very convenient technique for modifying the properties of graphene.In this thesis,we investigated the transport properties in graphene under an external driven field.The main results are summarized as follows:1.Using the Floquet theorem and non-equilibrium green's function methods,we study the electronic transport through ac driven graphene p-n junctions under a perpendicular magnetic field.It is found that subject to the transversely or longitudinally polarized ac field,in the p-n region,the conductance versus the on-site energy of the right electrode exhibits a characteristic structure with a zero value plateau and the followed oscillation peaks,whose widths are greatly suppressed by the ac field.In the n-n region,the conductance plateaus at G=(n +1/2)(4e2//h)(n is an integer)shrink for the transversely polarized ac field,whereas accompanied with the addition of the new quantized plateaus at G = n(4e2/h)for the longitudinally polarized ac field.The combined influence of the ac field with the disorder triggers a change in the mixing of the hole and electron states at the p-n interface,which leads to a destruction of the plateaus structure in the conductance versus the disorder strength with the emergence of new ones.2.We study the spin dependent transport in ac driven graphene spin valve devices in the presence of the magnetic field.With the increase of the amplitude of ac fields,the conductance plateaus shift,and new plateaus appeared for the very strong ac field.The reason is that saddle points were pulled down and two Dirac points become much close to each other.At sufficiently large the ac field strength,two Dirac points can merge into single one,in this case,each of the plateaus in the conductance will be split into two.We also calculate the band structure of graphene nanoribbon under the ac field,which give a further understanding for the observed transport feature.