Electrical Regulation Of The Polarization Properties In Bilayer Graphene

Since the successful separation of monolayer graphene in 2004,various unique properties of graphene have been studied.Recent studies on the valley-dependent transport properties of monolayer graphene and bilayer graphene have received widespread attention.In the first Brillouin zone of graphene,there are two valleys that are not equivalent,namely the K and 'K valleys.While studying the properties of monolayer graphene,people also have great interest in the properties of bilayer graphene.Bilayer graphene is a two-dimensional carbon material produced by stacking two layers of graphene in different methods(including AB stacking,AA stacking,etc.).Similar to monolayer graphene,the bilayer graphene also has two unequal K and 'K valleys.This has great value for the study of valley polarization properties.The main research contents of this thesis are as follows:Firstly,the structure,preparation methods,physical properties and applications of monolayer and bilayer graphene are briefly introduced respectively.The concept of valley and valley-related transport are also introduced.Then,the research methods of this thesis are introduced,including Green's function,tight-binding approximation and recursive Green's function.In this thesis,the transport properties of zigzag bilayer graphene are studied by the tight-binding approximation and Green's function.Our models are consisting of a scattering region of zigzag bilayer graphene connected to two semi-infinite bilayer graphene wires and the zigzag monolayer-bilayer-monolayer graphene by overlapping two semi-infinite monolayer graphene nanoribbons.Firstly,for the first model,the effect of interlayer staggered potential on the valley polarization properties of bilayer graphene is studied by applying the opposite potential to the upper and lower lattice points of bilayer graphene.The effects of the valley polarization are studied by changing the interlayer staggered potential,the length of the scattering region and the difference in the direction of the interlayer staggered potential.The electrical regulation of the valley polarization properties is realized.The research in this thesis shows that the valley polarization can be increased by increasing these three parameters.The disorder of the scattering region increases with the increase of these three parameters.The disorder increases the intervalley scattering and then increases the valley polarizaation.This shows that by controlling the interlayer staggered potential and the length of the scattering region,the electrical regulation of the valley polarization properties of bilayer graphene can be achieved.Secondly,for the second structure,the valley polarizaation is close to 100%,which is a perfect model for generating valley polarization and can be used as an efficient valley filter.The effects of the transverse and longitudinal dimensions of the bilayer graphene on the valley-polarized properties are also studied.The results show that the chang of the transverse dimension only causes the subband to move,so that the position of peaks of the valley polarization is shifted along the Fermi energy without significant effect on the strength of the valley polarization.The change in the longitudinal dimension results in a significant qualitative change in valley polarization.Both structures provide new ideas for studying the transport properties of bilayer graphene.