Ac Transport In The Multi-terminal Graphene Nanodevices

With the rapid development of modern science and technology, and the increasing integration of computer chips, the size of electronic components are designed to be smaller and smaller. Thus made the semiconductor technology face a huge challenge. On the other hand, as the size of electronic components becomes to a certain small dimension, some quantum phenomena appear, which can not be explained by the classical theory. So the introduction of new high-performance materials and the electronic transport properties investigation be-come a key point. In2004, when the individual graphene plane was isolated by using adhesive tape in the laboratory, the research of the properties of electronic transport in gaphene has became a hot topic in the area of mesoscopic quantum transport. In the thesis, a brief review of the mesoscopic physics background is represented, and we introduce the basic theory and methods in mesoscopic system, especially of the Buttiker’s ac transport theory. We mainly study the ac transport properties of multi-terminal graphene nanodevices.The main content of the thesis is organized as follows:In the second chapter, several fundamental theories are introduced, including scattering theory, nonequilibrium Green function theory and Landauer-Biittiker formula.In the third chapter, the Buttiker’s ac transport theory is discussed. Mean-while, the relation between local partial density of states (LPDOS) and Green function is derived.In the forth chapter, we describe the electronic properties of graphene and graphene nanoribbons (zigzag and armchair) in details. The wave function of the electron, named Dirac fermions in graphene, around Dirac point in the first Brillouin zone satisfies a massless2D Dirac equation. The dispersion relations of zigzag and armchair graphene nanoribbons are discussed. There exist edge state for zigzag graphene nanoribbon, which is metallic. Armchair graphene nanorib-bon is metallic or semi-conductive according to the width of ribbon. Meanwhile, the dc conductance of graphene nanoribbon is computed by using Green function and Landauer-Biittiker formula.In the fifth chapter, we study the topological asymmetry induced electronic transport in three terminal graphene nanoribbon structure. We find that the topological configurations play an important role in graphene nanoribbon based devices. We show that the topological asymmetries can form capacitive and inductive junctions in this three terminal structure. The transport properties are sensitive to the geometric features of the branches of the junctions and the coupling positions. We also demonstrated the asymmetric imaginary part of admittance as functions of the size of T-GNR device. It is shown that as the zigzag and armchair ribbon widths increase simultaneously, the asymmetric ac transport becomes not only robust but also more obvious.In the sixth chapters, we examined the ac transport attribute of the multi-terminal structures in the absence and presence of magnetic field. We found that the ac response depends on the structural configurations and that the admittance varies with the features of the attached nanoribbons. In the vicinity of Dirac point the dc conductance manifests a dip or peak and the imaginary part (emittance) vanishes or not, depending on whether the attached ribbon is semiconductive or metallic. In the presence of magnetic field, the emittance becomes asymmetric and obeys the reciprocal relation, reflecting the dynamic behaviors of electron and hole.The summary is given in the last chapter.