A Study On Quantum Transport Properties Of Bilayer Graphene And Magnetic Topological Insulator Thin Films

Two-dimensional layered materials represented by graphene have attracted considerable attention in condensed matter physics due to their novel material properties and excellent electromagnetic,optical and thermodynamic properties.Since the size in one dimension can be reduced to atomic layer thickness,two-dimensional layered materials show novel electronic transport properties in both in-plane and out-of-plane directions.For the studies of in-plane transport,researchers hope to realize electronic transport with low dissipation through the edge states of quantum anomalous Hall effect.Although the edge states protected by topology are robust,varieties of strong disorders in practical materials still hinder the design of electronic devices based on the edge states,which will cause Anderson localization.In this process,the spin flip of electrons caused by magnetic impurities makes the Anderson phase transition more complex.Especially in the large-Chen-number quantum anomalous Hall effect system,there are multiple edge states which make the Anderson phase transition even more complicated and changeable,which also hinders the scholars’researches on this topic to some extent.In the out-of-plane direction,graphene can provide more degrees of freedom which can be regulated,such as interlayer coupling distance,twisted angle between different layers,and interlayer voltage difference.Previous studies usually focus on the electron transmission of monolayer/bilayer graphene junction in the same plane,and lack the researches on the mechanisms of charge transfer and vertical transport between different layers,while the experiment results about vertical transports are on the increase.Therefore,it is of great significance to explore the functions of the factors like interlayer coupling and carrier concentration in the vertical transport of graphene.In view of the above problems in the transport of two-dimensional layered materials,our research mainly adopts the tight-binding model and Landauer-Büttiker formula to explore the transport properties of the system.This dissertation is divided into the following six parts:The first chapter mainly introduces the formation principle and classical model of quantum anomalous Hall effect,and two main systems that can realize large-Chernnumber insulators.In the aspect of in-plane transport,we sort out the current researches of Anderson localization caused by disorders in topological materials;at the same time,we also briefly summarize the researches on vertical transport in graphene system.In Chapter 2,we provide a brief introduction to the main research methods of this dissertation,namely tight-binding model and non-equilibrium Green’s function,and the ways to calculate the Berry curvature and Chen number in different parameter spaces.In Chapter 3 and Chapter 4,taking two large-Chern-number systems of bilayer graphene and magnetic topological insulator as examples,we give an insight into the Anderson localization of the whole system when magnetic impurities that can affect electronic spin are doped.In the bilayer graphene with Chern number 4,magnetic impurities promote the system to change from quantum anomalous Hall insulation phase into metallic phase,and then into Anderson insulating phase.In the magnetic topological insulator,with the increase of impurity strength,the integer Hall conductance plateau is destroyed one by one from high to low.The quantum anomalous Hall insulator with Chern number-N will first enter the metallic phase,and then change into the quantum anomalous Hall insulating phase with Chern number-(N-1),and finally turn into Anderson insulating phase after N-1 cycles.The above phenomena can be explained via the Berry curvature exchange carried by valence and conduction bands.At the end of this chapter,we provide a phenomenological model of topological charge to vividly illustrate the localization process.This research solves the Anderson localization problems of magnetic impurities which affect electronic spin in large-Chen-number systems.In Chapter 5,we use graphene to construct four-terminal devices to study the vertical transport.In the AB-stacked bilayer graphene with the armchair boundary,around the charge neutral point,the interference of the electronic eigenwave functions in the central region causes the periodic oscillation of conductivity,and the variation period is only influenced by the interlayer interaction,not by the size of the transverse direction of the central region.Here,the incident current is equally distributed to the other terminals by the electronic device.However,when Fermi energy is at the high energy level,the scattering between multiple channels makes the conductance of different terminals complementary.This research illustrates the functions of Fermi energy,interlayer coupling and system size in the vertical transport of graphene system.which provides theoretical guidance for the design of electronic devices that can be adapted to controlling the distribution and switching of current.At the end of this dissertation,we make a brief summary of this research,and look to the future development of the above two topics in Chapter 6.