Study On Topological And Transport Properties Of Two-dimensional Graphene-like Materials

Topological insulator is a distinctive matter with anti-interference characteristics,and its main characteristics are an insulating gap and topologically protected edges.Due to the special properties of topological insulators,the research on the topological properties of two-dimensional graphene-like materials is becoming more and more extensive.Compared with the research on the topological and corresponding transport properties of the conventional two-dimensional graphene-like material,the related research on the topological and corresponding properties under the Coulomb repulsion effect(Hubbard model)is still in the preliminary stage.In graphene-like systems,different topologies correspond to different topological edge states.For example,the quantum spin Hall effect corresponds to a pair of helical edge states with the effect of the spin filtering,and it has good robustness.Based on these properties,the helical edge state is naturally applied for spintronics.The study found that topological edge states not only retain their own characteristics but also have long-term spin-coherent transport properties.In addition,the transport properties of the topological edge states have high-efficiency and low-dissipation transport characteristics,which are very conducive to the storage and manipulation of information.Recently,the research on the transport properties of topological phases is in full swing,but the related topological transport mechanism is rarely studied.In addition,the valley transport properties of two-dimensional graphene-like materials are also particularly important,which directly gave birth to the field of the valley electronics.Since the concept of the valley filtering effect was proposed,the corresponding valley transport phenomena and applications have emerged in an endless stream,but there is still a huge space for research.In the graphene-like system,this paper mainly studies the transport mechanism of topological edge states the transport properties of the valleys,and the topology and related transport properties under the Coulomb repulsion effect.The main studies of this paper will be discussed below.In the graphene-like system under the Coulomb repulsion effect,since the distribution of spin-up and spin-down electron densities in the zigzag boundary system is different from that in the two-dimensional system,the topological bulk-edge correspondence of the system becomes inaccurate.On this basis,we propose a weakly coupled quasi-one-dimensional calculation model(equivalent two-dimensional model),and use the Berry curvature and Chen number after self-consistent iterative calculation to describe the corresponding topological properties,and give out an accurate topological bulk-edge correspondence.In a system with the Hubbard model,the zigzag graphene nanoribbons are usually a kind of magnetic insulator.However,under the framework of the weakly coupled calculation model,the quantum spin Hall effect and the spin-polarized quantum anomalous Hall effect can be found through the modulation of the external field.In addition,under the action of the magnetic exchange field,the system will also be accompanied by the changes in magnetism.Using the self-consistent iterative Green's function calculation method,we have also further studied the transport properties in the process of topological phase transition and magnetic change.The study found that the topological phase transition and the change of magnetism correspond to the change of related transport properties,and it is a direct correspondence.At present,the research of topological insulators in transport is relatively scarce,and this paper further studies the transport mechanism of the topological edge states.Two-dimensional graphene-like materials have very rich topological properties,corresponding to rich topological edge states.In order to study the related topological transport mechanism,we use two-dimensional graphene-like materials to construct a topological insulator heterojunction,where each part of the heterojunction is in the different or same topological phase.These different topological phases are modulated by the external field.By calculating the local current in the heterojunction of topological insulators,we found that the transport phenomenon of topological edge states is jointly dominated by four related mechanisms,including the famous Fano and Fabry-Perot resonance effects.Using these transport mechanisms,we have further designed many different types of spin filters and switchers.In order to verify the stability of these devices,some real effects have also been taken into consideration,such as dephasing and random disorder.The calculation results show that these devices have good robustness.Finally,we studied another transport property(valley transport)of the graphene-like system.Based on the two-dimensional graphene P/N heterojunction model,we proposed the concepts of unipolar and bipolar valley filtering effects.And under the framework of this concept,four types of bipolar and one type of unipolar valley filtering effects were found using the two-terminal graphene P/N model system,such as the valley-mixed bipolar spin filter effect,valley-mixed bipolar filter effect,valley-locked bipolar spin filter effect and valley-locked bipolar filter effect.Each of the different types of bipolar and unipolar valley filtering effects corresponds to a variety of the spin-valley current forms.In the complex valley filtering effect,the switch between these bipolar and unipolar valley filters is further realized through the modulation of the external field.In addition,for the temperature gradient interference of the two-terminal electrodes,the study found that both the bipolar and unipolar valley filtering effects have good robustness.