Theoretical Study Of Spin And Valley Transport Based On Graphene-like Nanoribbons

Due to the influence of energy consumption and quantum size effect,the development of electronic devices with charge as information carrier can no longer continue Moore’s law.In the post-Moore era,people began to explore the use of spin and valley degrees of freedom to store and transmit information,and has made great progress.In recent years,attention has shifted to the electrons of spin and valley transport in low-dimensional nanomaterials.Silicene,germanene and stanene are all graphene-like materials(also known as silicene-like materials)with honeycomb lattice structures.Moreover,compared with graphene,they have large spin-orbit coupling and are candidates for topological insulators(TI),which can theoretically realize quantum spin Hall effect(QSH effect).The topological insulator has the characteristics of internal insulation and surface(3D material)or edge(2D material)conduction,and the electrons transported on the surface or edge are protected by topology and are not disturbed by impurities and geometric shapes.Therefore,it is very robust.In order to apply graphene-like materials to circuits,it is often necessary to cut them into nanoribbons.Therefore,it is of great significance to study the transport properties of spin and valley electrons in nanoribbons for the design of new electronic devices in the future.In this paper,we first briefly introduce the concepts of spintronics,valleytronics,low-dimensional topological insulators,and describe the research progress of silicenelike materials in transport.And then introduce the theories,models and methods used in this paper in the theoretical part,in which some of the detailed derivation process is given.Finally,the transport properties of spin and spin-valley currents in three-terminal graphene-like nanoribbon heterojunctions are studied as follows:1.based on graphene-like nanoribbons,we propose a three-terminal spin/charge current router.The function of routers is to convert spin/charge currents to output from different output ports.In other words,spin/charge currents can be controlled in the device.To understand the working mechanism of these devices,we calculated the local current distribution and energy band.The results show that the helical edge states corresponding to the QSH effect of graphene-like nanoribbons and the band gap that can be adjusted by the external field are the core of the normal operation of the devices.At the same time,we find that the Rashba spin-orbit coupling(SOC)does not destroy the filtering characteristics of the system.Furthermore,we also use the LandauerButtiker formula to obtain the current-voltage curves of the first class of routers and study the transmissivity reciprocity of such three-terminal systems.2.In the previous work,we found that the topological edge state(outer edge)plays an important role in the normal operation of the device.Therefore,we would like to explore the transport behavior of the edge state current in silicene-like materials,which will be of guiding significance for the design of topological electronics devices.We utilize a three-terminal topological heterojunction system composed of graphene-like nanoribbons to investigate these edge-state current behaviors.Different external fields are applied at the electrode and central area(conductor)to obtain the desired topological edge states.By calculating the local current distribution,we find two important characteristics of topological edge state current transport in heterojunction system.First,in the absence of scattering,the edge-state current tends to flow into the nearest channel rather than other further channels.Secondly,group velocity mismatch as a scattering engineering affects the selection of current to channel.We can manipulate topological edge state currents through these two current transport properties.As an example,we also propose a three-terminal spin filter to show how to manipulate edge state currents using these two characteristics of topological edge state current.3.We have studied the transport properties of topological outer edge state current and designed a spin filter based on topological outer edge state current.Therefore,we naturally consider studying the edge state current in the topology.According to the existing research literature,we find that the interface between different topological nanoribbons may have spin-valley(inner edge state)current with spin,valley and momentum locking properties.Therefore,based on graphene-like nanoribbons,two types of three-terminal spin-valley filters are designed to study their spin-valley current filtering effect.The first category can separate and output two different spin-valley currents simultaneously.The second type can separate and output a spin-valley current.These two types of filters can be switched by adjusting the external field.By calculating the local current distribution,the transport process of electrons is visualized,which helps us to understand its transport mechanism.The mechanism is that when the spinvalley current channel of the electrode and the central region has the same direction,spin and valley,the current can be transmitted nearly freely.In addition,we find that the device size should be as small as possible under the premise that the heterojunction system has good inner-edge states.especially,we consider the effect of “disorder” on these filters and find that the system is robust.We believe that these filters have potential applications in future spintronics and valleytronics device designs.