First-principles Study Of Photo Pure Spin Currents Device In Two-dimensional Materials

Spintronics,a research that has received much international attention in recent years,focuses on the introduction of spin as a degree of freedom,making the charge and spin of the electron a carrier of energy and information transfer.Achieving pure spin current is one of the main goals of this research,a phenomenon characterized by the presence of only spin current and no charge current in the device.Precisely because there is no Joule heat generated by the charge current,the process is low dissipation.Because of its low energy consumption,it has an important role to play in the development of new spintronic devices and in alleviating the current growing energy problem.The study of pure spin current based on the photogalvanic effect(PGE)in 2D materials has attracted widespread interest as 2D materials exhibit a variety of exciting properties.Using density general function theory combined with a nonequilibrium Green’s function approach,this paper investigates the spin transport properties of two-dimensional devices constructed from graphene and silicone nanoribbons,specifically by carrying out the following work.(1)We present nano shrinkage engineering of armchair graphene nanoribbons(AGNRs)to construct optoelectronic nanodevices aimed at generating pure spin currents using the photogalvanic effect(PGE).We designed two devices with different symmetries,and the results show that both structures can generate pure spin currents without accompanying charge current,but in the Cs devices,pure spin currents can be generated only at specific polarization angles.While in the devices with spatial inversion symmetry,pure spin currents can be generated at any polarization angle.These results suggest that strong pure spin currents can be generated in non-magnetic systems based on PGE by local magnetic engineering.(2)We have investigated the photovoltaic behavior of magnetic graphene interconnects constructed from zigzag graphene nanoribbons(ZGNRs).We designed two interconnected structures by connecting two 6-ZGNRs through a carbon hexagon(C6)or a carbon tetragon(C4),respectively.Both structures have zero charge current under light irradiation due to the presence of spatial inversion symmetry.However,there is pure spin current in the C6 structure,while in the C4 structure,there is almost no pure spin current because the currents in both spin directions are small.This provides a reliable method for generating pure spin currents using PGEs and opens up new possibilities for the application of graphene in spintronics.(3)Based on these two works,we have further investigated the photoelectric spin transport properties of graphene nanoribbon devices connected by two connectors.This work designed two devices with spatial inversion symmetry,both consisting of three ZGNRs in which two carbon tetragons(“2-C4”)or two carbon hexagons(“2-C6”)are introduced as connectors.The results show that both structures produce a stable pure spin current with no charge current.In addition,the “2-C6” is modulated by a magnetic field,which modulates the edge states of the three ZGNRs from left to right as AFM-AFM-AFM or FM-AFM-FM,resulting in an excellent pure spin current independent of photon energy and polarization angle.However,when the edge state of “2-C6” is FM-FM-FM ordered,the photocurrent is hindered.This work provides a novel solution to obtain pure spin current in GNRs using PGE and magnetic field modulation and is an important reference for the design of spintronic devices with composite nanostructures.(4)Previously,optoelectronic devices that obtained stable pure spin current had a central region of the device that satisfied centrosymmetry,while the centre of the illuminated region coincided with the centre of the device.It is not yet known whether the optoelectronic properties change when the latter condition is not satisfied.We first studied the case of a sawtooth graphene nanoribbon junction where the illuminated region is not centrosymmetric and is located at different positions(irradiated area effect)and found that pure spin current is not obtained until the entire central region is irradiated.In addition,we have studied the superposition effect of the device.If we divide the central area into two parts that are not centrosymmetric,the sum of the photocurrents produced by irradiating each of these two parts is pure spin current(superposition effect).We find that the sum of the photocurrents produced by irradiating the two parts separately produces pure spin current when and only when the two parts have spatial inversion symmetry with respect to the centre of the device.Furthermore,the pure spin current thus obtained is smaller than that obtained by irradiating the entire central region,which means that the rule “1+2=3” does not hold and that the coupling effect between the two parts is important in photocurrent generation.(5)The response of silicene nanoribbon devices to external magnetic fields and their optoelectronic transport properties were investigated.It was found that the devices can switch between the conducting and insulating states by adjusting the magnetic field.We also considered the photocurrents obtained for devices with different magnetic fields under the PGE,and only the AP device obtained a robust pure spin current.This mechanism of conductance switching by tuning the orbital symmetry and the generation of pure spin current by different magnetic fields could be considered in the future design of silicon-based electronic devices.