Investigations Of The Transport Properties In Low-Dimensional Materials Induced By Circularly Polarized Light

The charge and spin of electrons are the core of electronics and spintronics.Their wide application,especially in ultra-large-scale integration and advanced information technology,provides great potential for the development of basic quantum physics and electronic equipment.Since the advent of graphene,the emergence of valley degrees of freedom has become the focus of continued research,resulting in a new research field called valleytronics.A essential problem in the development of spintronics and valleytronics is how to generate the spin polarization and valley polarization.In addition,other two-dimensional materials have appeared in people's field of vision,such as silicene,germanene and so on.The structural features of these materials are similar to that of graphene,but there are significant differences,so they will have strange properties.In the past few years,the academia,including many domestic universities and research institutes,has carried out a series of theoretical and experimental to study the transport properties of spin polarization and valley polarization in low-dimensional materials.Here,two polarization transport models for low-dimensional materials with induced by the circularly polarized light has been proposed,one is the normal/circularly polarized light and mass/normal graphene junction,and the other is ferromagnetic silicene superlattice induced by circularly polarized light.The main research contents are as follows:(1)Based on the existing research work,the paper propose a method to generate valley polarization without magnetic field and ferromagnetic strips.Taking the normal/circularly polarized light and mass/normal graphene junction as a model,the effects of the circularly polarized light field and mass term on the band structure and valley polarization transport of the graphene junction are studied in detail.We found that when circularly polarized light or mass term act alone,the degeneracy of valley degrees of freedom cannot be broken.At this time,only band gaps can be generated,but valley-polarized transport cannot be achieved;when both exist at the same time,the valley degeneracy is broken.At this time,not only can the valley-related band gap be generated,but also valley-polarized transport can be realized.With appropriate parameters,the valley polarization can reach 100%.In addition,when Fermi energy is given,the valley polarization direction can be reversed by adjusting the direction and size of the circularly polarized light and the quality item.Therefore,controllable full valley polarization and valley switching effects can be obtained in the normal/circularly polarized light and mass/normal graphene junction.(2)Based on the current research status of superlattices,the paper propose a method to achieve pure spin polarization and valley polarization in silicene superlattices simultaneously.Using the ferromagnetic silicene superlattice as the research model,the effects of circularly polarized light field,exchange field and external electric field on spin-polarized transport and valley-polarized transport are systematically studied.We found that the band structure and transport properties of silicene under periodic field modulation strongly depend on the spin and valley degrees of freedom.When other external fields are given,circularly polarized light can enhance spin polarization and valley polarization.When a right polarized light is applied,positive spin polarization irrespective of incident energy can be obtained.And when the intensity of the light field increases,valley polarization can also be changed from 100% to-100%.In addition,the number of crystal lattices can also a enhance the spin polarization and valley polarization.Therefore,by adjusting the circularly polarized light,the spin freedom and valley freedom can be controlled in the silicene superlattice.