Exploration And Regulation Of Valley Polarization In Two-dimensional Atomic Layer Materials

The valley degrees of freedom can be analogous to the spin and charge degrees of freedom to realize information encoding and storage as an information carrier.The key to manipulating the valley degrees of freedom is to achieve valley polarization.Intrinsic valley-polarized materials provide a new platform for the development of valley electronics.At present,the number of known valley-polarized materials is limited,and finding materials with larger valley-polarization can provide more options for valley electronic devices.In this work,we found that monolayer Gd X₂（X=Br,Cl）is a ferrovalley material.Their valley polarization values are 79 me V and 35 me V.Since monolayer Gd X₂（X=Br,Cl）has similar electronic structure,the valley physical properties of monolayer Gd Br₂ are studied as an example.The valley polarization of monolayer Gd Br₂ can be tuned by external strain,and the valley polarization value of monolayer Gd Br₂ can reach 107 me V when a tensile strain of 4%is applied.Furthermore,by reversing the spin direction of monolayer Gd Br₂,the valley polarization inversion of monolayer Gd Br₂ can be achieved,which is very necessary to utilize the valley degrees of freedom of electrons to encode and process information.In addition,the doping of electrons and holes has a significant effect on the valley polarization of monolayer Gd Br₂.Our results provide a new family of ferrovalley materials:Gd X₂（X=Br,Cl）.We theoretically design a new ferrovalley material-monolayer XI₂（X=Sc,Y）,and investigated the structure,electronic and magnetic properties of monolayer XI₂（X=Sc,Y）.The valley polarization values of monolayer Sc I₂ and monolayer YI₂ reach 97 me V and 108 me V,and their Curie temperatures are 138 K and 58 K,respectively.The regulation of the valley polarization of monolayer XI₂（X=Sc,Y）by strain engineering is very obvious,and the variation trend of the valley polarization of monolayer XI₂（X=Sc,Y）is opposite with the increase of strain.By applying a tensile strain of6%,the valley polarization of the monolayer YI₂ is as high as 113 me V.The valley polarization of the monolayer Sc I₂ is as high as 103 me V when the compressive strain of-6%is applied.The monolayer Sc I₂ still maintains an obvious valley polarization under the tensile strain of 6%,and has a high The Curie temperature is 182 K.In addition,the inversion of the valley polarization of the monolayer XI₂（X=Sc,Y）can be achieved by changing the orientation of the spins,so that this applies to valleytronics for the monolayer XI₂（X=Sc,Y）.The device is very favorable,and the valley polarization can be well adjusted by doping electrons and holes,and the transition from semiconductor to semi-metal is realized at the same time.Our findings demonstrate that monolayer XI₂（X=Sc,Y）is a potential material for valleytronics.In this work,we discovered a novel strain-regulated valley polarization outcome:the material’s valley polarization remains stable with increasing strain.By synthesizing magnetic materials with non-magnetic materials,the time-reversal symmetry is broken,and a valley polarization of up to 122me V is found in the single-layer Ti Te I.Through the adjustment of its valley polarization by strain,it is found that its valley polarization is insensitive to strain.The valley polarization of monolayer Ti Te I remains around 122 me V under strains from-3%to 3%.In addition,the Curie temperature of monolayer Ti Te I is as high as 269 K,which is close to room temperature.Such a high Curie temperature provides convenience for experimental research.There are opposite Berry curvatures in the K valley and K′valley.The Berry curvature can be regarded as an effective magnetic field.When an electric field is applied in the plane,the carriers in different valleys will move in opposite directions to realize the Anomalous Valley Hall Effect.Therefore,single-layer Ti Te I is an ferrovalley material with higher Curie temperature,which has great potential in the application of valley electronic devices.