Proposal for valleytronic materials:Ferrovalley metal and valley gapless semiconductor

Valleytronic materials can provide new degrees of freedom to future electronic devices.In this work,the concepts of the ferrovalley metal(FVM)and valley gapless semiconductor(VGS)are proposed,which can be achieved in valleytronic bilayer systems by electric field engineering.In valleytronic bilayer systems,the interaction between out-of-plane ferroelectricity and A-type antiferromagnetism can induce layer-polarized anomalous valley Hall(LP-AVH)effect.The K and−K valleys of FVM are both metallic,and electron and hole carriers simultaneously exist.In the extreme case,the FVM can become VGS by analogizing spin gapless semiconductor(SGS).Moreover,it is proposed that the valley splitting enhancement and valley polarization reversal can be achieved by electric field engineering in valleytronic bilayer systems.Taking the bilayer RuBr_(2)as an example,our proposal is confirmed by the first-principle calculations.The FVM and VGS can be achieved in bilayer RuBr_(2)by applying electric field.With appropriate electric field range,increasing electric field can enhance valley splitting,and the valley polarization can be reversed by flipping electric field direction.To effectively tune valley properties by electric field in bilayer systems,the parent monolayer should possess out-of-plane magnetization,and have large valley splitting.Our results shed light on the possible role of electric field in tuning valleytronic bilayer systems,and provide a way to design the ferrovalley-related material by electric field.

Correlation-driven threefold topological phase transition in monolayer OsBr_(2)

Spin–orbit coupling(SOC)combined with electronic correlation can induce topological phase transition,producing novel electronic states.Here,we investigate the impact of SOC combined with correlation effects on physical properties of monolayer OsBr_(2),based on first-principles calculations with generalized gradient approximation plus U(GGA+U)approach.With intrinsic out-of-plane magnetic anisotropy,OsBr_(2)undergoes threefold topological phase transition with increasing U,and valleypolarized quantum anomalous Hall insulator(VQAHI)to half-valley-metal dxy dx2−y2 dz2¯6m2(HVM)to ferrovalley insulator(FVI)to HVM to VQAHI to HVM to FVI transitions can be induced.These topological phase transitions are connected with sign-reversible Berry curvature and band inversion between/and orbitals.Due to symmetry,piezoelectric polarization of OsBr_(2)is confined along the in-plane armchair direction,and only one d11 is independent.For a given material,the correlation strength should be fixed,and OsBr_(2)may be a piezoelectric VQAHI(PVQAHI),piezoelectric HVM(PHVM)or piezoelectric FVI(PFVI).The valley polarization can be flipped by reversing the magnetization of Os atoms,and the ferrovalley(FV)and nontrivial topological properties will be suppressed by manipulating out-of-plane magnetization to in-plane one.In considered reasonable U range,the estimated Curie temperatures all are higher than room temperature.Our findings provide a comprehensive understanding on possible electronic states of OsBr_(2),and confirm that strong SOC combined with electronic correlation can induce multiple quantum phase transition.