Tunable Spin And Energy Valley Of Two-dimensional Janus-VSSe Monolayer And Heterostructure

The two-dimensional transition metal dichalcogenides（TMDs） have a wide range of applications in the fields of opto-electronics,nano-electronics,sensing,catalysis,energy and so on for its unique physical and chemical properties.The spatial inversion of the monolayer structure is broken,which leads to the opposite berry curvature of two unequal energy valleys in the Brillouin region.However,due to the lack of magnetism,it cannot induce spin polarization to produce degenerate valley,which limits its application in spin-valley electronic devices.Although a large number of studies have reported that magnetic doping,adsorption,defect can inject spin,there are still many problems.At present,a kind of intrinsic room temperature ferrovalley material VX₂（X=S,Se and Te） has been experimentally and theoretically proved to have spontaneous ferromagnetism and valley polarization,which provides a good platform for the realization of anomalous valley/spin Hall effect.Therefore,in this paper,Janus 2H-VSSe,as a new kind of ferrovalley material,is systematically investigated by first principle calculation.1.Based on the first-principle of DFT and Mede A-VASP software package,the effects of in-plane strain,interlayer coupling and vertical electric field on the valley polarization,magnetocrystalline anisotropy and dipole moment of Janus 2H-VSSe monolayer with stable room-temperature ferromagnetism are systematically investigated.The results show that the Janus 2H-VSSe monolayer with intrinsic spin-splitting and out-of-plane dipole moment can spontaneously achieve a large valley splitting of 94.8 meV due to the V 3d¹ electronic configuration and breaking of mirror symmetry,respectively.Combined with the two band k·p model,the strong spin-orbital coupling effect and magnetic exchange interaction of localized V 3d electrons are responsible for the observed valley polarization in the Janus 2H-VSSe monolayer.Under various bi-axis strains and layer thickness,the Janus 2H-VSSe monolayer still maintains the intrinsic in-plane ferromagnetism,and exhibits tunable valley polarization and magnetocrystalline anisotropy,which are associated with the in-plane d（x²-y²）/d_xy orbitals of V atoms near the Fermi level.Importantly,the stacking patterns in the layered Janus 2H-VSSe can influence the spatial inversion symmetry and further alter the valley polarization.Applied external electric-field can effectively control intrinsic built-in electric-field by charge redistribution in the vertical direction,which can remarkably change the dipole moment and slightly adjust the valley polarization for the Janus 2H-VSSe monolayer.2.The first-principle calculation is used to predict the magnetic proximity effect of CrI₃ layer on the magnetic properties and valley of 2H-VSSe monolayer.According to the k·p perturbation theory,the total spin splitting comes from the spin-orbit coupling,the intrinsic magnetic exchange field of 2H-VSSe monolayer and the Zeeman effect induced by the magnetic substrate（CrI₃）,which leads to the large valley polarization.In addition,the valley can be modulated by the type of interface atoms,the position of atom stacking,the distance between the interfaces,and even bi-axial strain.The results show that the heterostructures with Se-I atom interface has a slightly larger electrostatic potential difference than that with S-I atom interface,and presents a relatively large valley polarization;the indirect coupling strength of V-Cr atomic orbital is related to the V-Cr atomic distance and whether the V-Cr atom overlaps in vertical space direction;the layer spacing affects the charge transfer,the electrostatic potential difference of the interface,and the position of the energy band,and further change the valley polarization.The in-plane strain leads to obvious band displacement,especially the tensile strain will reduce the valley gap significantly.Finally,the sandwich structures are constructed to further confirm the influence of stacking pattern and layer spacing on the magnetic properties and valley polarization of 2H-VSSe monolayer.