Electronic Structure And Magnetic Regulation Of Two-dimensional 2H-VS₂ And Janus V₂X₃Y₃（X,Y=Br,Cl,I,and X≠Y） Ferromagnetic Monolayers

Two-dimensional materials have attracted considerable attention due to their unique physical properties and promising applications in spintronics and valleytronics devices.However,the Curie temperature of some two-dimensional materials with intrinsic ferromagnetism successfully stripped in the experiment is far lower than room temperature,which limits their application in spintronic devices.Valleytronics are expected to become the carrier of encoding and manipulating information in the next generation of information technology.Due to the difficulty in maintaining magnetic order in most materials,they cannot be applied in information storage and many spintronic devices.Therefore,the intrinsic ferrovalley materials with magnetic,high Curie temperature and large valley polarization are the urgent need to find candidate materials for the next generation of valleytronics.At the same time,as an important derivative of two-dimensional materials,Janus transition metal sulfide with asymmetric surface characteristics has become a research hotspot in the field of material science,electronics and other fields.The electronic structure and magnetic regulation of 2H-VS₂ and Janus V₂X₃Y₃（X,Y=Br,Cl,I,and X≠Y）monolayers are systematically investigated by first-principles calculations in this paper.1.The electronic structure,valley splitting and magnetic properties of 2H-VS₂monolayer are investigated in detail by first-principles calculations.The pristine 2H-VS₂ monolayer exhibit obvious room temperature ferromagnetic ground state with Curie temperature of 278 K,in-plane magnetic anisotropy and large spontaneous valley splitting of 75.1 me V.As the strain increases from-10%to 10%,the valley splitting and magnetic moment of V atom monotonously increase,revealing that the magnitude of magnetic moment is an important factor on the valley splitting.In addition,MAE of2H-VS₂ monolayer firstly increases and gradually decreases with the increasing strain from-10%to 10%.The 3d orbital-resolved MAE based on second-order perturbation theory proves that the in-plane magnetic anisotropy mainly comes from the matrix element differences between the d_xy and d_x-y²² orbitals of V atoms.In addition,2H-VS₂monolayer retains a large valley-contrasting Berry curvature,which indicates that the transverse velocity of carriers is opposite to the effect of in-plane longitudinal electric field.The non-zero anomalous Hall conductance is further demonstrated,and a valley and spin filter based on 2H-VS₂ is proposed.2.The effect of strain on the electronic structure and magnetic properties of Janus V₂X₃Y₃（X,Y=Br,Cl,I）monolayers are systematically studied through first principles calculation.The Janus V₂Br₃Cl₃,V₂I₃Cl₃,and V₂Br₃I₃ monolayers are half-metallicity and intrinsic ferromagnetism,with Curie temperatures of 242,289 and 261K,respectively.Janus V₂X₃Y₃ monolayers have Dirac points caused by V-d electrons around the Fermi level,and biaxial strains can effectively regulate the gap of Dirac points induced by SOC.The Janus V₂Br₃Cl₃ monolayer shows a perpendicular magnetic anisotropy,While the Janus V₂Br₃I₃,and V₂I₃Cl₃monolayers have an in-plane magnetic anisotropy.The MAE transition from perpendicular to in-plane magnetic anisotropy occurs atε=4%in the V₂Br₃Cl₃ monolayer.The strain-induced magnetic anisotropy transition between perpendicular and in-plane magnetic anisotropy in the Janus V₂Br₃Cl₃ monolayers can be mainly attributed to the changes of V d_xy and d_x-y²²orbits,Br p_y and p_z orbits and Cl p_y and p_z orbits hybridizations.These results greatly broaden the diversity of two-dimensional Janus monolayers,which are benefit to the developing high-performance electronic and multifunctional spintronic devices.