Research Of Regulation And Control On Magnetic And Energy Valley Properties Of Two-dimensional Materials

In partly,charge,spin and valley degrees of freedom can determine the basic properties of the materials.Two-dimensional（2D）magnetic and valleytronic materials can still retain the physical and chemical properties of light,electricity,magnetism,force,and other aspects under the atomic thickness.This advantage provides a new opportunity for realizing new spintronic devices with mechanical flexibility,fast response rate and high switching ratio.Based on this,this paper aims to study the regulation of magnetism and valley degrees of freedom.We have developed the following two research work:1.Strain-tunable phase transition and doping-induced magnetism in iodinene.Inspired by the experimental report that the number of layers of iodinene materials changed can induce structural phase transition.Based on the data reported in the experiment,we have built monolayer and bilayer iodinene structural models.Based on the density functional theory,we systematically studied the electronic properties,doping induced magnetism and strain effect of bilayer and monolayer iodinene.Through molecular dynamics simulation,it is found that monolayer and bilayer iodinene can exist stably at room temperature and below room temperature,and materials will melt above room temperature.The electronic energy band shows that there is a flat band near the Fermi level of the monolayer and the bilayer materials,and doped carriers induce magnetism.At a certain carrier doping concentration,the ferromagnetic Curie temperature much higher than room temperature.Further more,it was found that the structure of monolayer and bilayer iodinene changed from buckling phase to flat phase by tensile strain.The structure phase transition can regulate both doping magnetism and ferromagnetic Curie temperature,and the magnetic and structural phase transition can occur simultaneously at the appropriate doping concentration range.In addition,tensile strain can regulate the electronic band structure,carrier effective mass,transport properties and optical absorption of iodinene.This study reveals the potential of iodinene in the future tunable spintronic equipments,and provides a feasible approach for the design of 2D magnetic functional materials.2.Strain-driven valley states and phase transitions in Janus VSi Ge N₄ monolayer.Based on the first-principles calculation and the wannier function,we predict a new ferrovalley material VSi Ge N₄.By calculating its binding energy,molecular dynamics,and phonon spectrum,it is proved that VSi Ge N₄ has good thermodynamic stability against the elemental phases.In the future,it can substitut atoms in a Mo Si₂N₄ monolayer（Mo Si₂N₄ have synthesized）.Due to transition metal element V,the material may be considered as intrinsic magnetic material.Through the calculate of magnetism,electronic and topological properties,it is found that the monolayer material is an out-of-plane ferromagnetic semiconductor,and there is an intrinsic valley polarization quantum anomalous Hall effect.The ferromagnetic Curie temperature is113K.At the same time,there is a pair of degenerate and unequivalent energy valleys near the K point in the monolayer material.Spin orbital coupling effect can break the degeneracy of the energy valley and induce the spontaneous valley polarization.When-1%～1%biaxial strain is applied,a topological phase transition occurs.In this process,the band gap of K valley will close and re-open.Which attribute to crystal field distortion cused by strain.The topological phase transition changes the chirality of the K valley,so the polarization light absorption state is also changed.The special optical absorption properties can be applied to the next generation of optoelectronic devices.Our work provides a good material platform for studying the interaction between valleytronics,spintronics and topology.