Study On The Electrically Controlled Spin Texture In Two-dimensional Ferroelectrics And Multiferroics

Spin-orbit coupling establishes a close connection between the intrinsic spin properties of electrons and electromagnetic interactions and plays an important role in the field of spintronics.Spin-textured ferroelectrics realize the cross-fusion of spin-orbit coupling effect and ferroelectricity,making it possible to electrically control spin polarization in a non-volatile manner.In this thesis,we take two-dimensional ferroelectrics and multiferroics as the research objects and use first-principles calculations to make research on the coupling relationship between electron intrinsic charge,spin and spin texture in reciprocal space,as well as the nonlinear optical properties of the system.The main contents are as follows:1.It is predicted that the Nb OI₂monolayer with orthogonal structure is a kind of semiconductor with coexistence of ferroelectricity and Dresselhaus effect.This system belongs to the C_2v point group,has a Dresselhaus-type band splitting near the bottom of the conduction band,and has a unidirectional out-of-plane spin texture,so it will produce a persistent spin helix.When the in-plane ferroelectric polarization direction of Nb OI₂ is reversed,the spin texture in the out-of-plane direction will also be reversed.In addition,applying uniaxial strain can effectively adjust the bandgap value and the degree of spin splitting of the Nb OI₂ monolayer.With the increase of the compressive stress,a huge energy band splitting will appear in the valence band at theΓpoint,and there is still a unidirectional spin texture near theΓpoint,maintaining a persistent spin helix.2.Introducing the spin-orbit coupling effect into the two-dimensional multiferroic material VOX₂（X=Cl,Br）monolayer with ferroelectric-antiferromagnetic ground state structure,the system will have band splitting and the band splitting degree of the minimum conduction band increases with the increase of the halogen X atomic number.Both the conduction band minimum and the valence band maximum have a unidirectional out-of-plane spin texture,and the distribution of the spin texture will be reversed with the reversal of the in-plane electric dipole moment.The two metastable ferroelectric-antiferromagnetic structures of VOX₂ also have unidirectional out-of-plane spin texture.In addition,we predict that the VOCl Br monolayer is a two-dimensional multiferroics Janus material with ferroelectric-antiferromagnetic ground state.Through systematic research on the second harmonic generation（SHG）of VOX₂and VOCl Br monolayer,it is found that both VOX₂ and VOCl Br have larger SHG coefficient zero-frequency values,and the SHG zero-frequency values of different magnetic structures have obvious differences.