A Theoretical Study On Symmetry Breaking And Light Field Regulation Effect Of Two-Dimensional Materials

In recent decades,the related research of two-dimensional(2D)materials has developed rapidly.The discovery and synthesis of a large number of materials are enriching and improving the family of 2D materials at an amazing speed.The difference of dimensions leads to the existence of various electronic properties of materials of the same composition.Motivated by the great success of monolayer graphene,considerable efforts have been devoted to the study of 2D materials.Among them,the more representative 2D materials include hexagonal boron nitride(h-BN),transition metal dichalcogenides,black phosphorus and so on.These 2D materials hold great promise in the fields of electronic properties,mechanical properties,heat conduction and light absorption.In order to pursue higher performance of 2D materials,researchers are constantly looking for new 2D materials,which requires first of all to understand and grasp the essence of the material performance mechanism,such as tight binding approximation,symmetry analysis and other methods play a role at this time;On the other hand,many strategies have developed to regulate the electronic properties of 2D materials,such as electrostatic field,stress regulation,carrier doping,which greatly broden the application range of 2D materials.In this thesis,the effects of symmetry breaking and periodic laser field on the electronic structure of 2D materials are investigated by means of first-principles calculations on combination with tight-binding method and Floquet theory.The main research contents include:Four types of distorted lattices with specific symmetry breaking based on the traditional Ruby lattice were considered.It is found that the Dirac cone exists in the lattices where the inversion symmetry(i-Ruby lattice)and mirror symmetry(m-Ruby lattice)are preserved separately.It is proved that the Dirac cone can be protected by a single symmetry.Further breaking of the remaining symmetry can open the band gap of the Dirac cone.While the other lattice with inversion symmetry broken(b-Ruby lattice)keeps the mirror symmetry,which differs from the mirror position of m-Ruby lattice,but shows structural characteristics similar to h-BN.In this case,the Dirac cone is opened.The symmetry protection mechanism of the distorted lattice can be proved by the unitary transformation of the position vector,and such a distortion operation can even connect the Ruby lattice and the Star lattice.This explains why both of them have flat bands.This work reduces the symmetry limit of the Dirac cone and provides convenience for the synthesis of the Dirac material.Then,the non-equilibrium electronic structure of 2D GaAs monolayer is studied using the Floquet theory.It is found that when circularly polarized light is applied to the 2D GaAs monolayer in a specific direction,the Rashba splitting at the conduction band minimum(CBM)is greatly enhanced and the Rashba splitting at the valence band maximum(VBM)is induced.Further analysis shows that such Rashba splitting is nonlinear and anisotropic,which can be described by a nonlinear Rashba model containing the third order term of the wave-vector.The nonlinear behavior of Rashba splitting varies with the amplitude of the light.Moreover,the direction of laser propagation can determine whether the Rashba splitting in two directions is equivalent.Such tunable nonlinear and anisotropic Rashba splitting in 2D materials holds great promise in spintronic devices.