Studies On The Electronic Structure And Spin-Orbit Coupling Of Janus Two-Dimensional Materials

The symmetry breaking of Janus two-dimensional materials can lead to strong spin-orbit coupling.The study of its unique electronic structure and spin/valley characteristics is a frontier topics in the field of material science and condensed matter physics.Taking transition metal dichalcogenides and new two-dimensional WSi₂N₄ family as objects,based on first principle calculation,this paper deeply studies the regulation effect and regulation mechanism of interlayer interaction,electric field,doping,strain and other factors on spin and valley splitting in Janus two-dimensional materials,establishes a tight-binding theoretical model,and reveals the origin of Rashba spin-orbit coupling in Janus two-dimensional materials.The main content of the paper is divided into the following four parts:（1）The effects of interlayer electrostatic interaction and magnetic proximity on the electronic structure and spin/valley splitting of 2H phase monolayer Janus WSSe were studied.It was found that the interlayer dipole interaction will lead to the layer-dependent Rashba splitting and inhibit the effect of the van der Waals interaction on the p_z orbit.Mn O（111）magnetic substrate introduces valley splitting to Janus monolayer WSSe,which is positively correlated with the vertical magnetic moment of surface Mn atoms and up to 410 me V.The magnetic proximity also affects the Rashba band,opening an energy gap and eliminating the degeneracy of in-plane spin.The equivalent magnetic fields felt by K andΓstates are opposite,and the difference is about two orders of magnitude.（2）The ferroelectric properties of d1T-phase monolayer transition metal dichalcogenides are studied,and the first derivative of the splitting energy to wave vector is used to measure the Rashba coupling strength.It was found that the spin-orbit coupling varies linearly with the electric field,but the relationship with charge doping is nonlinear.By studying the effect of strain on the Rashba spin-orbit coupling of d1T-phase monolayer WSSe,it is analyzed that the two factors that determine the Rashba intensity are the macroscopic charge transfer and the microscopic electronic orbit composition.Charge transfer reduces the gradient of potential.The larger the polarization is,the weaker the Rashba spin-orbit coupling is.The atomic orbital in z direction plays a leading role in Rashba coupling.（3）The spin-orbit coupling and valley properties of monolayer WSi₂N₄ family are studied.By changing the stacking pattern of the atomic layers,the symmetry is changed and the Rashba spin-orbit field is introduced.Vertical polarization and out-of-plane orbit play important roles in in-plane spin splitting,while in-plane polarization and orbit determine out-of-plane spin splitting.Combined with valley polarization,the characteristics of Rashba semiconductor help to control the spin direction of valley polarized electrons in two-dimensional materials.The intrinsic Rashba coupling of different monolayer Janus WSi₂N₄ family is analyzed by designing component.Replacing Group VA atoms bonded to W atoms can produce greater Rashba splitting.（4）Through tight binding model and perturbation theory,the origin of the Rashba spin-orbit coupling of monolayer Janus transition metal dichalcogenides is analyzed.The difference of on-site energy of p_z orbital of chalcogenide atoms and the coupling between d orbital and p_z orbital determine the strength of Rashba spin-orbit coupling,in which d_xz and d_yzorbits can not be ignored.The results of tight-binding and first-principle calculation are consistent.The strain mainly changes the coupling between d orbit and p_z orbit,and the electric field mainly changes the energy of p_zorbit.The results of this thesis are helpful to deeply understand the electronic structure and spin/valley characteristics of Janus two-dimensional materials,and are of great significance to the theoretical research and device application of spin-orbitronics in two-dimensional materials.