Theoretical Research On Electric Control Of Magnetism And Valley Polarization In Two-Dimensional Spin-Orbit Coupling Systems

Spintronics is one of the emerging fields in the scope of condensed matter physics,which mainly studies the characteristics of electron spins.Spintronic devices are one of the candidates for next-generation nanoelectronic devices.Identifying and designing materials that can be used to generate,detect and manipulate spin degree of freedom is the key step to realize practical applications.Non-magnetic materials characterized by strong spin-orbit coupling（SOC）interaction and magnetic materials characterized by magnetic moments are the two cornerstones of spintronics.This thesis is belong to the scope of spintronics.Combined with first-principles calculations,group theory analysis and other means,the electric control of magnetism will be studied in two-dimensional systems（including interfaces,surfaces,and two-dimensional materials）.In chapter 1,we briefly review spin-orbit coupling interaction and device applications based on Rashba effect in non-magnetic systems.In addition,the research progress of valley physics is also introduced.Finally,we present the external field control of magnetism,mainly including electric control of magnetism and light control of magnetism.In chapter 2,VASP and Density Functional Theory will be introduced.During the doctoral period,The main research work is divided into four sections,which are written in chapters 3-6.Below is a brief overview of four sections:In chapter 3,the multiferroic superlattice model is constructed by ferroelectric and antiferromagnetic.Taking Ge Te/Mn Te superlattice as an example,multiferroicity and electric field control of interface magnetism are investigated.Using first-principles calculations,we demonstrate that the ferroelectricity of Ge Te and the antiferromagnetism of Mn Te can coexist to form multiferroics in the ground state.Meanwhile,The ferroelectric polarization state of Ge Te can generate two-dimensional electron gas（2DEGs）and two-dimensional hole gas（2DHGs）with the same spin-polarized state at alternate adjacent interfaces.When the direction of ferroelectric polarization is reversed,the 2DEGs and 2DHGs at the interfaces exchange positions with each other,and simultaneously,the spin-polarized state is also reversed.When Ge Te is paraelectric state,the spin channel at the interface will be closed.So,we can generate or eliminate purely spin-polarized two-dimensional conductive channels by ferroelectric polarization or depolarization in Ge Te.The spin-polarized state also can be switched by flipping the ferroelectric polarization（between pure spin-up state and pure spin-down state）.As above,we can design a multi-channel spin field-effect transistor with 100%spin-polarized state.In chapter 4,an extension of the previous work will be discussed.Surface magnetoelectric response is studied on Mn Te films with a certain thickness（≈2 nm）under the action of electric field.Using first-principles calculations,we prove that electric field lead to the generation of spin-polarized conductive channels at both surfaces,while the inner part of Mn Te film still maintains the semiconducting properties.The conductive state and spin-polarized state of the channel depend on the direction and intensity of external electric field.This result corroborates with the conclusion of chapter 3.At the same time,the research results show that the electric field can promote the in-plane easy magnetization axis to turn to the out-of-plane direction in Mn Te thin film.In addition,external electric field strengthen the potential asymmetry at both surfaces.So the two-dimensional electron gas generated at the surfaces exhibits Rashba-type spin texture,but this is only presented at a certain surface.Besides,we also find that the electronic states accrossing Fermi level in the valence band have a non-zero Berry curvature.By flipping the direction of the applied electric field,the surface positions,the conductive spin states,chiral Rashba spin texture,and Berry curvature can be switched.Finally,we propose that Mn Te films have hidden band properties of bipolar magnetic semiconductors.This study will deepen the understanding of antiferromagnetic Mn Te and provide new application guidance for low-power,high-efficiency antiferromagnetic spintronic devices.In chapter 5,we study two-dimensional transition metal sulfides with strong spin-orbit coupling interaction.Taking Mo X₂（X=S,Se,Te）as an example,the electronic properties in Mo X₂（X=S,Se,Te）compounds are discussed under electrostatic doping.Studies show that weak ferromagnetism induced by hole-doping can be obtained in Mo Te₂,but not obtained in Mo S₂ and Mo Se₂.This phenomenon can be explained by the Stoner criterion.Furthermore,Our research shows that weak ferromagnetism will induce valley polarization in Mo Te₂.At an appropriate hole-doping concentration,the coexistence of valley Hall current,valley Hall voltage,and circularly polarized excitation can be obtained.This work will provide a deeper understanding of valley’s fundamental properties and valley polarization research.In Chapter 6,we try to find valleytronic materials with spin-valley locking in three-dimensional materials connected by strong covalent bonds.In the high-pressure phase Cr N₂system,we demonstrate the spin-valley locking by first-principles calculations.But spatial inversion symmetry makes the optical selectivity of the valleys indistinguishable.Therefore,we replace a layer of Cr atom with Mo atom to make spatial inversion broken.Group theory analysis shows that the spatial inversion asymmetry leads to obvious and distinguishable spin-valley locking.So,in this system,the spin-polarized state can be dynamically controlled by the optical pumping of circularly polarized light.This work will provide useful guidance for finding and designing valleytronic materials with spin-valley locking in three-dimensional materials.In chapter 7,we make a systematic summary of the research work during the doctoral period and point out the direction and content that can be further explored in the future.