Theoretical Studies On Magnetoelectric Coupling In Low-Dimensional Long-Period Crystal System

The fundmental researches of materials physics promotes the rapid progress of electronics and information technology.Materials design is based on the deep understanding of the intrinsic properties of atoms,molecules and electrons of the microscopic world,e.g.charge,spin,orbital degrees of freedom,so as to the further development of new electronic devices.The studies of magnetoelectric coupling effects in low-dimensional material systems is the central issue.By introducing single quantum states,such as defect states and impurity states,the material system can be modified,and the single-atom magnetism in the host material is controlled by an external electric field,and various exotic magnetoelectric effect has been discovered.Currently,related researches on effective control of single-electron spin are focused on different dimensional systems.The studies of single quantum states in low-dimensional systems involves introducing defect centers into two-dimensional systems,doping with dilute magnetic impurities,etc.,including single-electron spin control with single magnetic impurity atoms or single vacancies as the research object.It is generally believed that both experimentally and theoretically,applying an electric field or a stress field can observe the magnetoelectric effect effectively.Low-dimensional materials systems are characteristic of obviously higher degree of freedom of operation than that of bulk materials,and it is especially suitable for integration in spintronic devices to realize the control of single quantum states by external fields.Recently,as a novel degree of freedom of operation,interlayer twisting can be well used to design van der Waals bilayer systems with arbitrary stackings.The emergence of exotic phenomena and fantastic physics in this regard has broadened the understanding of magnetoelectric effects.In this thesis,we will mainly use the first-principles calculation method to briefly discuss the monolayer system that we do dilute magnetic doping 3d transition metal in Mo S2.A giant flexomagnetoelectric effect can be found in such system as we expect. A major feature of this system is that it breaks the short-range translational symmetry of the unit lattice and forms a long period in a superlattice.Similar examples can be extended to the studies of the twisted bilayer system.By theoretical calculations we also predict that for the antiferromagnetically coupled bilayer,the chirality dependent twist can cause dramatically differently dielectric behaviors of electron under vertical electric field.The discovery of such novel phenomena makes our subsequent research goes deep into the model analysis.For a simple twist small angle called ”magic angle”system,presetted by antiferromagnetic coupling between layers,we demonstrate that it has a topological magnetoelectric effect.The main research results are as follows.· Spintronics rooted in the spin degree of freedom is of both theoretical and technological importance.The development of some fantastic properties for electrically controlling this degree of freedom encourages enormous effort to the research on magnetic systems which possess sensitive magnetic response to the electric field.Here,a giant flexomagnetoelectric effect is predicted in a typical dilute magnetic monolayer Mn-dopoed Mo S2.Combining lattice bending and magnetic doping,it is shown that the magnetic response and magnetic anisotropy can be greatly amplified under the applied electric field.Further investigations reveal that such an effect stems from the orbit-dependent response of the single magnetic dopant.Physically,the electric field-induced orbital polarization causes the spatial distribution change of the Mn-3d orbital wavefunction,which is sensitive to the change of the orbital hybridization with the bent lattice.Hence the corresponding3 d energy levels can be controlled to shift near Fermi level via external electric field.These findings open a new route toward functional 2D materials design for flexible devices.· Twisted van der Waals bilayers provide an ideal platform to study the electron correlation in solids.Of particular interest is the 30°twisted bilayer honeycomb lattice system,which possesses an incommensurate moiré pattern and uncommon electronic behaviors may appear due to the absence of phase coherence.Such system is extremely sensitive to further twist and many intriguing phenomena will occur.In this work,based on first-principles calculations we show that,for further twist near 30°,there could induce dramatically different dielectric behaviors of electron between left and right twisted cases.Specifically,it is found that the left and right twists show suppressed and amplified dielectric response under vertical electric field,respectively.Further analysis demonstrate that such exotic dielectric property can be attributed to the stacking dependent charge distribution of the twisted bilayer,which can be illustrated by the spin textures.We will show that small electric field could hardly change the pseudospin pattern.As a result,for the right twisted case,there is almost no electric dipole formation exceeding the monolayer thickness when the electric field is applied.Whereas for the left case,the system could even demonstrate negative susceptibility,i.e.the induced polarization is opposite to the applied field,which is very rare in the nature.Such findings not only enrich our understanding on twisted systems but also open an appealing route toward functional 2D materials design for electronic,optical and even energy storage devices.· We show that the movement of electrons in solids,or in other words,the quasiparticles formed by the collective motions of electrons,can be hierarchical.Examples are valley electrons,which move in hyperorbits in a honeycomb lattice and forms a valley pseudospin,or the self-rotation of the wave packet.Here we demonstrate that twist can induce higher level motions of valley electrons around the moiré superlattice of bilayer systems.Such collective movement of the electron,can be regarded as the self-rotation(spin)of a higher-level quasiparticle,or what we call super-valley electron.This quasiparticle,in principle,may have macroscopic size as the moiré supercell can be very large.It could result in fascinating properties like topological and chiral transport,superfluid,etc.,even though these properties are absent in the original untwisted system.Using twist-ed antiferromagnetically coupled bilayer with honeycomb lattice as example,we find that there forms a Haldane-like superlattice.Interestingly,the system becomes quantum spin Hall insulator accompanied by the formation of Berry flux±2? per moiré supercell for each layer.Further analyses reveal that the supervalley electron possesses opposite spin(chirality of the rotation)in the top and bottom layer,and can be described as two components(Weyl fermion)of Dirac fermion in real-space.The massive Dirac fermion of the bilayer system possesses spontaneous topological magnetoelectric polarization,which could be understood as axion coupling of the spin with the permanent electric dipole moment.