Study On Physical Properties And Their Control In Moiré Pattern Of Two-dimensional Materials

When there exists a small lattice mismatch or relative twist between two-dimensional(2D)layered materials,a long-range periodic structure,i.e.moire pattern,will be formed.Moire patterns have become a powerful tool for designing new physics in the field of condensed matter physics,showing great advantages in modulating the electrical,optical and magnetic properties of materials.Systematic study of the moire patterns at the microscopical level is critical to understand its novel physical properties,which also provides valuale insights for the construction of moire quantum devices.In this thesis,we take the moire pattern as the main theme,and systematically study the 2D magnetic moires and semiconducting moires.In the moire patterns formed by 2D ferromagnetic materials,we study the stacking dependent interlayer magnetic interactions by using first-principles calculations combined with micromagnetics simulations,and design a series of non-uniform magnetic structures by defining a spatially changing effective magnetic field,including the topologically protected magnetic skyrmions,and tune these interesting magnetic structures via applying external magnetic fields.In the moire patterns formed by 2D antiferromagnetic materials,we introduce a tunable strong magnetic anisotropy in 2D antiferromagnetic materials by using the unique stacking degrees of freedom of 2D materials,and use it to stabilize and tune 2D antiferromagnetic order and its dynamics.In the moire patterns formed by 2D semiconductor materials,we systematically studied the evolution of the morphology of the moire pattern with applied strain and the mechanism of the associated energy band modulation using molecular dynamics simulations combined with first-principles calculations.The details are listed as follows:First,we study the interlayer magnetic interactions in bilayer CrI3 and CrBr3 for different stacking configurations through first principles calculations.With the change of interlayer stacking configurations,we find that the interlayer ground state transforms between ferromagnetic and antiferromagnetic one,and give its full phase diagram.We use this stacking dependence to introduce a spatially varying effective magnetic field.This effective magnetic field produces various nonuniform magnetizations in the moire.Using micromagnetic simulation,we calculate the steady state magnetic configurations in twisted bilayer CrI3 and CrBr3.We find that the moire supercell contains three magnetic domains,and the stable magnetic configurations with different topological numbers can be designed by changing the arrangement of magnetic moments around the domain walls.In addition,we also realize the control over these nonuniform textures through applying the external magnetic field,and demonstrate a unique double-loop hysteresis.These interesting nonuniform textures can be produced by quenching from the paramagnetic state.Second,we study the tunability of magnetic order and its dynamics in 2D antiferromagnetic materials.Via stacking a 2D antiferromagnetic materials on a magnetic substrate,we find that the interlayer magnetic interaction can introduce a sublattice dependent effective field in the antiferromagnetic layer.This effective field can be divided into an effective anisotropic field which stabilizes the antiferromagnetic order and an effective Zeeman field which breaks the degeneracy of the magnon.In addition,we also found that the effective anisotropic field sensitively depends on the interlayer stacking order and the magnetic order of the magnetic substrate,providing a new mechanical and magnetic means for controlling the antiferromagnetic order and its dynamics.Both methods can flip the antiferromagnetic order,but have different effects on the spin wave dynamics.Specifically,the ratio of precession amplitude of antiferromagnetic sublattice can be changed by changing the stacking configuration mechanically;While the chirality of antiferromagnetic sublattice precession can be changed by adjusting the magnetic order of the ferromagnetic substrate with an external magnetic field.We study the effective anisotropic field in MnPS3/CrCl3 heterobilayer by first-principles calculations.The effective anisotropy energy introduced in MnPS3 is about 40 times of its intrinsic one.Using the spatially changing effective anisotropic field,antiferromagnetic texture are prepared in the moire patterns formed by MnPS3/CrCl3.The low-energy magnon feels an effective potential contributed from both antiferromagnetic texture and effective fields,and is periodically localized in the magnetic domain.By applying interlayer translation or uniform external magnetic field,we can effectively control these antiferromagnetic textures.Third,we use molecular dynamics simulation to study the evolution of the morphology of the long-period 2D semiconductor(WS2)moire patterns with the applied strain.We find that with the increase of the strain,the morphology of the moire patterns gradually deformes from its original regular hexagonal shape.This change is mainly due to the competition between the external strain and the internal lattice reconstruction.This competition leads to the introduction of severe in-plane inhomogeneous strain in the moire patterns.Using firstprinciples calculations,we study the modulation behavior of the electronic band structure with the in-plane inhomogeneous strain under different stacking configurations,and find that this non-uniform in-plane strain results in a large energy modulation at the conduction band K point,and is more prominent in the heavily deformed moire patterns.We find that this modulation behavior originates from the coupling between atomic orbitals within the monolayer,which is different from the interlayer origin of the modulation behavior at the edge of the valence band reported in the literature.