Study Of Quantum Anomalous Hall Effect And Valley Properties In Two-Dimensional Materials

Two-dimensional materials（2D）represented by graphene have good electrical,mechanical and other physical properties.In recent years,they have been applied to communication,biology and other fields,and have become a research hotspot.As an important branch of 2D materials,2D intrinsic magnetic materials can maintain long range magnetic order and be regulated by the field under 2D limit conditions,which provides an ideal platform for the study of novel quantum physical effects.Quantum anomalous Hall effect is characterized by unimpeded electron transport and has great development prospects in low power electronic devices.The quantum anomalous Hall effect is often realized by magnetic doping and heterojunction construction,but it is only observed in a few materials at low temperature.Therefore,researchers hope to obtain the intrinsic magnetic topological insulator with high Curie temperature to realize the quantum anomalous Hall effect.Similar to the degree of freedom of charge and spin,the valley degree of freedom in 2D materials can also realize information encoding and storage.The key to manipulate valley freedom is to break the degeneracy between different valleys and realize valley polarization.Anomalous valley Hall effect can be realized by adjusting the unequal valley in Brillouin region of valley electronics materials.Optical pumping and magnetic atom doping are commonly used methods for regulating valley polarization,but there are still some problems in practical applications.Optical pumping is difficult to control in device applications,and magnetic atom doping can easily cause impurity scattering.At the same time,the intrinsic valley polarized materials provide a new opportunity for the development of valley electronics.Currently,the known valley polarized materials are limited.The search for larger valley polarized materials is helpful to develop more materials suitable for the preparation of electronic devices.In addition,magnetic valley materials with various Hall effects have also attracted the attention of researchers.Quantum anomalous Valley Hall effect is a novel valley related effect,which has the interaction between valley and energy band topology.The combination of valley and quantum anomalous Hall effect is expected to realize the application of high-performance quantum devices,and further promote the development of valley electronics and spintronics.However,the realization of quantum anomalous Valley Hall effect requires many conditions,which challenges the research of 2D materials.Based on the calculation of first principles,the topological properties and valley correlation effects of 2D hexagonal lattice materials are investigated by studying the electronic properties and magnetism of 2D hexagonal lattice materials.Firstly,2D MXenes structure has received a lot of attention in recent years as a kind of 2D transition metal carbide chemically stripped from the layered MAX phase.It has become a hot research direction to search for the intrinsic topological insulators with high Curie temperature in 2D MXenes.We systematically investigate the electrical,magnetic and topological properties of 2D MXene Mo YN₂CSCl.The results show that Mo YN₂CSCl monolayer is a semi-metallic material with long range ferromagnetic sequence.The Curie temperature is 619.1 K,which is expected to realize the high temperature quantum anomalous Hall effect.Symmetry breaking leads to spin orbit coupling（SOC）effect,and a topologically nontrivial band gap of about 37.3meV with the Chern number C=1 appears in the band,which is due to the band inversion between d_{x z} andd_yz orbitals.The topological properties of MXene Mo YN₂CSCl are further confirmed by the calculation of the edge state with a conductive channel and the non-zero Berry curvature.The topological phase transition of MXene Mo YN₂CSCl can be achieved by applying strain and adjusting magnetization direction.It is worth noting that MXene Mo YN₂CSCl can realize the semi-metallic state of nodule line in the regulation process of electron correlation interaction U,and can still realize the quantum anomalous Hall effect with C=1 in the case of SOC.Secondly,the concept of ferrovalley is proposed in 2D material 2H VSe₂.ferrovalley material has spontaneous valley polarization and intrinsic magnetism,which provides a new direction for the regulation of valley freedom.At present,ferrovalley materials with coexisting spin and valley polarization only exist in a few materials.In this article,we propose that 2H Janus LuClF is a 2D ferrovalley material with ferromagnetic semiconductor as its ground state.Under the action of time inversion breaking and SOC,the spontaneous valley polarization of valence band reaches 33.6 meV.In Berry curvature,the absolute values of K and K’ valley are different and the signs are opposite.When an external electric field is applied,the single layer LuClF will realize anomalous Valley Hall effect.The biaxial strain obviously regulates the valley polarization of the single layer LuClF,and the valley polarization of the single layer LuClF reaches 75.88 meV under-3%compression strain.Different U values also have significant influence on energy band.When U value changes from 0 eV to 4 eV,valley polarization is robust.Therefore,monolayer LuClF is a potential valley electronics material.Finally,the quantum anomalous Valley Hall effect,which combines valley electronics with band topology,has attracted the attention of researchers.The realization of quantum anomalous Valley Hall effect needs high requirements on materials.People are trying to find 2D materials which are easy to synthesize in experiments.We studied the change of energy band properties of 2D magnetic material 2H ScBr₂ under different electron correlation U.For vertical magnetic anisotropy,when U=3 eV,the valley polarization of 2H ScBr₂ becomes 60.74 meV considering SOC.The reversal of valley polarization can be regulated by reversing the direction of magnetization,and the anomalous Valley Hall effect can be realized by applying electric field.With the increase of U value,2H ScBr₂undergoes a phase transition from ferrovalley state to half-valley metal state to quantum anomalous Valley Hall effect to half-valley metal state and then to ferrovalley state.In the range of 3.68 eV to 3.80 eV,there is a reversal ofd_xy/d_x2-y2and d_z2 orbital components in the energy band,and the symbolically reversible Berry curvature is observed.When U=3.742 eV,ScBr₂ has a Chern number C=1 and a conductive channel connecting the conduction band and valence band.We study the intrinsic magnetic anisotropy under different U value conditions.There is no quantum anomalous Valley Hall effect or valley polarization when considering intrinsic magnetic anisotropy.This work mainly explores the influence of electron correlation U value and magnetic anisotropy on monolayer 2H ScBr₂ band structure.The study of quantum anomalous Valley Hall effect provides ideas for regulating 2D magnetic materials with valley dependent quantum devices.