The Design And Quantum Properties Reserch Of Transition Metal Materials In The Two-dimensional Tetragonal Lattice

Searching for two-dimensional materials with excellent quantum properties has become the focus of the design of spintronic devices with high performance,low power consumption and high storage density in the information field.We perform density functional theory（DFT）calculations to study the quantum physical properties of two-dimensional tetragonal lattice transition metal materials in this article.By analyzing the geometric structure and electronic properties of the four types of materials,the principle of the topological phase transition and the origin of the magnetic anisotropy are revealed,which provides a new way for the preparation of multifunctional and tunable spintronic devices.First of all,quantum spin Hall effect（QSH）can realize the non-dissipative transmission of electrons,so it is our goal to find two-dimensional topological materials with quantum spin Hall effect.Through first-principles calculations,we find that the quantum spin Hall effect can be achieved in two-dimensional Cu N materials.In addition,through the analysis of Young’s modulus and Poisson’s ratio,it can be found that two-dimensional Cu N materials can resist external strain pressure well.The energy band of Cu N has a Dirac point far away from the high symmetry point Y,which exhibits semi-metallic properties.Considering the spin-orbit coupling,the energy band of Cu N opens a band gap of 160 me V at point D.Through band composition analysis,we find that there is a energy band in p_x-p_z,and the local density of states at the edge can prove that there is a topological phase transition.Its topological properties are verified by Z₂=1.The heterostructure formed by epitaxially growing Cu N material on the semiconductor Al As substrate still maintains the topological properties of Cu N.These findings provide a new way for the design of spintronic devices.Secondly,the research on topological insulators has made great breakthroughs,but there are relatively few researches that combine topology with other properties.For this reason,we propose a new two-dimensional material-Hf C,in which two quantum states of ferroelasticity and topological properties can exist at the same time.The two-dimensional Hf C material is a semiconductor material with a band gap of 0.23 e V.Cu N produces topological phase transition through spin-orbit coupling,and the mechanism of topological effect of Hf C is different from that of Cu N.By applying stress to control the Hf C energy band,it is found that the band gap has an opening-closing-opening process,resulting in a topological phase transition.Edge state analysis and Z₂ invariants can prove this conclusion.Under the action of stress,the Cu N anisotropic structure can be rotated 90°,causing the x and y directions to switch mutually,which shows that the Cu N material has ferroelasticity and the ferroelastic reversible strain is 17.4%.Since Cu N possesses both ferroelastic and topological properties,Cu N is expected to be a candidate material for anisotropy and quantum Hall effect through strain control in experiments.Next,magnetic materials play an important role in the storage and transmission of information,but most two-dimensional materials are not magnetic,so looking for two-dimensional materials with intrinsic magnetic properties has always been a hot spot in the field of spintronics.We have constructed three structures of VX（X=N,P,As）based on a two-dimensional tetragonal lattice,and found that all three materials have intrinsic magnetism with a magnetic moment of 4μ_B,and their kinetic and thermodynamic stability are excellent.Band structure analysis can reveal that the two-dimensional VN and VP films are magnetic semiconductors,and the two-dimensional VAs film is magnetic halfmetal,and the magnetic properties are mainly provided by V atoms.Since the ferromagnetic transition temperature will affect the electromagnetic effect of the material,the higher Curie temperature of the two-dimensional VX material provides conditions for its application in magnetic devices.When considering spin-orbit coupling,the energy of VN and VP is the lowest on the z-axis,so they have the out-of-plane magnetic anisotropy.When considering spin-orbit coupling,the energy of VAs is the lowest on the y-axis,so the easy magnetization direction of VAs is in-plane.The magnetic anisotropy energy of the two-dimensional VN crystal is 2130μe V.The magnetic anisotropy energy of the VP crystal is 1638μe V.And the magnetic anisotropy energy of the VN crystal is 1599μe V.Through the analysis of atomic and orbital resolved magnetic anisotropy,it is found that the generation of magnetic anisotropy is related to the d orbital of the V atom.As the atomic number of X increases,the in-plane magnetization increases,while the out-of-plane magnetization decreases.Finally,we have performed a more in-depth study of magnetic material after studying the mechanism of the magnetic anisotropy of magnetic materials.We find the law of energy change by adjusting the direction of electron spin in the whole space.And we use carrier doping to achieve magnetic switching.Here we propose a two-dimensional tetragonal Cr X（X=P,As）material.By comparing the energy of ferromagnetism and antiferromagnetism,it is found that Cr X is a semiconductor material with intrinsic ferromagnetism.The 90°super exchange interaction between two magnetic atoms is the source of magnetism in Cr X materials.The Curie temperature of Cr P is 255 K,and the Curie temperature of Cr As is 855 K.The higher Curie temperature indicates the feasibility of experimental preparation.When the magnetization direction changes in the whole space,the easy magnetization axis of Cr P is on the z-axis,while the easy magnetization axis of Cr As is in the plane.The magnetic anisotropy has a strong dependence on the polar angle,but the azimuth angle dependence is very weak.And there is no energy barrier in the plane.The switching of out-of-plane magnetization and in-plane magnetization is realized by carrier doping,which creates conditions for the preparation of two-dimensional field effect transistors.