The Design And Performance Research Of New Two-dimensional Topological And Magnetic Materials

Two-dimensional topological and magnetic materials are the research hotspots of spintronics.In this paper,four new types of two-dimensional materials are designed and their structure and electronic properties are studied based on first-principles calculations.On the basis of studying the non-trivial topological state of topological materials and the intrinsic ferromagnetism of magnetic materials,the physical properties of materials can be controlled through external adjustments such as rotation,stress,electric field or substrate on the structure.We believe that these unique properties in materials provide a solid theoretical foundation for the development of topological insulators and the application of high-efficiency and multifunctional spintronic devices.First,the functionalization of chemical groups for two-dimensional materials is an effective way to regulate their properties.Therefore,we constructed a two-dimensional hexagonal lattice Pb-CH₂OCH₃ by using two dimethyl ether groups(CH₂OCH₃)to adsorb Plumbene.The research results show that the material is a new type of ferroelectric topological insulator,which has a large band gap of 0.80 e V under the effect of spin-orbit coupling.The system has a non-trivial topological state based on the orbital filtering effect,which is confirmed by two helical conductive edge states with opposite spin polarization and the Z₂invariant of value 1.Disrupting the symmetry of the structure by rotating the ligand molecules on one side introduces the in-plane ferroelectric polarization,the intensity and direction of which depend on the relative position of the CH₂OCH₃ molecule.The topological phase of the system always exists robustly when rotating the ligand molecule,which realizes the coexistence of ferroelectricity and topological properties in Pb-CH₂OCH₃.We constructed a Pb-CH₂OCH₃/Si C(111)heterojunction in order to increase the experimental operability of Pb-CH₂OCH₃ and found that its electronic properties are only slightly affected by the coupling between the substrate layers,and the topological state is still maintained.The coupling of ferroelectricity and topology in material provides theoretical basis for combination of ferroelectric polarization and spintronics.Secondly,topological semimetals are a new class of topological quantum states,which are different from topological insulators.Nodal-line semimetals are a kind of branch of topological semimetals,and the intersections of its bands form a continuous closed curve in the lattice momentum space.We designed the two-dimensional hexagonal lattice Ta₂C₃.The research results show that there are two crossing bands around the K point at the Fermi level,which are contributed by the orbitals of Ta atom and the p_z orbital of C atom.The opposite parity of wave functions of these two bands prevents the interaction between them.Therefore,the crossing bands appear as a nodal loop centered on the K point in the Brillouin zone of system,which proves that Ta₂C₃ monolayer is a new class of nodal-line semimetal.The spin-orbit coupling effect leads to obvious spin-splitting of 268.87 me V and 61.90 me V for the conduction band minimum and valence band maximum.Massless Dirac fermions in unequal valleys at K and K'points have opposite Berry curvature and spin index.The coupling of spin and valley causes the material to produce spin-valley Hall effect when the planar electric field is applied,thereby generating spin current and spin-valley polarization.This enables low power consumption and fast electron transfer,and also provides greater development space for combination of spintronics and valley electronics.In addition,an interesting negative differential resistance effect was discovered when studying the transport characteristics of Ta₂C₃,which is helpful for the application of material in diodes,amplifiers and oscillator circuits.The quantum anomalous Hall effect is different from the above quantum spin Hall effect.It does not require any external magnetic field but is generated by the spontaneous magnetization of the material itself.It can realize the quantum Hall state in the zero magnetic field,and it is easier to apply to the electronic devices that people need daily.Therefore,we designed and studied the two-dimensional quantum anomalous Hall insulator Ru I.The research results show that the two-dimensional tetragonal lattice Ru I has Curie temperature as high as 282 K,which confirms the intrinsic strong ferromagnetism of material.The out-of-plane magnetization direction and perpendicular magnetic anisotropy of material are verified by the calculation of magnetocrystalline anisotropy.The spin-orbit coupling opens a topological non-trivial band gap of 196 me V.The Chern number is determined to be|C|=2by integrating the Berry curvature of frequency band occupied in k-space.The two topologically protected gapless edge states further confirm that the material is a quantum anomalous Hall insulator with high Chern number.In addition,Ru I has robust quantum anomalous Hall effect and perpendicular magnetic anisotropy under biaxial stress of-1%to6%.These findings provide an opportunity to obtain viable two-dimensional ferromagnetic Chern insulators in spintronics applications.Finally,we designed a two-dimensional tetragonal lattice structure XOBr(X=Tc,Ru).The research results show that XOBr(X=Tc,Ru)is a kind of two-dimensional anisotropic material with intrinsic ferromagnetism,whose magnetism comes from transition metal atoms Tc and Ru.The magnetic moments of Tc OBr and Ru OBr unit cells are 4.0 and 2.0?_B,and their Curie temperatures are 195 K and 82 K,respectively.The materials behave as ferroelastic if the anisotropy and lattice structure in two-dimensional materials can be controlled by strain engineering,and the unique ferroelasticity provides an opportunity to control the anisotropy of material.We found that XOBr has excellent ferroelasticity,and its ferroelastic phase transition can be achieved by applying external stress to drive the crystal lattice orientation to rotate 90°.The moderate conversion barrier of 77 me V/atom/136me V/atom in Tc OBr/Ru OBr makes the material valuable in the application of shape memory devices.In addition,there is a significant difference in the carrier mobility of XOBr in x and y direction,and ferroelastic phase transition causes transfer direction of electrons and holes to rotate by 90°.The characteristic of ferroelastic phase transition to control the directional of anisotropic electronic behavior makes XOBr a powerful candidate material for designing controllable electronic devices.