Design Of Two-dimensional Ferrovalley Materials And Regulation Of Quantum Properties

To explore and predict the novel degree of freedom of electron and its application is one of the important research directions in condensed matter physics.Traditional electronics industry mainly relies on the charge degree of freedom of electrons,and processes information by manipulating charge degree of freedom,which gained important development in the 20th century.However,considering the influence of energy consumption and quantum effect,Moore’s law approaches the limit,which makes the circuit integration and device running speed also approach the theoretical limit,the miniaturization of devices is becoming more and more difficult,and the traditional transistor devices meet the development bottleneck,and the key to lead the development of modern electronic novel degree of freedom is to explore and predict the electronic novel degree of freedom,and can be successfully made and applied.With the gradual deepening of the research on crystal materials,people have found the spin angular momentum of electrons and the strong coupling effect between charge and spin through the research on magnetic nano multilayer films and giant magnetoresistance effect.A series of studies on the degree of freedom of spin of electrons has become a new discipline,namely spintronics.In addition to charge and spin degrees of freedom,it has been found in recent years that the energy valley based on berry phase effect in crystalline materials with special symmetry can be used as a novel degree of freedom.The energy valley is the extreme point of the electronic band of the crystal Bloch.The energy valley degree of freedom is used as an information carrier to encode and manipulate information,and the related field is valleytronics.The development of spintronics and the rise of energy valley electronics will become the reserve force of traditional electronics.The polarization of carrier in energy valley is very important in the application of valley electronics devices.In order to control the freedom of energy valley,the energy degeneracy between the two valleys should be broken to realize the polarization of energy valley carriers.Compared with introducing magnetism through external conditions,including magnetic doping,adsorption,magnetic proximity effect and other methods to break the symmetry of time inversion and thus break the energy valley energy degeneracy,For example,cluster problems and carrier scattering problems,the energy valley polarization of ferrovalley materials with intrinsic ferromagnetism is spontaneous,which can avoid many problems caused by magnetic introduction by external conditions,and provide a good platform for the realization of anomalous valley Hall effect.Since ferrovalley materials have both intrinsic ferromagnetism and valley-polarization properties,it is possible to observe multiple Hall effects in the same material,which can further broaden the horizon of valley-dependent physical mechanisms of two-dimensional materials,and also offer the prospect of multi-choice valley-spin coupled quantum devices in the field of valley-electronics.Therefore,it is of great significance to explore two-dimensional ferrovalley materials and study their quantum property regulation.In this paper,based on density functional theory and calculated from the first-principles calculation,we study the energy valley electronics properties in two-dimensional magnetic ferrovalley material structure,and conduct a regulatory study on its quantum properties.The main research contents of this paper are as follows:Firstly,the exploration and design of two-dimensional ferrovalley materials,because of its intrinsic energy valley polarization to avoid some problems caused by external conditions,is the current research hotspot in the field of energy valley electronics.However,the coexistence of multiple Hall effects in the same ferrovalley material is a more bizarre phenomenon,which has attracted wide attention.By first principles calculations,we study the electron and valley electron properties of a two-dimensional magnetic H-Ru Cl Br,and investigate the effects of electron correlation effect and strain regulation on the energy valley polarization,magnetic ground state,magnetic crystal anisotropy energy and berry curvature of a H-Ru Cl Br monolayer.The results show that the H-Ru Cl Br monolayer has good mechanical and dynamic stability,the magnetic ground state is ferromagnetic.With the increase of electron correlation effect,the out-of-plane magnetic crystal anisotropy decreases and turns to in-plane.The light absorption coefficient of H-Ru Cl Br appears blue shift with the increase of electron correlation,and the prominent peaks are about 1.63 e V and 7.5 e V,indicating that the material has obvious light absorption in infrared and ultraviolet bands.Without considering the electron correlation effect U,the single-layer Ru Cl Br is an indirect band-gap dual-polarized semiconductor.Its conduction band and valence band have a pair of spin-down and spin-up energy valleys,respectively.With the enhancement of electron correlation effect,the spin-down energy band of valence band gradually shifts upward under the action of spin orbit coupling（SOC）.Its K and K’valleys cross with the conduction band to close and then open the band gap,and then sink into the spin-up valence band.The H-Ru Cl Br monolayers undergo a series of electronic phase transitions in the U value from 0 to 3 e V,it includes ferrovalley state,half-valley-metal state and quantum anomalous Valley Hall state,in which the electron correlation regulates the quantum anomalous Valley Hall state from 1.93 e V to2.13 e V.In addition,the above electronic phase transition can also be achieved by stress regulation.The quantum anomalous Valley state appearing by strain regulation ranges from0.9%to 2.3%of compressive stress.As a two-dimensional ferrovalley material,single-layer H-Ru Cl Br has high energy valley polarization,and can realize electron phase transition under the regulation of electron correlation and strain.The quantum anomalous valley Hall state has both energy valley polarization and quantum anomalous Hall effect（QAHE）,which realizes the combination of energy valley physics and topological physics,and opens up a new prospect for the research direction of multi-functional spin-valley quantum devices.Secondly,it is of great scientific and experimental value to find more types of quantum anomalous Hall insulators which are not limited to the regulation of electron correlation.Monolayer jacutingaite（Pt₂Hg Se₃）is a recently discovered layered platinum group mineral.Recent experimental studies show that it has the properties of quantum spin Hall insulator（QSHI）.Related theoretical calculations also show that the dynamic stable monolayer Pt₂Hg Se₃ is a topological insulator with a band gap of 160 me V.Inspired by this structure,we theoretically predict a new class of quantum anomalous valley hall insulator Pt₄Hg₂Cl₃Br₃,which is also dynamic stable and has a ferromagnetic ground state.The light absorption spectrum has a small absorption peak around 0.55 e V,indicating that the material has a certain absorption of infrared light.It reaches a peak value around 6.09 e V,which indicates that the material has excellent absorption properties for ultraviolet wavelengths.Without considering the effect of SOC,the band gap is 160.12 me V.After considering SOC,the two unequal energy valleys merge and realize energy valley polarization.The band gap difference between K and K’valley is 305.69 me V and 26.60 me V respectively,and the polarization values of valence band and conduction band are very large,they were 120.79me V and 158.30me V,respectively.In addition,the semi-infinite edge state has a conductive channel connecting the upper and lower states,and berry curvature direction is different and the direction is the same.These properties further indicate that two-dimensional Pt₄Hg₂Cl₃Br₃ has both QAHE and anomalous Valley Hall effect（AVHE）,which makes multiple Hall effects can be realized in the same material.The premise of the above research is that the magnetic axis is out of plane,but the magnetic crystal anisotropy energy is 1.6me V lower in plane than out plane,and the small magnetic crystal anisotropy energy can be reversed by an external magnetic field,reversing the magnetic axis out of plane.However,we also found that the magnetic axis can be successfully regulated from in plane to out plane by applying 8%biaxial tensile stress,two-dimensional Pt₄Hg₂Cl₃Br₃ under 8%biaxial tensile strain still has QAHE.In this work,a new two-dimensional ferrovalley material is proposed,which is inspired by the existing layered platinum-group minerals,has certain synthesis possibilities,and the energy valley polarization value is very large.It has both QAHE and AVHE,which is a quantum anomalous valley hall insulator.This study has broaden the range of poly-Hall effect materials,presenting a new avenue for combining and utilizing spintronics,valleytronics and topological nanoelectronics.It provides a new platform for further material research in these frontier fields.