First-principles Study On The Valleytronic Properties Of 2D Magnetic Janus Structures

Due to the thermodynamic and quantum mechanic limits,the traditional electronic devices that process information based on charge degree of freedom of electrons are facing a development bottleneck.Therefore,new quantum degrees of freedom for information processing are actively explored.Recently,a new valley degree of freedom based on the Berry phase effect has been discovered in some graphene-like semiconductors:the inequivalent Dirac valleys have opposite electrical,magnetic and optical properties.Valleytonics uses valley degree of freedom,in combination with the charge and spin degrees of freedom to encode information,to provide a richer,more stable and efficient solution of information processing.The spontaneous valley polarization is a key to stable information coding.Common valleytronic materials such as Mo S₂ and WSe₂ has nonzero Berry curvature as a result of the breaking of spatial inversion symmetry.However,the inequivalent valleys are degenerate in energy and therefore not spontaneously polarized.With the continuous discovery of the two-dimensional magnetic materials MX₂(M:magnetic metal element,X:non-metal element),time reversal symmetry breaking by magnetism has become an important route towards the valley spontaneous polarization,which is a focus of the present valletronics research.The Janus structure H-MXY(X?Y)further breaks the mirror symmetry in z-direction,building an internal electric field and making its response to applied strain,electric field and magnetic field more notable and effective.There are a diversity of nterlayer stackings,producing richer magnetic properties.In this dissertation,based on first-principles calculations,we use more rigorous methods,i.e.,hybrid functional method,quasiparticle GW method and Bethe-Salpeter equation to study the valleytronic properties of the two-dimensional magnetic Janus structure.The major findings of are as follows:1.The valleytronic properties and modulation of the monolayer magnetic Janus structure H-VSSe.In comparison with the results of conventional density functional theory(DFT),the band dispersion calculated by the GW method has a significant change,and the valley band gap is approximately doubled,indicating that the quasiparticle effects in VSSe are strong.Calculations confirm that VSSe is ferromagnetic,and hence the time reversal symmetry is broken,so that Berry curvature no longer has an odd parity in momentum space.Ferromagnetism results in the different magnitude of Berry curvature at the inequivalent valleys,which leads to considerable Hall anomalous conductivity.The optical response of VSSe exhibits valley selective circular dichroism.Ferromagnetism also causes large valley Zeeman splitting,making the energy of inequivalent valleys significantly different.Therefore,non-polarized light can be used to achieve valley selective excitation.Due to the symmetry breaking,Janus VSSe can be more effectively tuned through the external fields.Since the time reversal symmetry is broken,the valley-Zeeman splitting can be continuously adjusted by changing the magnetization direction.The loss of spatial mirror symmetry in VSSe makes it possible to control the band gap in both directions.Strain can linearly adjust energy valley splitting in a considerable range.Berry curvature and anomalous Hall conductance can also be effectively adjusted in the external field.2.Exciton splitting of monolayer Janus H-VSSe.Optical excitation is an important way to achieve selective excitation of valleys.In a monolayer the electron-hole interaction is strong,and hence the optical spectra calculated by ordinary density functional theory method have large large error.Therefore,we use the GW method to accurately calculate the band gap,and then pass the GW The BSE(Bethe Salpeter Equation)calculation takes into account the electron-hole interaction spectrum.After considering the electron-hole interaction with BSE,the calculated imaginary part of the dielectric function of the monolayer H-VSSe has a significant red shift compared with the result of GW-RPA(Random phase approximation),indicating that H-VSSe has strong electron-hole interaction and that the binding energy of excitons is large.Considering spin orbital coupling(SOC),valley splitting is produced and the first exciton peak is split into two sub-peaks with an interval of140 me V.The two sub-peaks correspond to the optical transitions in the two inequivalent valleys.The study further found that the exciton binding energies of the inequivalent valleys are different,which are 0.688 and 0.727 e V,respectively.Compared with H-VSSe,the splitting of the first exciton peak of H-VSe₂ increases to0.230 e V,which is due to the larger valley splitting of VSe₂.The binding energy of the two sub-peaks of VSe₂is significantly smaller than that of VSSe,indicating that the screening effect of the non-mirror symmetric structure of Janus is stronger.3.The valleytronic properties of the monolayer Janus H-FeClBr.Calculations show that FeClBr is ferromagnetic.The smallest band gap calculated by PBE and PBE-GW is 0.310 and 1.415 e V,respectively.The latter is more than four times that of the former,indicating that the quasiparticle effect in FeClBr is very strong.Orbital analysis based on PBE calculation shows that the highest valence band and the lowest conduction band have band inversion at the valleys.The highest valence and the lowest conduction bands dominated by the Fe-d orbital contribution are relatively flat,which means that the d orbital of Fe is very localized,and therefore the electronic correlation effect needs to be considered.The calculation found that similar results are obtained by using the hybrid functional HSE06 and DFT+U obtained,the minimum band gap is increased to 2.717 e V,and the band inversion disappears.On the basis of our HSE06 results,we continued to calculate with the GW method and found that the minimum band gap increased to 4.043 e V,and the valley splitting reached 123 me V.This shows that FeClBr is a system with strong quasi-particle and electronic correlation effects.We further calculated the Berry curvature of FeClBr,which has opposite signs but different magnitudes at the inequivalent valleys.The calculated circular polarizability of light shows that circularly polarized light of different polarities can be used to selectively excite unequal energy valleys.The optical properties were calculated using the BSE equation,and it was found that the first exciton peak was split into two peaks when with SCO,and the corresponding exciton binding energies were large,which were 1.542 and 1.595 e V at two valleys,respectively.4.Interlalyer magnetism and valleytronic properties in bilayer Janus H-VSSe.The bilayer Janus H-VSSe has three different interfaces:Se VS/SVSe?SVSe/Se VS and Se VS/Se VS,and each interface has six different stackings.In addition,the interlayer spacing can be controlled by hydrostatic pressure.Using the HSE06 method,it is found that the bilayer Janus H-VSSe remains to be magnetic semiconductor and there are still two inequivalent valleys.However,the interlayer magnetic order and valleytronic properties of the bilayer Janus H-VSSe vary with the interface,stacking and interlayer strain.For SVSe/Se VS interface,the interlayer magnetic order of the six stackings is all antiferromagnetic.When compressive strain is applied,the interlayer antiferromagnetic order is maintained.When tensile strain is applied,the interlayer magnetic order of AA1 and AB2 stackings become ferromagnetic.For the SVSe/Se VS interface,the interlayer magnetic order of the six stackings is remained to be antiferromagnetic no matter whether there is or not strain,or whether the strain is tensile or compressive.For the Se VS/Se VS interface:when no strain is applied,except AA1 stack has interlayer ferromagnetic order,the other stackings have interlayer antiferromagnetic order.After the compressive strain is applied,the six stacking magnetic sequences become antiferromagnetic.When a certain degree of tensile strain is applied,AA1 and AA2 stackings become ferromagnetic.The total berry curvature of the valence bands of AB1 and AB2 stackings is zero,while AA1and AA2 have salient berry curvature.