Theoretical Studies Of Ferroelectricity And Valley Properties Of Two-Dimensional Materials

The charge,spin and valley degrees of freedom of electrons spawn the traditional electronics,spintrocisc and valleytronics.The dual states can be used as information carriers to maniputate and process data,which construct the basis of modern information industry.The boom in traditional electronics has laid the foundation for the information industry in the past few decades.However,with the development of traditional electronics,the limitation of thermodynamics becomes more obvious.Spintronics and valleytronics which are based on spin and valley degrees of freedoms will emerge as promising route to overcome the above limitation.The appearance of multiferroicity and the coupling between the multiferroicity and these degrees of freedom provide an ideal platform for the mutual regulation of various degrees of freedom and promote the cross fusion o fmultiple research fields.At the beginning of this thesis,we introduce the development process of valleytronics,as well as the composition of the valleytronics materials,and summarizes the various properties of ideal valleytronics materials.Based on the first-principle calculation,the research is carried out from two main aspects:firstly,the discovery of new materilas with excellent properties that can be used in valleytronics and ferroelectricity;Secondly,the regulation of degrees of freedom is carried out througu new strategies on the basis of existing research.Combind with theoretical models and other means to broaden research ideas and enrich research connotation,which can provide theoretical basis for relevant experimental research,and promoted the application of new degrees of freedom.We summaried the main research contents and conclusions of this thesis:1.Two-dimensional grapheme-like materials with the hexagonal lattice structure similar to graphene,which can be regarded as the substitution of some carbon atoms in graphene or the hybridization of graphene and another hexagonal lattice.Such materials can inherit or surpass the the excellent properties of graphene.Our theoretical calculations show that g-BC6N monolayer and GaAsC6 monolayer posses the valleyselective circular dichroism and high carrier mobility,which better for the application of valleytronics.Firstly,g-BC6N monolayer and GaAsC6 monolayer have a direct band gap at K valley,and the band gap(g-BC6N:1.833 eV;GaAsC6:1.937 eV)is in the visible light range.Secondly,the system has a pair of inequivalent valleys due to the inversion symmetry breaking.They also have high carrier mobility and more conductive to the response of devices,which provide a good platform for the study of valleytronics devices.More interesting,due to the difference in the atom radius of GaAsC6 monolayer,an out-of-plane spontaneous electric polarizaiotn was obtained.The integration of valley polarization and ferroelectric properties in a single material offers a promising platform for the design of electronic devices.2.In multiferroic materials,the valley polarization can be tuned by the ferroicity.Multiferroics contain two or more ferroicities,which are ideal materials for information storage.In recent years,magnetoelectric coupling of ferroelectric and ferromagnetic properties is the most widely studied field.It is urgent to study the valley degree of freedom in multiferroic materials coexist ferroelectric and ferromagnetic.In the paper,it is proved that CuCrP2Te6 monolayer is a two-dimensional multiferroic material with intrinsic valley polarization and spin polarization by the first-principle calculation.Due to the inversion symmetry breaking,the corners of Brillouin zone has the characteristics of conduction valley,and the Berry curvature at two valleys have opposite sign.The absence of time-reversal symmetry,a valley splitting of 110.4 meV at the valley,which has a great advantage to realize the anomalous valley Hall effect.In addition,the valley splitting can be tuned by various methods,such as modulating the spin polarization,electric polarization and external strain.This work reveals the interaction between valley polarization and multiferroicity and provides a unique strategy for valleytronics.3.Valley splitting stems from spin-orbit coupling ang time-reversal symmetry breaking.The modulation of valley splitting is not only of fundamental but is also crucial to valleytronics.In this paper,we propose a static and reversible valley splitting in an antiferromagnetic bilayer/ferroelectric heterostructure.With the antiferrovalley VTe2 bilayer and ferroelectric substrate Ga2S3 monolayer formed VTe2/Ga2S3 heterostructure as example,the valley degeneracy can be break of the substrate with valley splitting.With the increase of substrate,the valley splitting can reach 132 meV.More interestingly,both the Berry curvature and the caeeier spin are reversed by switching the ferroelectric direction of the substrate,resulting in tunable anomalous valley Hall effect.Our results offer a promising strategy for all-electrical reading and writing memory devices.4.In valleytronics,with the band gap of one valley is closed,but the band gap of another valley still exists,this state is similar to the spin half-metal state and is called half-valley-metal state,but materials with intrinsic half-valley-metal state are very rare.In the paper,using Floquet theory,we demonstrate half-valley-metal states in periodic laser-engineered two-dimensional gaped Dirac materials.The results show that 100%valley and spin polarization can be achieved with a circular laser is applied to a twodimensional gaped Dirac material,which is important fro the realization of the valley Hall effect.The band gap can of the two valleys can be tuned by changing the photon energy,amplitude and chirality of the laser.In the work,a promising candidate material BSb monolayer is proposed,in which one valley can be closed and another is opened under certain photon energy and laser amplitude.The Berry curvature in the valley can be tuned bu changing the laser parameters.