First-Principles Study On The Singular Physical Properties Of Two-Dimensional Ferroelectric-Type Ferrovalley Materials

In some crystalline materials,the edge of the conduction or valence band will appear extreme point of energy,which is similar to the shape of a valley,so it can be called valley.These positions have degenerate energy and different momentum.Due to the separation of momentum space,the scattering between the carriers on the valley is very weak,and there is enough time to manipulate it.Therefore,the valley is regarded as a new degree of freedom,and the valleytronics which manipulates the valley degree of freedom is born.In the previous studies,the valley degrees of freedom could not form a strong coupling with the external field,which was difficult to directly manipulate.However,with the development of valleytronics,it is found that the valley degrees of freedom in Mo S₂ monolayer can be coupled to the external field strength through the valley Hall effect and valley dependent selective absorption of circularly polarized light.Furthermore,ferrovalley materials with spontaneous valley polarization represented by VSe₂ monolayer have also been proposed.However,the means to control the valley polarization in VSe₂ monolayer is magnetic field,which is not conducive to the miniaturization and integration of modern devices.On this basis,ferroelectric-type ferrovalley materials represented by Ge Se are proposed,and the valley polarization and ferroelectricity are strongly coupled.The appearance of ferroelectric-type ferrovalley materials expands the concept of ferrovalley materials and promotes the development of valleytronics and ferroelectric field.Therefore,further research on ferroelectric-type ferrovalley materials is necessary.In the thesis,the properties of ferroelectric-type ferrovalley materials are studied by first-principles calculations.The thesis is divided into six chapters.Chapter 1 is a brief introduction of the research progress of two-dimensional ferroelectric materials and valley materials in recent years.Chapter 2 introduces the theoretical basis and research tools of the thesis.The theoretical foundation part includes the development of band theory of solid and density functional theory.Chapter 3 to Chapter 5 are the work content.Chapter 6 is the summary and prospect.The work content is as follows:Chapter 3 is about the flexoelectric effect in Ge Se monolayer.It is found that the highest occupied molecular orbital（HOMO）and the lowest unoccupied molecular orbital（LUMO）will separate in real space and form a type-Ⅱband alignment when the Ge Se monolayer is bent along the armchair（polarization）direction.When curved along the zigzag（non-polarized）direction,the HOMO and LUMO coincide in real space,forming a type-I band alignment.Then we analyze the cause of the type-Ⅱband alignment.The flexoelectric field will be generated,and the in-plane dipoles generated by the ferroelectric polarization will be arranged along the tangential direction of the bending,and the opposite out-of-plane components will be generated on both sides.Additional opposite electrostatic energies are generated on both sides of the structure,resulting in typeⅡ-band alignment.The existence of a flexoelectric field is proved by calculating the response of the bending structure to external electric field in the±z direction.We believe that this phenomenon is not limited to Ge Se monolayer,and similar phenomena will occur under the influence of flexoelectric effect whenever there is an in-plane dipole.Subsequently,we analyze the electrical,optical and transport properties of bending Ge Se monolayers.The Ge Se monolayer,which is curved along the polarization direction,exhibits similar rectification properties as the p-n junction.And it is expected that the flipping polarization direction,the HOMO（n-type）and the LUMO（p-type）positions will also exchange.Chapter 4 focuses on the valley property of Ge Se monolayer.As a ferroelectric-type ferrovalley material,the two valleys of Ge Se monolayer with valley polarization occurs are V_x and V_y valleys linked by lattice symmetry,and the energy of V₊and V_-valleys linked by time inversion is still degenerate.In this chapter,we analyze the anisotropic band splitting caused by spin-orbit coupling in Ge Se monolayers,and reveal the conditions for the valley polarization of V₊and V_-valleys in Ge Se monolayers.Then,valley polarization between V_+x,V_-x and V_+yand V_-y are realized in Ge Se monolayer by doping and heterojunction design,and four valleys polarization states that can be switched between the external electric field and the magnetic field are obtained.In addition,the Boltzmann equation is used to analyze the special transport property of Ge Se monolayer In Ge Se monolayer,the carriers on V₊and V_-valley will produce opposite transverse valley flow,which makes the separation of carriers with different momenta can be considered as a new Hall effect.Finally,the influence of doping and heterogeneous association on the transport property is also analyzed.In Chapter 5,the research object is CIPS（Cu In P₂S₆）monolayer,another material with ferroelectric and valley properties.First,we calculate the ferroelectric polarizability of and the selective absorption of circularly polarized light to determine its ferroelectric and valley properties.It is found that although CIPS monolayer has both ferroelectric and valley properties,there is no strong coupling between them.However,there are pseudospin textures strongly coupled to ferroelectric polarization on the valley of CIPS monolayer.When the ferroelectric polarization is flipped,the chirality of the pseudospin texture will also change.This indicates that CIPS monolayer can be considered as a more generalized ferroelectric ferrovalley material,and it also proves that CIPS monolayer is a novel ferroelectric Rashba semiconductor.Further analysis of k·p model shows that there are two types of pseudospin texture in CIPS monolayer,one is the chiral Rashba-type pseudospin texture,and the other is a divergent Dresslhaus-like pseudospin texture.More interestingly,when we analyze the change of pseudospin texture during the transition from paraelectric to ferroelectric phase,we find that the Dresslhaus-like pseudospin texture at point K will disappear and the Zeeman splitting at point K will also disappear.This indicates that there is an in-plane dipole cancellation during the phase transition,which means that there may be in-plane ferroelectric in the CIPS monolayer.