Theoretical Study On Novel Physics Of Two-dimensional Ferroic Materials

Two-dimensional materials possess unique structures and rich physics,which not only provide an ideal platform for exploring novel physical effects,but also lay a solid foundation for developing high-performance,intelligent and miniaturized devices.Among them,two-dimensional ferroic materials(ferroelectric materials,ferromagnetic materials,and ferroelastic materials)have attracted great attention due to their enormous potential in the next-generation non-volatile information storage technology.Ferroelasticity is of particular interest among the ferroic properties of two-dimensional materials because it can couple with anisotropic physical properties,providing the possibility for developing novel controllable devices.Moreover,the coupling between different ferroic properties of two-dimensional materials can create multiferroicity,which has important applications in high-density storage,energy conversion and signal generation.In addition,multiferroicity and new degrees of freedom such as valley can induce interesting coupling effects in certain special twodimensional systems,resulting in novel physics,such as spontaneous valley polarization and layer Hall effect.The exploration of multiferroic-valley coupling effect lays the foundation for the development of novel valleytronic devices in the post-Moore era.In this dissertation,we systematically study a series of novel physics in two-dimensional ferroic materials,discussing the relationship between ferroelasticity and anisotropic physical properties,multiferroic coupling effect,and multiferroic-valley coupling effect.We explore the application potential of two-dimensional ferroic materials in controllable,non-volatile storage and valleytronic devices.These results have theoretical significance for further experimental research and application.This dissertation is divided into six chapters:in chapter one,we briefly introduce the ferroic properties of two-dimensional materials(including ferroelectricity,ferromagnetism and ferroelasticity),the coupling between ferroicity and electronic properties,multiferroicity,valley physics and the corresponding research progress;in chapter two,we introduce the theoretical basis,calculation methods and software used in this dissertation;in chapter three,we study the relationship between ferroelasticity and anisotropic physical properties in two-dimensional ferroic materials;in chapter four,we investigate the construction of multiferroic coupling mechanism via weak interlayer interaction;in chapter five,the multiferroic-valley coupling effect based on weak interlayer interaction is explored;in chapter six,we summarize the main conclusions and innovations of this dissertation,and provide the outlook for further research.The detailed research contents are listed as follows:(1)Realizing the directional control of electronic behaviors in two-dimensional materials is of great significance for the development of novel controllable nanodevices.At present,the major problem in this field is the lack of effective control means.Two-dimensional ferroic materials with the coupling effect of ferroelasticity and anisotropic electronic properties bring a new opportunity for the directional control of electronic behaviors.Based on this,we propose a mechanism to control the direction of anisotropic electronic behaviors by using the ferroelastic phase transition.We find that single-layer α-MPI(M=Zr,Hf)not only exhibits ultrahigh carrier mobility with an anisotropic character,but also harbors excellent ferroelasticity.They show moderate switching barriers and distinct ferroelastic signals,which are conducive to the occurrence of ferroelastic phase transition.Meanwhile,the carrier mobility of singlelayer α-MPI is extremely high along the y direction,while low along the x direction,resulting in anisotropic carrier migration behavior.Based on the coexistence of ferroelasticity and anisotropic electronic properties,we achieve precise directional control of carriers’ migration by 90° reversible ferroelastic phase transition,which provides a feasible method for the design of novel controllable electronic devices.We further apply this mechanism to single-layer Nb2ATe4(A=Si,Ge).Single-layer Nb2ATe4 can be exfoliated from its layered bulk,exhibiting excellent dynamical,thermodynamic and mechanical stability.The high reversible ferroelastic strain and moderate ferroelastic barrier endow single-layer Nb2ATe4 with excellent ferroelasticity.In addition,single-layer Nb2ATe4 also exhibits remarkable anisotropic properties,including anisotropic carrier mobility and optical properties.More importantly,the anisotropic properties of single-layer Nb2ATe4 can be efficiently controlled through ferroelastic switching.The discovery of the relationship between ferroelasticity and anisotropic properties provides an ideal scheme for the realization of novel controllable devices.(2)Two-dimensional multiferroic materials with coexistence of two or more ferroic orders show great advantages in high-density multi-state storage.At present,the research on two-dimensional multiferroic materials is mainly based on the paradigm of unique symmetries in single-layer lattices,which severely limits their development.Based on this,we propose a design principle for realizing ferroelastic-ferroelectric multiferroicity in van der Waals bilayer lattice using weak interlayer interaction as perturbation.Through layer-stacking,we demonstrate that bilayer ZrI3 not only exhibits 120° ferroelasticity induced by crystal symmetry,but also in-plane and out-of-plane ferroelectric polarization introduced by interlayer charge redistribution.Thus,ferroelastic-ferroelectric multiferroicity is realized in bilayer lattice.The reversal of out-of-plane polarization relates to interlayer sliding,while the reversal of in-plane polarization correlates with 120° ferroelastic switching.The coupling between ferroelasticity and ferroelectricity endows bilayer ZrI2 with six-logic-state multiferroicity.Undoubtedly,multiferroicity has important basic research and practical application value in the field of condensed matter physics,especially triferroicity with three ferroic orders at the same time.However,two-dimensional intrinsic triferroicity,especially ferromagnetic triferroicity,has rarely been explored.Here,based on weak interlayer interaction,we propose a design scheme to realize ferromagnetic triferroicity starting from the mechanism of interlayer sliding in two-dimensional van der Waals lattice.Further,we demonstrate the feasibility of this scheme in van der Waals bilayer T’-VTe2,which exhibits ferromagnetism,ferroelasticity and ferroelectricity simultaneously,thus achieving triferroicity.In addition,we also predict novel physical phenomena such as ferroelastic control of magnetization orientation and ferroelectric control of magnetic moment distribution in bilayer T’-VTe2.These findings enrich the research on two-dimensional multiferroics.(3)Exploring the coupling effect between multiferroicity and valley properties in two-dimensional materials will benefit the development of next-generation information storage technologies.Currently,spontaneous valley polarization in two-dimensional materials can only be obtained from inversion asymmetric single-layer crystals,while inversion symmetric single-layer lattices are unable to generate spontaneous valley polarization.Here,starting from inversion symmetric single-layer lattices,we map out a physical mechanism for realizing two-dimensional spontaneous valley polarization based on weak interlayer interaction.We further demonstrate the feasibility of this mechanism in T-FeCl2.Remarkably,the interlayer A-type antiferromagnetism and valley properties are related through sliding ferroelectricity,thereby realizing ferroelectric-valley coupling and magnetoelectric coupling.Layer Hall effect,derived from the coupling between Berry curvature and layer degree of freedom,is of great significance in both fundamental physics and device applications.However,current research on the layer Hall effect is limited to topological systems,making this effect rather scarce.Based on this,through kp model analysis,we propose a physical mechanism to realize the layer Hall effect in two-dimensional magnetic valley systems:based on weak interlayer interaction,we construct antiferromagnetic bilayer valley systems,where the out-of-plane ferroelectricity and A-type antiferromagnetism are related to the valley properties.This multiferroic-valley coupling effect generates layer-locked Berry curvature,thereby realizing the layer Hall effect.Interestingly,the layer Hall effect can be strongly coupled with sliding ferroelectricity,making the layer Hall effect ferroelectrically controllable.We further demonstrate the feasibility of this mechanism in a series of two-dimensional valleytronic materials,including bilayer VSi2P4,VSi2N4,FeCl2,RuBr2 and VClBr.These works not only provide important insights into the research on multiferroic-valley coupling effect in two-dimensional materials,but also broaden the range of two-dimensional materials that can be used to realize spontaneous valley polarization or layer Hall effect.