Design Of Novel Two-dimensional Van Der Waals Heterojunction And Control Of Quantum Propertie

Traditional electronic devices fail to meet the demands of individuals in their daily life with the advancement of society and progress in science and technology.Consequently,there is a growing expectation for multifunctional,miniaturized,and energy-efficient electronic devices to overcome these limitations.In this pursuit of constructing high-performance novel electronic devices,new magnetic functional materials play an indispensable role.Therefore,active exploration of magnetic functional materials exhibiting multiple exceptional properties has emerged as a prominent research focus in the field of materials science.Currently,multi-ferroic materials have garnered significant attention due to their remarkable characteristics such as ferroelectricity,ferromagnetism,and ferroelasticity.However,practical applications have revealed that conventional single-phase multi-ferroic materials face challenges in weak magnetoelectric coupling effects and low-temperature working environments.Henceforth,it has become crucial to identify and design multi-ferroic heterostructure materials with diverse functionalities.Four ferromagnetic（FM）/antiferromagnetic（AFM）heterostructures are investigated by using density functional theory（DFT）,tight binding model analysis,and Monte Carlo simulations to explore their potential application value in spintronic and valley electronics devices.The primary research contents and conclusions are summarized as follows:1.Design and magnetic prediction of a high-temperature valley-polarized AFM van der Waals heterostructure.AFM materials hold significant potential for spintronics devices due to various fascinating features,such as absence of parasitic stray field,robustness against external magnetic field perturbation and ultrafast dynamics absence of stray fields.Based on the aforementioned reasons,a heterostructure based on two-dimensional（2D）monolayers MoSi₂N₄ and MnPS₃ is presented,investigating the magnetic proximity effect between MnPS₃ and MoSi₂N₄ and analyzing the electronic structure and magnetic properties of the MoSi₂N₄/MnPS₃ heterostructure.The result demonstrates that the 2D MoSi₂N₄/MnPS₃ heterostructure exhibits a type Ⅱ band structure.The electronic states nearest to the fermi level in conduction and valence bands are separately stemmed from the Mn-d_x2-y2 and Mo-dz2+d_x2-y2,s-px,y orbitals.Notably,the Neel temperature of the MoSi₂N₄/MnPS₃ heterostructure reaches approximately 340 K,surpassing those of many other 2D AFM materials and thus holding significant application value.Theoretical analysis further reveals that coupling between monolayer AFM MnPS₃ and MoSi₂N₄ break time-inversion symmetry in MoSi₂N₄,resulting in energy degeneracy at K/K’ valleys with spontaneous valley polarization values of 9.15 meV in valence band maximum and 9.11 meV in conduction band minimum,respectively.Moreover,application of biaxial strain,modification of interlayer spacing,and manipulation of magnetization direction effectively regulate valley polarization in the MoSi₂N₄/MnPS₃ heterostructure.Therefore,designing a 2D AFM MoSi₂N₄/MnPS₃ heterostructure holds great significance for developing high-temperature AFM van der Waals heterostructures while promoting their practical applications in valleytronic devices.2.Design and theoretical prediction of AFM van der Waals heterostructure with anomalous Valley Hall（AVH）effect.For 2D valleytronic devices,the controllability and non-volatility of valley polarization are crucial challenges in the application of AVH effect devices.Therefore,it is imperative to actively explore new 2D valleytronic materials and implement their applications.Based on the aforementioned reasons,the 2D AFM HfN₂/MnPSe₃ heterostructure is proposed to realize the AVH effect.The intrinsic magnetic field provided by the 2D AFM MnPSe₃ breaks the time-inversion symmetry of the HfN₂ layer in HfN₂/MnPSe₃ heterostructure,resulting in valley polarization.Simultaneously,introducing an HfN₂ layer breaks the spatial-inversion symmetry of MnPSe₃,leading to energy degeneracy at K+/K-valleys.Theoretical results demonstrate strong AVH effects in both 2D MnPSe₃ and HfN₂ structures with time and spatial symmetries broken in HfN₂/MnPSe₃ heterostructure.To effectively manipulate electron valley polarization characteristics in the HfN₂/MnPSe₃ heterostructure,we further introduce a ferroelectric substrate SC₂CO₂ and construct a three-layered HfN₂/MnPSe₃/Sc₂CO₂ heterostructure.This design enables multistate regulation involving ferrov alley,ferroelectricity,and ferromagnetism simultaneously.In addition,we use the ferroelectric polarization of SC₂CO₂ to regulate the AVH effect in the HfN₂/MnPSe₃ heterostructure.Our proposed three-layered HfN₂/MnPSe₃/Sc₂CO₂ heterostructure not only facilitates studies on AVH effects in 2D AFM systems but also promotes practical applications of AVH effect-based heterostructure in valleytronics.3.Design and performance control of a multiferroic van der Waals heterostructure with both multiferroic and valley polarization characteristics.Exploring the coupling properties between multiferroic and valley properties in 2D materials holds great significance for the development of next-generation memory devices.In this study,monolayer ScI₂ with spatial-inversion symmetry broken utilizes interlayer van der Waals interaction to induce valley polarization,and validate the feasibility of this mechanism in a bilayer ScI₂ system.By constructing various types of bilayer ScI₂,we analyze the mechanisms between interlayer magnetic coupling and valley characteristics.Our findings reveal that interlayer ferroelectric slipping plays a crucial role in magnetoelectric coupling as well as ferroelectric-valley coupling.Consequently,valley polarization can be jointly manipulated by the direction of magnetization and ferroelectric polarization in bilayer ScI₂.Notably,we achieve coexistence of valley polarization with 114 meV and ferroelectric polarization in 3R-stacked bilayer ScI₂,surpassing previous reports on related 2D heterostructures.Additionally,we calculate the slipping energy barrier and plane-averaged electrostatic potential in different stacking configurations.A high ferroelectric polarization value with 3.0×10^-11 C/m is obtained in 3R-stacked bilayer ScI₂ that lack of spatial-inversion symmetry.This work expands the application scope of 2D multiferroic heterostructures in spintronics while providing feasible design ideas for multi-state memory devices based on electroreading and electrowriting.4.Design and theoretical prediction of high-temperature quantum anomalous Hall（QAH）effect in van der Waals heterostructures.Germanene has been successfully synthesized in experiments and exhibits characteristics of a topological insulator,possessing a large band gap.The realization of germanene spin polarization with quantum anomalous Hall efect remains a formidable challenge.In this work,we fabricate Ge/NiI₂ van der Waals heterostructure and demonstrate that the heterostructure exhibit Chern insulator behavior with out-of-plane magnetization.Monte Carlo simulation reveals that the Curie temperature of the Ge/NiI₂ heterostructure can reach up to 238 K,significantly higher than that of bulk NiI₂,making it promising for spintronics applications.Analysis of atomic-resolved magnetic anisotropy energy（MAE）,orbital-resolved MAE and density of state demonstrates that the enhanced MAE in the Ge/NiI₂ van der Waals heterojunction is primarily attributed to competition between interface Ⅰ atoms py,Px and py,pz hybrids.Furthermore,we propose an effective k·p model to explain the intrinsic QAH effect in Ge/NiI₂ heterostructure.Additionally,by reducing the interlayer distance of the Ge/NiI₂ van der Waals heterostructure,we can effectively enhance the out-of-plane MAE of monolayer NiI₂ and thereby increase its Curie temperature.Our proposed method for manipulating MAE in 2D FM semiconductors through regulation of interlayer distance in van der Waals heterostructures provides an effective approach for designing new Chen insulators.