First-Principles Study Of Valley Splitting And Modulation In Two-Dimensional Materials

With the continuous advancement of technology,improvements in chip performance are gradually approaching physical limits,and Moore’s Law is facing unprecedented challenges.Against this backdrop,the emergence of valleytronics in two-dimensional materials offers a promising solution to this bottleneck.Valleytronics leverages the valley degree of freedom to provide a novel method for information processing and storage.This technology not only has the potential to surpass the performance limitations of traditional semiconductors but also offers more efficient and flexible solutions for future electronic devices.Therefore,the research and development of valleytronics in two-dimensional materials has become a crucial focus at the forefront of contemporary science and technology.In current research in the field of valleytronics,Transitional Metal Dichalcogenides（TMDs）are recognized as high-quality valleytronic materials due to their exceptional physical properties and have garnered considerable attention from researchers.TMDs not only exhibit strong spin-orbit coupling but also possess valley-related Berry curvature and significant valley splitting characteristics under external fields.Valley splitting,the energy separation between different valleys in multivalley semiconductors,breaks valley degeneracy and is crucial for the application of valleytronics,providing a choice for electrons between different valleys.Despite TMDs’significant potential in the field of valleytronics,exploring and researching a broader range of materials is equally crucial for advancing the field.Such"new"materials may also possess excellent valleytronic performance.Researching these materials can not only break the existing paradigm and enrich our understanding of valleytronic phenomena but also reveal new valley-related physical effects,promoting the development of more efficient and multifunctional valleytronic devices.Based on the research background provided,this thesis primarily explores novel valleytronic materials using density functional theory calculations based on first principles.Our research is not limited to specific structural types or singular valleytronic physical phenomena.Through theoretical calculations,we propose several two-dimensional valleytronic materials with valley splitting and demonstrate the control of valley splitting through external fields.These studies aim to reveal the complex relationship between the unique electronic structures and valleytronic properties of different materials,hoping to positively impact the design and application of future valleytronic devices.The specific research contents of this thesis are as follows:1.Spontaneous Valley Splitting and Its Regulation in Two-Dimensional MXenes Material Y3N2O2The scarcity of spontaneous valley splitting materials is a key factor limiting the rapid development and widespread application of valleytronics.We propose a monolayer transition metal nitride material,Y₃N₂O₂,through first-principles calculations.The calculations show that Y₃N₂O₂ can spontaneously generate a valley splitting of 21.3 me V without external manipulation.The spontaneous valley splitting arises from the combined effect of the inherent magnetic effect of the atoms and the spin-orbit coupling（SOC）effect.We analyze the related properties of valley splitting through an effective SOC model and confirm the presence of the anomalous valley Hall effect in the system through Berry curvature calculations.Meanwhile,we achieve control over valley splitting by altering the magnetization direction,applying biaxial strain,adjusting inter-electron interactions,and changing the system’s SOC strength.Finally,based on the positive correlation between valley splitting and SOC strength,we design the La₃N₂O₂ material,finding its spontaneous valley splitting value can reach 100 me V.These studies provide new insights into designing and developing novel valleytronic materials and devices with specific functions.2.Chiral Breathing Valley Lock Effect in Two-Dimensional Kagome LatticesSwitching valley states is a necessary condition for achieving precise control of valleytronic information transmission,but this process typically requires altering the system’s magnetization direction.This study introduces an innovative concept known as the"chiral breathing valley lock"effect,primarily analyzed through first-principles calculations on the two-dimensional Kagome lattice structure Ta₃I₈.This system exhibits two chirally symmetric breathing phases（α-phase andβ-phase）,which can transition into each other under certain conditions.Further research reveals that these two chirally symmetric breathing phases correspond one-to-one with the two valley indices（K and-K）.This phenomenon provides an additional degree of freedom for the valley Hall effect,allowing the effect to switch between the K and-K valleys while fully preserving the material’s electronic spin characteristics.Simultaneously,through the analysis of the SOC effective model,we detail the relationship between valley splitting values and SOC strength,and successfully achieve regulation of valley splitting through external fields.Lastly,leveraging the material’s response to external electric fields and the electronegativity difference between the upper and lower atoms,we construct a Janus Ta₃I₄X₄ material with a sub-built-in electric field.This sub-built-in electric field can interact with its unique breathing mode to achieve coordinated regulation,reaching a maximum valley splitting value of 246 me V.This study successfully expands the boundaries of our understanding of valley-related physical phenomena and establishes a foundational platform supporting future device applications and physical mechanism exploration in the field of valleytronics.3.Spontaneous Valley Splitting Induced by Valley Layer Coupling Effects in Janus HfZrSiCO₂Valleytronics research primarily focuses on hexagonal crystal system magnetic materials,which have weak resistance to magnetic interference.Through the coupling of valley-layer effects and built-in electric fields,we successfully achieve spontaneous valley splitting in the Janus HfZrSiCO₂ system.Interestingly,in HfZrSiCO₂,the electrons in different valleys are contributed by different atomic layers,leading to the breaking of time-reversal symmetry in the system.Computations also revealed the presence of two types of valley excitons:inter-valley excitons and intra-valley excitons.The electrons and holes of intra-valley excitons originate from different atomic layers,hence referred to as inter-layer excitons,while those of inter-valley excitons come from the same atomic layer,hence referred to as intra-layer excitons.The location of these two types of excitons in momentum space and real space suggests they have a long valley exciton lifetime.Additionally,we discover that the valley splitting in the Janus HfZrSiCO ₂ system is tunable and achievable through the application of external electric fields and biaxial strain.The ground state of valley excitons can also switch from inter-layer（intra-valley）bright excitons to intra-layer（inter-valley）dark excitons with the control of external fields.Lastly,we conduct studies on the valley-contrast linear dichroism of this system.These studies indicate that Janus HfZrSiCO₂,which possesses both spontaneous valley splitting and a long valley exciton lifetime,has significant implications for valleytronics research.In summary,using first-principles calculations based on density functional theory,we design and predict a series of new valleytronic materials and conduct in-depth studies on valley splitting and its control.This provides greater possibilities for exploring unknown valleytronic materials and discovering new valley-related physical phenomena.