First-principles Functional Material Design Combining Ferroelectric,Magnetic,and Two-Dimensional Structure

To find materials with specified functionalities is crucial to modern technol-ogy.However,perfect materials with specific technology-enabling functionalities are quite rare.I have been focusing on how can combine different functionalities-ferroelectric,magnetic,topological,and two-dimensional-into one system to make multifunctional materials during my Ph.D.study.In this thesis,I will present three works I have finished during my Ph.D.study.My first work is reforming the famous BiFeO₃?BFO?into Bi2FeMo06?BF-MO?to induce both strong polarization and macroscopic magnetism in this sys-tem.The ferroelectric polarization from berry phase method is 85 ?C/cm² as high as BiFeO₃.The structure parameters and Born effective charges indicate that the origin of the ferroelectric distortion comes from the Bi 63 lone pair,which is the same as BiFeO₃.Moreover,BMFO has been proved to be a room temperature ferrimagnetic semiconductor with net magnetic moment 2?B/f.u.,Curie temperature 650 K and energy band gap 0.54 eV.Further,simulations shows that the magnetic moment has an easy axis collinear with ferroelectric polarization,which can be used to switch the magnetic moment by electric field.By combining ferromagnetism,topological band structure and two-dimensional structure,we systematically study the the ferromagnetic properties and quantum anomalous Hall?QAH?effect in the monolayer CoBr₂.we predicted that mono-layer CoBr₂ can have a ferromagnetic phase under 27 K with magnetic moment 3?B/f.u..Further DFT combined with tight-binding?TB?modeling identify its topological non-trivial band structure with chern number 4.So,we believe that the monolayer CoBr₂ can be a QAH insulator with the temperature as high as 27 K which is far larger than the observation temperature in the experiment.In addition,we have calculate the revolution of the topological band structure according to the extern strain and the width of the nanoribbon.We find that the edge states can be 100%spin-polarized when the nanoribbon is narrow enough,which can be a promising candidate in the modern electronic technology.By combining the mechanical flexibility in two dimensional material and the strain tunable ferroelectric polarization in ferroelectric material,a room temper-ature flexible two-dimensional ferroelectric BiN is predicted.It has a very large ferroelectric polarization,especially when it is stacked into Van der Waals bulk structure,which can be 88 ?C/cm².Further simulations show that both the band structure and energy band gap can be easily tuned by extern strain,which make us believe two-dimensional BiN can be used as a good candidate in the flexible electronic devices.Also,we systematically study the oxygen-octahedra rotations in the per-ovskites with DFT calculations combining Landau second order phase transition models.As we all know,the octahedra tilts characterize many of the most impor-tant perovskite compounds.Because the tilting determines the overlap between the electronic orbitals,many novel properties of such materials are related to such oxygen-octahedra rotations.Our work thus provides a comprehensive un-derstanding and reference on this family of the material,which will be useful to identify and design new strategies for materials engineering.