Engineering Electronic And Topological Properties In Two-dimensional Antiferromagnet NaMnBi

The research of condensed matter physics is to discover and describe new states of matter,and to study the processes and properties of their phase transitions.For example,topological insulators are new states of quantum matters that are protected by time-reversal symmetry and characterized by gapless edge or surface states and full insulating gaps in the bulk.Topological states of matter are generally described by topological invariants which overturn the method of describing states of matter by symmetry or the onset of a local order parameter.For example,topological insulators can be labeled with Z2 invariants which can be realized in both two-dimensional(2D)and three-dimensional systems.The introduction of magnetism in topological insulators can realize novel quantum phenomena such as the quantum anomalous Hall effect,which provides a new platform for the innovative design of low-dissipation spintronics devices.Magnetic materials with topologically nontrivial bands have recently attracted significant attention in physics and materials science.Remarkably,antiferromagnets,which have several crucial advantages such as greater stability and higher-speed operation over ferromagnets for spintronics devices.The three-dimensional anti ferromagnetic topological insulators have received special attention and have been widely studied both theoretically and experimentally with exciting quantum phenomena.However,the understanding of 2D antiferromagnetic topological insulators is still limited because the property of three-dimensional systems cannot be generalized to 2D systems.The 2D antiferromagnetic topological insulators are highly desirable and of great importance to be explored.In the paper,we systematically study the electronic properties,magnetic order and topological properties of 2D materials.We deeply analyze the theoretical mechanism of the phenomena,and provide theoretical predictions for experimental research.There are five chapters in the thesis.The first chapter mainly introduces the properties and applications of 2D materials.In the second chapter,we provide the theoretical basis of the first-principles calculation and several first-principles software packages.The theoretical basis includes the approximate method of first-principles calculation and the construction of the maximum localized Wannier function.The approximate method includes adiabatic approximation,density functional theory and exchange correlation functional approximation.The third chapter is the focus of the paper,which mainly introduces the control of the 2D intrinsic antiferromagnetic insulator NaMnBi,including electronic property modulation and topological phase transitions.In the fourth chapter,we present the effects of the magnetic orientation and film thickness on the topological properties of the stress-controlled 2D NaMnBi QL.The last chapter makes a summary of the full text and the prospect of further research in the future.The main results and innovative ideas of the thesis are as follows:(1)An intrinsic 2D antiferromagnetic insulator NaMnBi QL is theoretically predicted.Its exfoliation energy is 44.1 meV?-2 and the thermodynamic stability and the kinetic stability of NaMnBi QL are also demonstrated by phonon dispersion and Ab Initio Molecular Dynamics(AIMD)simulation.The Neel temperature of 2D NaMnBi QL is up to 530 K which show the feasibility of room temperature applications.As it remains magnetic order unbroken in the two dimensions,2D NaMnBi QL provides an intriguing platform for exploring and modulating low-dimensional magnetic structures.(2)The intrinsic 2D antiferromagnetic insulator NaMnBi undergoes a topological phase transition from a trivial insulator to an antiferromagnetic topological insulator at a small compression strain of 0.58%where the band inversion occurs between the d-d orbitals at the Γ point.The non-zero topological invariants of 2D NaMnBi QL with phase transition can be calculated by spin Hall conductance,spin Chern number and the Wannier charge center.The topological invariants for 2D NaMnBi QL are Cs=1 and Z2=1.In addition,the system has a pair of edge states.(3)The topological properties in different magnetic directions are investigated and it is found that the antiferromagnetic topological insulator phase remains unchanged despite the change in magnetic direction.The non-trivial topology is highly robust to the magnetization direction.We provide an in-depth analysis of the theoretical mechanisms,which provides some implications for experimental studies.