Strain-engineered Topological Phase Transition And Two-Dimensional Weak Topological Insulators With Inversion Symmetry

Since the successful exfoliation of graphene in experiments in 2004,research into twodimensional(2D)materials has gained significant momentum in the past two decades due to their high specific surface area and excellent mechanical,electrical,thermal,and optical properties.Compared with bulk materials,the utilization rate of surface atoms in 2D materials is greatly improved,facilitating the manipulation of their physical properties through surface modification,strain engineering,and doping,providing new opportunities for the development of novel spintronic devices.In addition,discovering and controlling new physical states is also crucial in condensed matter physics.Recently,research on topological states in 2D materials has also been flourishing,unveiling a variety of non-trivial topological states and enigmatic topological quantum phenomena.These states have significant implications for low energy consumption electronic devices,as their topologically protected edge states provide a platform for low-energy-consumption electronic devices.Therefore,in this thesis,we are committed to exploring and discovering new electronic and topological properties of two-dimensional materials,revealing the intrinsic mechanisms of induced topological phase transitions,and making theoretical contributions to the development of two-dimensional topological states.In this thesis,we systematically study the switchable topological phase transition in 2D ferromagnetic Chern insulator EuCd2As2 quintuple layers(QLs)based on density functional theory and band theory of solids.Furthermore,we also propose a new type of 2D weak topological insulator state realized in RhBi2.Chapter 1 provides an overview of 2D materials and topological states,describing the types of topological states and their protection mechanisms.Chapter 2 introduces the first-principles approximation methods,the basics of density functional theory,and the program packages used in this work.Chapter 3 focuses on the strain-induced magnetic ground state transition and topological phase transition in EuCd2As2 QLs,achieving robust quantum spin Hall effect with magnetic configurations.Chapter 4 discusses the possibility of realizing the weak topological insulator phase in 2D materials,considering the effects of spin-orbit coupling on electronic and topological properties,and verifying them from band structure,topological invariants,and edge states.Chapter 5 summarizes and outlines the research issues that can be further explored and studied in this work.The main research contents and innovative aspects of this paper are as follows:(1)The study explores the electronic,magnetic,and topological properties of the twodimensional intrinsic ferromagnet EuCd2As2 QLs,which exhibit the quantum anomalous Hall effect,only occurring in the spin-up subband with energy band inversion,possessing non-zero Chern number and chiral edge states.The research indicates that these properties can be effectively manipulated by strain engineering.The system can transform into a conventional ferromagnetic insulator under compressive strain.While under tensile strain,it exhibits the quantum spin Hall effect.Besides,the study also reveals that EuCd2As2 QLs can undergo a magnetic ground state transition from an out-of-plane ferromagnetic to in-plane antiferromagnetic state under a tensile strain as small as 1%.Remarkably,the obtained QSH effect is highly robust against the magnetic configurations,including FM and AFM configurations with both out-of-plane and in-plane directions.Hereby,we promote EuCd2As2 as a wonderful candidate for understanding and utilizing the complex coupling mechanism between magnetic order and topological states.(2)This study conducted a systematic exploration of the structural,electronic,and topological properties of RhBi2,a 2D nonmagnetic material.The findings of the study suggest the possibility of realizing weak topological insulator phase in 2D materials.Dirac semimetal was observed in 2D RhBi2 without considering spin-orbit coupling.Similar to graphene,the system opens a band gap at the Dirac points when considering spin-orbit coupling effect.RhBi2 was found to possess time-reversal and inversion symmetry,and its parity at the time-reversal invariant momentum points was calculated.The three topological numbers(v0:v1v2)of the material were found to be(0:01),with v0(Z2 invariant)being 0,and v1 and v2 being 0 and 1,respectively.Furthermore,gapless edge states only appear at the(01)boundary,revealing the weak topological properties of the 2D RhBi2.We realize the weak topological insulator phase in two-dimensional electronic materials,which contributes to the further understanding of topological states and expanding the topological classification of two-dimensional materials.