Electronic Structures And Properties Of Ternary Quantum Spin Hall Insulators

Topological insulator is a new type of quantum state with internal insulation and edge conduction.Since the discovery of quantum spin Hall states in graphene,namely two-dimensional topological insulators,it has aroused extensive research interest in the fields of condensed matter physics and materials science.The unique feature of the topological insulator is to produce a pair of gapless edge state in the bulk gap,where the edge state is topologically protected by time-reversal symmetry(TRS),and have relatively strong resistance to backscattering,and this kind of peculiar property has more extensive application prospects in spintronics and quantum computing.Recently,a lot of topological insulators have been theoretically predicted,and some of them have been confirmed by experiments.At the same time,the multi-character topological materials formed by topological properties coupled with other properties,or the novel physical phenomena formed have been favored by researchers.When topological properties and other physical properties are coupled,some interesting phenomena will appear.For example,the coupling of electric field,stress and topological order can realize topological phase transition,and the coupling of ferroelastic order and topological order can realize ferroelastic topological insulator.What is interesting is that when the TRS is broken by the magnetic order,the entanglement of magnetism and topology will produce some novel topological quantum phenomena,such as anomalous quantum Hall effect,magnetic Dirac semimetal,magnetic Weyl semimetal and so on,the combination of topology and spintronics provides a new platform for the design and application of dissipationless spintronics devices.In this paper,based on the first-principles calculation method of density functional theory,we systematically discussed the response of topological properties to ferroelastic properties in the ZrAsX(X=Br/Cl)monolayer,and the realization of antiferromagnetic quantum spin Hall effect in RbCuSe/CsMnP heterojunction.This thesis contains five chapters in total.The first chapter briefly summarizes the research and development status of two-dimensional topological insulators in recent years.The second chapter briefly introduces the theory of first-principles calculation involved in this thesis basics,methods and corresponding software packages.In the third chapter,the electronic structure and topological properties of the ZrAsX(X=Br/Cl)monolayer and the mechanical properties related to ferroelastic are studied in detail.In the fourth chapter,we study the magnetic properties of the RbCuSe/CsMnP heterostructure and its related topological properties,and the physical mechanism is further explained through the four-band tight-binding model.In Chapter 5,we summarized the main content and innovations of this paper,and made a prospect for the research field of topological insulators.The main research contents and conclusions of this paper are as follows:(1)Here we propose a two-dimensional ferroelastic topological insulator monolayer ZrAsX(X=Br/Cl)with a tunable topological edge state,the topological edge state of which can be significantly adjusted by switching the ferroelastic ground state.Our research shows that single-layer ZrAsX(X=Br and Cl)is a two-dimensional ferroelastic topological insulator,in which ferroelastic and topological insulator phases appear simultaneously.In order to show its topological characteristics,we calculated the spin Chen number and topological edge states of ZrAsBr and ZrAsCl,and the results directly represented the phases of the two-dimensional topological insulators with band gaps of 57.25 meV and 69.18 meV,respectively.It is worth noting that,due to the ferroelastic properties,ZrAsX exhibit not only a low transition barrier but also a strong ferroelastic signal,which is beneficial for application in nonvolatile storage devices These interesting behaviors have important theoretical and experimental research significance for the further development of the basic exploration and practical application of two-dimensional ferroelastic and two-dimensional topological insulators.(2)Based on first-principles calculations,we found that there are gapless edge states in antiferromagnetic heterjunction RbCuSe/CsMnP,which revolutionized people's understanding of time-reversal symmetrical breaks and topological edge states,and revealed the possibility of the magnetic quantum spin Hall effect.The quantized spin Hall conductance,Wannier charge center and gapless edge states prove the existence of topological nontrivial phase of RbCuSe and RbCuSe/CsMnP.In addition,we established a general tight-binding model and gave the phase diagram of the antiferromagnetic quantum spin Hall effect in a two-dimensional square lattice.The research results reveal the new possibility of the quantum spin Hall effect under the time-reversal symmetry breaking,opening up a new direction for the design of low-dimensional magnetic topological materials.