Quantum Spin Hall Effect In Antiferromagnetic Heterobilayer KCuSe/NaMnBi

The discovery of topological insulators(TIs)is a milestone in the history of condensed matter physics.Its peculiar properties,in which the bulk is insulating while the edges are conducting,have revolutionized people’s understanding of the states of matter and attracted widespread attention in fields such as condensed matter physics and materials science.The main difference between topological insulators and traditional insulators is that within the insulating bulk energy gap of a topological insulator,there exist gapless edge states which are protected by the time-reversal symmetry(TRS)of the system.Therefore,the movement of electrons at the boundary can effectively resist backscattering,which has huge potential applications in spintronics devices and quantum computing.Interestingly,when TRS is broken by magnetic order,the complex interaction between magnetic order and topological properties can give rise to numerous novel quantum phenomena,such as the quantum anomalous Hall effect,magnetic Weyl semimetal,antiferromagnetic topological insulator,axion insulator,etc.These not only greatly enrich our understanding and comprehension of topological states but also provide new opportunities and design platforms for achieving high-speed,non-dissipative electronic devices.At the same time,two-dimensional materials have received much attention in the field of spintronics due to their high surface-to-volume ratio and excellent controllability in recent years.The combination of two-dimensional materials with magnetic order and topological properties has significantly enriched our exploration of topological states.However,research on two-dimensional magnetic topological states is still in its early stages and requires further exploration.In this paper,we systematically studied the electronic,magnetic and topological properties of KCuSe quintuple layers(QLs)and KCuSe/NaMnBi van der Waals(vdW)heterostructures based on first-principles calculations.We revealed the influence and the laws of action of magnetic order on topological properties,and explored the possibility of realizing quantum spin Hall effect(also known as 2D topological insulator)in two-dimensional antiferromagnetic materials.This paper consists of four chapters.In Chapter 1,we introduced the research process and development status of topological insulators in recent years,including 2D and 3D topological insulators and magnetic topological insulators,and briefly described their characteristics.In Chapter 2,we provided a detailed introduction to the theoretical basis,research methods and corresponding software and codes involved in first-principles calculations.In Chapter 3,we studied the crystal structure,electronic properties and topological characteristics of KCuSe QLs material in detail,discussed its stability and the possibility of experimental preparation.In Chapter 4,we constructed the antiferromagnetic heterostructure KCuSe/NaMnBi,systematically studied its magnetic and topological properties,and further discussed the interaction and influence between magnetic and topological properties.At last,we summarized the innovation points of the whole paper and made prospects for future research work.The main research contents and conclusions of this paper are as follows:(1)Based on first-principles calculations,a new type of 2D topological insulator material KCuSe QLs was predicted and its structure,electronic and topological properties were systematically studied.The exfoliation energy is only 12 meV(?)-2,indicating the possibility of obtaining KCuSe QLs through mechanical exfoliation.The phonon spectrum calculation also verifies the dynamic stability of KCuSe QLs.KCuSe QLs is a non-magnetic 2D material,and neglecting the spin-orbit coupling,it is a 2D Dirac semimetal.Similar to graphene,the spinorbit coupling opens a band gap of 58 meV at the original Dirac point.The calculation of spin Hall conductivity and Wannier charge center suggests that the system’s Z2 topological invariant is Z2=1.At the same time,the study of KCuSe nanoribbons shows that they have a pair of gapless edge states.(2)By coupling a 2D topological insulator with a 2D antiferromagnetic insulator,we constructed a 2D antiferromagnetic vdW heterostructure,KCuSe/NaMnBi.The system has weak interlayer van der Waals interactions.The bands near the Fermi level are mainly contributed by KCuSe and exhibit a gap induced by spin-orbit coupling,revealing the possibility of realizing antiferromagnetic quantum spin Hall effect in KCuSe/NaMnBi heterostructures.The study of Wannier charge center and edge states further proves the topological properties of this vdW heterostructure.Based on this,we constructed other similar vdW heterostructures and obtained non-zero topological invariant Z2=1 and gapless edge states,demonstrating the universality of this scheme and providing a new idea for experimental design of magnetic topological insulators.