强三维拓扑绝缘体与磁性拓扑绝缘体的角分辨光电子能谱学研究进展

Angle resolved photoemission spectroscopy studies on three dimensional strong topological insulators and magnetic topological insulators

拓扑材料的发现标志着凝聚态物理学和材料科学的又一次革命.从电学属性来说,人们不再仅仅以导电性的强弱(能隙的有无)把材料划分为导体、半导体和绝缘体,而是进一步通过系统的整体拓扑不变量把材料划分为拓扑平庸的和拓扑不平庸的.拓扑绝缘体是最早发现的拓扑非平庸系统,以负能隙的体材料和无能隙的拓扑边缘态为标志.强三维拓扑绝缘体拥有连接导带和价带的狄拉克锥拓扑表面态,而引入铁磁性会使拓扑表面态打开一个特殊的磁性能隙.这些新颖的材料在自旋电子学、非线性光学等广泛的领域有潜在的应用价值,更是将来的拓扑量子计算中不可或缺的核心材料.作为应用最广泛的一种直接观察k空间的实验手段和表面物理的重要分析工具,角分辨光电子能谱(ARPES)在拓扑材料的研究中一直处于举足轻重的地位.从拓扑绝缘体的最初发现到现在,利用ARPES研究强三维拓扑绝缘体和磁性拓扑绝缘体的文章已数以千计,不胜枚举.本文试从材料分类的角度对这两类材料的部分ARPES研究作一综述,侧重于描述利用ARPES研究此类材料的一般方法和过程,力求使读者对这一领域的研究现状有一个基本的概念.本文假定读者具有ARPES的基础知识,因此对ARPES的基本原理和系统构成不作讨论.

The discovery of topological materials-condensed matter systems that have nontrivial topological invariants-marked the commencement of a new era in condensed matter physics and materials science.Three dimensional topological insulators(3 D TIs)are one of the first discovered and the most studied among all topological materials.The bulk material of the TIs have the characteristics of the insulator,having a complete energy gap.Their surface electronic states,on the other hand,have the characteristics of a conductor,with energy band passes continuously through the Fermi surface.The conductivity of this topological surface state(TSS)is protected by the time reversal symmetry of the bulk material.The TSS is highly spin-polarized and form a special spin-helical configuration that allows electrons with specific spin to migrate only in a specific direction on the surface.By this means,surface electrons in TIs can"bypass"the influence of local impurities,achieving a lossless transmission of spin-polarized current.The existence of TIs directly leads to a variety of novel transport,magnetic,electrical,and optical phenomena,such as non-local quantum transport,quantum spin Hall effect,etc.,promising wide application prospects.Recently,several research groups have searched all 230 non-magnetic crystal space groups,exhausting all the found or undiscovered strong/weak TIs,topological crystalline insulators(TCI),and topological semimetals.This series of work marks that theoretical understanding of non-magnetic topological materials has gone through a period of one-by-one prediction and verification,and entered the stage of the large-area material screening and optimization.Parallel to non-magnetic TIs,magnetic topological materials constructed by ferromagnetic or antiferromagnetic long range orders in topological systems have always been an important direction attracting theoretical and experimental efforts.In magnetic TIs,the lack of time reversal symmetry brings about new physical phenomena.For example,when a ferromagnetic order is introduced into a three-dimensional TI,the Dirac TSS that originally intersected at one point will open a magnetic gap.When the Fermi surface is placed just in the gap,the quantum anomalous Hall effect can be implemented.At present,the research on magnetic topology systems is still in the ascendant.It is foreseeable that these systems will be the main focus and breakthrough point of topology material research in the next few years.Angle-resolved photoemission spectroscopy(ARPES)is one of the most successful experimental methods of solid state physics.Its unique k-space-resolved single-electron detection capability and simple and easy-to-read data format make it a popular choice for both theoretists and experimentalists.In the field of topological materials,ARPES has always been an important experimetnal technique.It is able to directly observe the bulk and surface band structure of crystalline materials,and in a very intuitive way.With ARPES,it is incontrovertible to conclude whether a material is topological,and which type of topological material it belongs to.This paper reviews the progress of ARPES research on TIs since 2008,focusing on the experimental energy band characteristics of each series of TIs and the general method of using ARPES to study this series of materials.Due to space limitations,this paper only discusses the research progress of ARPES for strong 3 D TIs(focusing on the Bi2Se3 series)and magnetic TIs(focusing on the MnBi2Te4 series).Researches involving TCIs,topological Kondo insulators,weak 3 D TIs,topological superconductors and heterostructures based on topological insulators will not be discussed.This paper assumes that the reader has the basic knowledge of ARPES,so the basic principles and system components of ARPES are not discussed.