Topological Insulators in Bismuth-Halogen and Related Systems: Design, Synthesis, Optimization and Properties

在第一个资助期内，我们采用了“受限金属”的概念，建立了基于晶体结构特征的新拓扑绝缘体定向搜索的粗略指南。作为低维结构片段嵌入到富铋金属-盐杂化物的本体结构中的2D TI片段可以解释整个化合物的弱或强3D TI性质。盐状部分可以由高度极性的碘阴离子构成，与硫属化物相比，碘阴离子有利于更无缺陷的化学计量化合物。这种方法现在通过两个3D弱拓扑绝缘体（Bi 14 Rh 3 I9，Bi 2 TeI）和3D强拓扑绝缘体β-Bi 4 I4来举例说明，其电子结构接近于弱3D TI相和平凡绝缘体相。这些块体材料由两种不同的2D TI片段构建：结构上类似于石墨烯的装饰蜂窝状金属间化合物层和铋双层，这是元素铋结构的构建单元。Bi 14 Rh 3 I9是迄今为止唯一已知的3D弱TI，其拓扑边缘状态的发生被实验证实。β-Bi 4 I4是强拓扑类Z2=1;（1，1，0）的唯一代表，具有高度各向异性的狄拉克锥。我们的理论考虑的Bi 2 TeI电子结构预测异国情调的拓扑表面状态超出了Z2-形式主义。在优先计划的第二个资助期内，我们的目标是以上述化合物为起点，合成，探索和功能化新的TI家族。真实的材料的可用性将促进三维弱TI的理论和实验进展，到目前为止，由于缺乏标本，强对应物也黯然失色。实现这些目标的研究计划中的基本步骤是：1）通过碘掺杂对用于本征电子和磁电传输的可用大Bi 14 Rh 3 I9单晶进行微调，以及对薄Bi 14 Rh 3 I9层的剥离、门控和接触进行实验和理论评估，以便获得基于3D弱TI的第一个简单器件。2)通过合成化学、基于DFT的电子结构和拓扑不变量的计算，探索和扩展由β-Bi 4 I4衍生的下一代强TI。3)致力于将Bi 2 TeI的单晶生长推向最高可达到的水平，以研究弱3D TI相和受镜像晶体对称性保护的拓扑晶体绝缘体相的可能共存。4)通过对晶体结构的“受限金属”概念的应用以及通过第一原理电子结构计算和拓扑不变量的直接计算，识别具有不同结构类型的新TI候选物。

Employing the concept of "confined metals" in the first funding period, we have established rough guidelines towards the directed search of new topological insulators based on crystal-structure features. A 2D TI fragment embedded as a low-dimensional structural fragment into the bulk structures of bismuth-rich metal-salt hybrids can account for weak or strong 3D TI properties of the entire compound. The salt-like part can be constructed from the highly polar iodide anions that favor for more defect-free, stoichiometric compounds as opposed to chalcogenides. This approach is by now exemplified by two 3D weak topological insulators (Bi14Rh3I9, Bi2TeI) and a 3D strong topological insulator, beta-Bi4I4, which electronic structure is in proximity of both the weak 3D TI phase and the trivial insulator phase. These bulk materials are built by two different 2D TI fragments: a decorated honeycomb intermetallic layer that structurally resembles graphene and a bismuth bilayer, which is a building unit of the elemental bismuth structure. Bi14Rh3I9 is so far the only known 3D weak TI, for which the occurrence of topological edge states was experimentally confirmed. beta-Bi4I4 is the only representative of the strong topological class Z2=1;(1,1,0) and features a highly anisotropic Dirac cone. Our theoretical consideration of the Bi2TeI electronic structure predicts exotic topological surface states beyond the Z2-formalism. Our goal within the second funding period of the Priority Program is to synthesize, explore and functionalize new families of TIs, taking the above mentioned compounds as a starting point. The availability of real materials will promote theoretical and experimental advances for 3D weak TIs that were so far overshadowed by the strong counterparts also due to the lack of specimens. The essential steps in the research program to realize these goals are: 1) Fine-tuning of the available large Bi14Rh3I9 single-crystals for intrinsic electronic and magneto-electronic transport by means of iodine doping, as well as experimental and theoretical assessment of exfoliation, gating and contacting of thin Bi14Rh3I9 layers in order to get access to first simple devices based on a 3D weak TI. 2) Exploration and expansion of a next generation of strong TIs derived from beta-Bi4I4 by synthetic chemistry, DFT-based calculations of electronic structure and topological invariants. 3) Dedicated efforts to push single-crystal growth of Bi2TeI to the highest attainable level in order to investigate the possible co-existence of the weak 3D TI phase and the topological-crystalline-insulator phase that is protected by the mirror crystal symmetry. 4) Identification of new TI candidates with differing structural types via application of the "confined metals"-concept toward the crystal structure and via first-principle electronic structure calculations and direct calculation of topological invariants.