Topological band engineering of graphene nanoribbons
Topological band engineering of graphene nanoribbons
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DOI：
10.1038/s41586-018-0376-8
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发表时间：
2018-08-09
期刊：
影响因子：
64.8
通讯作者：
Fischer, Felix R.
中科院分区：
文献类型：
--
作者：
Rizzo, Daniel J.;Veber, Gregory;Fischer, Felix R.
拓扑绝缘体是一类新兴的材料，在保持绝缘体(1，2)的同时，具有高度坚固的带隙内表面或界面状态。这一领域的最大进展集中在二维(4-6)和三维(7-10)的拓扑绝缘体和相关的拓扑晶体绝缘体(3)，但最近的理论工作预测了石墨烯纳米带(GNRs)(11)中存在一维对称保护的拓扑相。这些横向受限的半导体石墨烯的拓扑相由它们的宽度、边缘形状和终止的晶体晶胞决定，并由Z(2)不变量(12)(即指数0或1，表示两个拓扑类-类似于准一维孤子系统(13-16))来表征。由不同Z(2)值表征的不同拓扑结构的GNR之间的界面被预测为支持半填充、带隙内局域电子态，这些电子态原则上可以用作材料工程的工具(11)。在这里，我们提出了一种拓扑工程GNR超晶格的合理设计和实验实现，该超晶格拥有这样的状态的一维阵列，从而产生否则无法获得的电子结构。这一策略还使得新的端态可以直接被设计到一维GNR超晶格的末端。在超高真空条件下，通过分子前驱体在金表面Au(111)表面合成了原子精确的拓扑GNR超晶格，并用低温扫描隧道显微镜和能谱对其进行了表征。我们的实验结果和第一性原理计算表明，这些GNR超晶格的前线能带结构(包括充满态和空态的能带)完全由相邻的拓扑界面态之间的耦合决定。这种非平凡的一维拓扑相的表现为一维材料的能带工程提供了一条基于对其电子拓扑的精确控制的途径，并为一维量子自旋物理的研究提供了一个有前途的平台。
Topological insulators are an emerging class of materials that host highly robust in-gap surface or interface states while maintaining an insulating bulk(1,2). Most advances in this field have focused on topological insulators and related topological crystalline insulators(3) in two dimensions(4-6) and three dimensions(7-10), but more recent theoretical work has predicted the existence of one-dimensional symmetry-protected topological phases in graphene nanoribbons (GNRs)(11). The topological phase of these laterally confined, semiconducting strips of graphene is determined by their width, edge shape and terminating crystallographic unit cell and is characterized by a Z(2) invariant(12) (that is, an index of either 0 or 1, indicating two topological classes-similar to quasi-one-dimensional solitonic systems(13-16)). Interfaces between topologically distinct GNRs characterized by different values of Z(2) are predicted to support half-filled, in-gap localized electronic states that could, in principle, be used as a tool for material engineering(11). Here we present the rational design and experimental realization of a topologically engineered GNR superlattice that hosts a one-dimensional array of such states, thus generating otherwise inaccessible electronic structures. This strategy also enables new end states to be engineered directly into the termini of the one-dimensional GNR superlattice. Atomically precise topological GNR superlattices were synthesized from molecular precursors on a gold surface, Au(111), under ultrahigh-vacuum conditions and characterized by low-temperature scanning tunnelling microscopy and spectroscopy. Our experimental results and first-principles calculations reveal that the frontier band structure (the bands bracketing filled and empty states) of these GNR superlattices is defined purely by the coupling between adjacent topological interface states. This manifestation of non-trivial one-dimensional topological phases presents a route to band engineering in one-dimensional materials based on precise control of their electronic topology, and is a promising platform for studies of one-dimensional quantum spin physics.