[论文解读] Designer magnetic topological graphene nanoribbons

Shaotang Song, Pei Wen Ng+16 more|arXiv (Cornell University)|Apr 27, 2022

Graphene research and applications被引用 4

一句话总结

本文通过统一实空间（Clar规则）与倒空间（能带拓扑）方法，提出了一种磁性拓扑石墨烯纳米带（MT-GNRs）的理性设计策略。通过表面合成定制的分子前体，作者实现了具有强健端部π-磁性的磁性拓扑石墨烯纳米带，并利用单镍茂自旋探测技术直接观测到其拓扑边缘态；通过调控纳米带长度，实现了从反铁磁性到类顺磁性行为的可调自旋耦合。

ABSTRACT

The interplay of magnetism and topology lies at the heart of condensed matter physics, which offers great opportunities to design intrinsic magnetic topological materials hosting a variety of exotic topological quantum states including the quantum anomalous Hall effect (QAHE), axion insulator state, and Majorana bound states. Extending this concept to one-dimension (1D) systems offers additional rich quantum spin physics with great promise for molecular-scale spintronics. Despite recent progress in the discovery of symmetry-protected topological quantum phases in 1D graphene nanoribbons (GNRs), the rational design and realization of magnetic topological GNRs (MT-GNRs) represents a grand challenge, as one must tackle multiple dimensions of complexity including time-reversal symmetry (TRS), spatial symmetry (width, edge, end geometry) and many-electron correlations. Here, we devised a new route involving the real- and reciprocal-space descriptions by unifying the chemists and physicists perspectives, for the design of such MT-GNRs with non-trivial electronic topology and robust magnetic terminal. Classic Clar's rule offers a conceptually qualitative real-space picture to predict the transition from closed-shell to open-shell with terminal magnetism, and band gap reopening with possible non-trivial electronic topology in a series of wave-like GNRs, which are further verified by first principle calculations of band-structure topology in a momentum-space. With the advance of on-surface synthesis and careful design of molecular precursors, we have fabricated these MT-GNRs with observation of topological edge bands, whose terminal pi-magnetism can be directly captured using a single-nickelocene spin sensor. Moreover, the transition from strong anti-ferromagnetic to weak coupling (paramagnetism-like) between terminal spins can be controlled by tuning the length of MT-GNRs.

研究动机与目标

• 为克服理性设计具有非平凡拓扑结构与强健端部磁性的磁性拓扑石墨烯纳米带（MT-GNRs）这一重大挑战。
• 将化学家的实空间直觉（Clar规则）与物理学家的倒空间能带结构分析统一，实现可预测设计。
• 通过精确的分子前体工程，实现具有受控电子拓扑结构与磁性边缘态的MT-GNRs。
• 利用先进的自旋探测技术，实验验证拓扑边缘能带与端部磁性的存在。
• 通过改变纳米带长度，实现自旋耦合（从反铁磁性到类顺磁性）的可调性。

提出的方法

• 基于实空间共振结构，应用Clar规则预测波状石墨烯纳米带中的开壳特征与端部磁性。
• 通过第一性原理计算分析动量空间中的能带拓扑结构，确认非平凡拓扑。
• 设计并合成特定宽度、边缘几何结构与端部结构的定制分子前体，用于表面制备石墨烯纳米带。
• 利用低温扫描隧道显微镜（STM）与单镍茂自旋传感器，直接成像并探测端部π-磁性。
• 通过调控纳米带长度，控制端部自旋之间的磁耦合，实现从强反铁磁性到弱耦合（类顺磁性）状态的转变。
• 结合实验观测与理论建模，验证拓扑边缘态与强健磁矩的出现。

实验结果

研究问题

• RQ1 实空间中的Clar规则能否可靠预测波状石墨烯纳米带中端部磁性的出现与开壳特征？
• RQ2 纳米带长度与MT-GNRs中端部自旋之间磁耦合强度之间存在何种关系？
• RQ3 能否利用自旋敏感探针在设计的磁性石墨烯纳米带中实验观测到拓扑边缘态？
• RQ4 在一维MT-GNRs中，时间反演对称性、空间对称性与电子关联的相互作用在多大程度上可被调控？
• RQ5 从闭壳到开壳电子结构的转变在MT-GNRs的能带结构与自旋纹理中如何体现？

主要发现

• 作者通过表面合成设计的分子前体，成功制备了具有非平凡电子拓扑结构与强健端部π-磁性的石墨烯纳米带。
• 在MT-GNRs中实验观测到拓扑边缘态，证实了受保护边缘模式的存在。
• 利用单镍茂自旋传感器直接成像并量化了端部磁性，为局域磁矩的存在提供了明确证据。
• 随着纳米带长度增加，端部自旋之间的磁耦合从强反铁磁性转变为弱耦合（类顺磁性）状态。
• 第一性原理计算证实了动量空间中能隙重新打开与非平凡拓扑，与Clar规则的实空间预测一致。
• 实空间与倒空间相结合的方法为设计具有定制量子特性的低维磁性拓扑材料提供了可预测的框架。