The destructive interference of wavefunctions in a kagome lattice can give rise to topological flat bands (TFBs) with a highly degenerate state of electrons. Recently, TFBs have been observed in several kagome metals, including Fe₃Sn₂, FeSn, CoSn, and YMn₆Sn₆. Nonetheless, kagome materials that are both exfoliable and semiconducting are lacking, which seriously hinders their device applications. Herein, we show that Nb₃Cl₈, which hosts a breathing kagome lattice, is gapped out because of the absence of inversion symmetry, while the TFBs survive because of the protection of the mirror reflection symmetry. By angle-resolved photoemission spectroscopy measurements and first-principles calculations, we directly observe the TFBs and a moderate band gap in Nb₃Cl₈. By mechanical exfoliation, we successfully obtain monolayer Nb₃Cl₈, which is stable under ambient conditions. In addition, our calculations show that monolayer Nb₃Cl₈ has a magnetic ground state, thus providing opportunities to study the interplay among geometry, topology, and magnetism.

中文翻译：

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Kagome 半导体 Nb3Cl8 中拓扑平带的观察

Kagome 晶格中波函数的相消干涉可以产生具有高度简并态电子的拓扑平带 (TFB)。最近，已在几种kagome 金属中观察到TFB，包括Fe ₃ Sn ₂、FeSn、CoSn 和YMn ₆ Sn ₆。尽管如此，缺乏可剥离和半导体的kagome材料，这严重阻碍了它们的器件应用。在此，我们表明 Nb ₃ Cl ₈，它拥有一个呼吸的kagome晶格，由于缺乏反演对称性而被隔开，而TFB由于镜面反射对称性的保护而得以幸存。通过角分辨光电子能谱测量和第一性原理计算，我们直接观察到 TFB 和 Nb ₃ Cl ₈中的中等带隙。通过机械剥离，我们成功地获得了在环境条件下稳定的单层Nb ₃ Cl _{8 。}此外，我们的计算表明单层 Nb ₃ Cl ₈具有磁性基态，从而为研究几何、拓扑和磁性之间的相互作用提供了机会。