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 TFB and a moderate band gap in Nb3Cl8. By mechanical exfoliation, we successfully obtain monolayers of Nb3Cl8 and confirm that they are stable under ambient conditions. In addition, our calculations show that monolayers of Nb3Cl8 have a magnetic ground state, thus providing opportunities to study the interplay between geometry, topology, and magnetism.
@article{arxiv.2111.01583,
title = {Observation of topological flat bands in the kagome semiconductor Nb$_3$Cl$_8$},
author = {Zhenyu Sun and Hui Zhou and Cuixiang Wang and Shiv Kumar and Daiyu Geng and Shaosheng Yue and Xin Han and Yuya Haraguchi and Kenya Shimada and Peng Cheng and Lan Chen and Youguo Shi and Kehui Wu and Sheng Meng and Baojie Feng},
journal= {arXiv preprint arXiv:2111.01583},
year = {2022}
}