Observation of Topological Flat Bands in the Kagome Semiconductor NbCl.

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 FeSn, FeSn, CoSn, and YMnSn. Nonetheless, kagome materials that are both exfoliable and semiconducting are lacking, which seriously hinders their device applications. Herein, we show that NbCl, 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 NbCl. By mechanical exfoliation, we successfully obtain monolayer NbCl, which is stable under ambient conditions. In addition, our calculations show that monolayer NbCl has a magnetic ground state, thus providing opportunities to study the interplay among geometry, topology, and magnetism.