Observation of Topological Flat Bands in the Kagome Semiconductor Nb 3 Cl 8 .

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 Fe 3 Sn 2 , FeSn, CoSn, and YMn 6 Sn 6 . Nonetheless, kagome materials that are both exfoliable and semiconducting are lacking, which seriously hinders their device applications. Herein, we show that Nb 3 Cl 8 , 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 Nb 3 Cl 8 . By mechanical exfoliation, we successfully obtain monolayer Nb 3 Cl 8 , which is stable under ambient conditions. In addition, our calculations show that monolayer Nb 3 Cl 8 has a magnetic ground state, thus providing opportunities to study the interplay among geometry, topology, and magnetism.