Observation of topological flat bands in the kagome semiconductor Nb${}_3$Cl${}_8$

Abstract

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 TFB and a moderate band gap in Nb${}_3$Cl${}_8$. By mechanical exfoliation, we successfully obtain monolayers of Nb${}_3$Cl${}_8$ and confirm that they are stable under ambient conditions. In addition, our calculations show that monolayers of Nb${}_3$Cl${}_8$ have a magnetic ground state, thus providing opportunities to study the interplay between geometry, topology, and magnetism.

6 pages, 4 figures

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