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.
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