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

TL;DR

This study reports the experimental observation and theoretical confirmation of topological flat bands in the kagome semiconductor Nb$_3$Cl$_8$, which is exfoliable and stable, opening avenues for device applications and magnetic studies.

Contribution

The paper demonstrates the existence of topological flat bands in Nb$_3$Cl$_8$, a semiconducting kagome material that can be exfoliated into monolayers, with potential magnetic properties.

Findings

Direct observation of TFBs in Nb$_3$Cl$_8$ via ARPES and calculations.

Successful mechanical exfoliation of Nb$_3$Cl$_8$ monolayers.

Monolayers exhibit a magnetic ground state, enabling studies of topology and magnetism.

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.