Observation of Topological Flat Bands in the Kagome Semiconductor Nb₃Cl₈

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 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.

Correlated flat bands and quantum spin liquid state in a cluster Mott insulator

“…3(c). Setting the chemical potential µ below the upper Hubbard band, we immediately see that the lower Hubbard band and ϵ 3,4 all shift to higher binding energy in the same amount of U [31,32].…”

“…As long as the flat bands are below the Fermi level and are reasonably within the energy range accessible for ARPES, their signal should be captured. Due to this reason, we believe the argument of rigid energy shift [31] is less favorable.…”

“…We start our argument by examining the inconsistency recently observed in density functional theory (DFT) calculations and the angle-resolved photoemission spectroscopy (ARPES) for Nb 3 Cl 8 [31,32]. We emphasize that such an inconsistency indicates a remarkable correlated and molecular nature of Nb 3 Cl 8 , whose low-energy gap corresponds to a cluster Mott insulator.…”

Spectroscopic evidence of flat bands in breathing kagome semiconductor Nb3I8

“…Note. During the preparation of the manuscript, the authors became aware of similar work on a similar compound Nb 3 Cl 8 38 .…”

Epitaxial Growth of Single-Layer Kagome Nanoflakes with Topological Band Inversion

“…Although substrate hybridization could largely complicate the band structure, the nontrivial topology of the 1-UC FeGe/Ag(111) resembles that of the freestanding one. Following the recent success in observing topological flat bands in breathing kagome bulk crystals, 41 our efforts to prepare the "real" 2D kagome lattice illustrate the possibility of realizing topological phases and flat band in 2D kagome lattice. Our work can motivate further studies on 2D kagome lattice with nontrivial topological phases built from strongly interacting electrons and may have potential applications in room-temperature dissipationless topological electronics.…”