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.
Dirac
materials, which feature Dirac cones in the reciprocal space,
have been one of the hottest topics in condensed matter physics in
the past decade. To date, 2D and 3D Dirac Fermions have been extensively
studied, while their 1D counterparts are rare. Recently, Si nanoribbons
(SiNRs), which are composed of alternating pentagonal Si rings, have
attracted intensive attention. However, the electronic structure and
topological properties of SiNRs are still elusive. Here, by angle-resolved
photoemission spectroscopy, scanning tunneling microscopy/spectroscopy
measurements, first-principles calculations, and tight-binding model
analysis, we demonstrate the existence of 1D Dirac Fermions in SiNRs.
Our theoretical analysis shows that the Dirac cones derive from the
armchairlike Si chain in the center of the nanoribbon and can be described
by the Su–Schrieffer–Heeger model. These results establish
SiNRs as a platform for studying the novel physical properties in
1D Dirac materials.
The Su-Schrieffer-Heeger (SSH) model in a two-dimensional rectangular lattice features gapless or gapped Dirac cones with topological edge states along specific peripheries. While such a simple model has been recently realized in photonic/acoustic lattices and electric circuits, its material realization in condensed matter systems is still lacking. Here, we study the atomic and electronic structure of a rectangular Si lattice on Ag(001) by angle-resolved photoemission spectroscopy and theoretical calculations. We demonstrate that the Si lattice hosts gapped Dirac cones at the Brillouin zone corners. Our tight-binding analysis reveals that the Dirac bands can be described by a 2D SSH model with anisotropic polarizations. The gap of the Dirac cone is driven by alternative hopping amplitudes in one direction and staggered potential energies in the other one and hosts topological edge states. Our results establish an ideal platform to explore the rich physical properties of the 2D SSH model.
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