Layered Kagome Compound Na$_2$Ni$_3$S$_4$ with Topological Flat Band

TL;DR

This paper investigates Na$_2$Ni$_3$S$_4$, revealing its layered kagome structure, weak paramagnetic insulating behavior, and the presence of topological flat bands arising from its unique electronic and structural properties.

Contribution

It reports the discovery of topological flat bands in a layered kagome compound, combining experimental characterization with band structure calculations.

Findings

Na$_2$Ni$_3$S$_4$ is a weak paramagnetic insulator with a 0.5 eV gap.

The compound features a Ni-based kagome lattice with a square planar S environment.

Topological flat bands are identified from the rotated d$_{xz}$-orbital in the band structure.

Abstract

We report structural and electronic properties of Na$_2$Ni$_3$S$_4$, a quasi-two-dimensional compound composed of alternating layers of [Ni$_3$S$_4$]$^{2-}$ and Na$^{+}$. The compound features a remarkable Ni-based kagome lattice with a square planar configuration of four surrounding S atoms for each Ni atom. Magnetization and electrical measurements reveal a weak paramagnetic insulator with a gap of about 0.5 eV. Our band structure calculation highlights a set of topological flat bands of the kagome lattice derived from the rotated d$_{xz}$-orbital with $C_\mathrm{3}$ + $T$ symmetry in the presence of crystal-field splitting.