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

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 $C_3$ + $T$ symmetry in the presence of crystal-field splitting.