Valence-bond insulator in proximity to excitonic instability

Abstract

${\mathrm{Ta}}_{2}{\mathrm{NiS}}_{5}$ is supposed to be a simple semiconductor in which excitonic instability of ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$ is suppressed due to its large band gap. However, the Ni $2p$ core-level photoemission of ${\mathrm{Ta}}_{2}{\mathrm{NiS}}_{5}$ exhibits a satellite indicating Ni $3d$ orbitals are mixed into its conduction band as expected in an excitonic insulator. The valence band does not show dispersion flattening and spectral sharpening which are fingerprints of an excitonic insulator. Instead, Ni $3p\ensuremath{-}3d$ resonant photoemission indicates Mottness of the Ni $3d$ electron in ${\mathrm{Ta}}_{2}{\mathrm{NiS}}_{5}$ with negative charge-transfer energy. The present results show that ${\mathrm{Ta}}_{2}{\mathrm{NiS}}_{5}$ can be viewed as a valence bond insulator with a band gap exceeding the exciton binding energy.