Effect of S Substitution for Se on the Ta–Ni Hybridization in an Excitonic Insulator Ta₂NiSe₅

We have investigated electronic structure of Ta2Ni(Se1−xSx)5 (x = 0.2) by means of photoemission spectroscopy and band structure calculations. Ta2NiSe5 exhibits a transition from a band insulator (orthorhombic) to an excitonic insulator (monoclinic), and the transition temperature decreases with the S substitution for Se [Lu et al., Nature Commun. 8, 14408 (2017)]. While the overall electronic structure of Ta2Ni(Se0.8S0.2)5 probed by Ni 3p-3d resonant photoemission spectroscopy is a mixture of those of Ta2NiSe5 and Ta2NiS5, the flat top valence band observed in the monoclinic Ta2Ni(Se0.8S0.2)5 is similar to that of Ta2NiSe5, apparently inconsistent with the suppression of the excitonic insulator state by the S substitution. The experimental and theoretical results indicate that the S substitution for Se enhances local Ta 5d-Ni 3d hybridization to flatten the valence band top while the atomic disorder tends to suppress coupling between the Ni chains and the long-range order.