Anomalous spectral weight in photoemission spectra of the hole-doped Haldane chain Y_2-xSr_xBaNiO₅

In this paper, we present photoemission experiments on the hole-doped Haldane chain compound Y_2-xSr_xBaNiO₅. By using the photon-energy dependence of the photoemission cross section, we identified the symmetry of the first ionization states (d type). Hole doping in this system leads to a significant increase in the spectral weight at the top of the valence band without any change in the vicinity of the Fermi energy. This behavior, not observed in other charge-transfer oxides at low doping level, could result from the inhomogeneous character of the doped system and from a Ni 3d–O2p hybridization enhancement due to the shortening of the relevant Ni-O distance in the localized hole-doped regions.