Large-cluster-model calculation of O 1s XAS for Y2-xCaxBaNiO5

The electronic structure of a quasi-one-dimensional nickelate, Y-2-xCaxBaNiO5, is discussed on the basis of numerically-exact diagonalization calculations of O 1s XAS using a four-site one-dimensional cluster model. In the undoped nickelate (x = 0), the band dispersion along the chain direction is suppressed owing the intra-atomic Coulomb and exchange interactions between 3d holes. As a result, the width of the 0 Is XAS peak that corresponds to the transition to the upper Hubbard band (UHB) is very narrow. Upon hole-doping, two new peaks appear about 2 eV below the UHB in accordance with the experiment. The dependence on the incident-photon polarization is also consistent with the experiment. (c) 2004 Elsevier B.V. All rights reserved.