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

The electronic structure of a quasi-one-dimensional nickelate, Y2-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 O 1s 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-bar eV below the UHB in accordance with the experiment. The dependence on the incident-photon polarization is also consistent with the experiment