Effects of Hund's coupling on oxygen is X-ray absorption and resonant X-ray emission in one-dimensional nickelates

The electronic structure of quasi-one-dimensional nickelates, where the formal electron configuration of Ni ions is d⁸, is discussed on the basis of numerically-exact diagonalization calculations of the O 1s X-ray absorption spectrum (XAS) and X-ray emission spectrum (XES) using a four-site one-dimensional cluster model. In the undoped nickelate, the band dispersion along the chain direction is suppressed owing to the intra-atomic Coulomb interaction (U) and Hund's coupling (J) on the Ni 3d shell. 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 that are split by J appear about 2 eV below the UHB peak in accordance with the recent experiment of Y_2-xCa_xBaNiO₅. In the O 1s XES of the undoped nickelate, we expect a peak that is caused by the spin exchange between neighboring Ni sites.