Orbital polarization, charge transfer, and fluorescence in reduced-valence nickelates

This paper presents a simple formalism for calculating x-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS) that has as input orbital-resolved density of states from a single-particle or many-body ab initio calculation and is designed to capture itinerant like features. We use this formalism to calculate both the XAS and RIXS with input from density functional theory (DFT) and DFT+DMFT for the recently studied reduced valence nickelates R4Ni3O8 and RNiO2 (R=rare earth), and these results are then contrasted with those for the cuprate CaCuO2 and the unreduced nickelate R4Ni3O10. In contrast to the unreduced R4Ni3O10, the reduced valence nickelates as well as the cuprate show strong orbital polarization due to the dominance of x2-y2 orbitals for the unoccupied 3d states. We also reproduce two key aspects of a recent RIXS experiment for R4Ni3O8: (i) a charge-transfer feature between 3d and oxygen 2p states whose energy we find to decrease as one goes from RNiO2 to R4Ni3O8 to the cuprate, and (ii) an energy-dependent polarization reversal of the fluorescence line that arises from hybridization of the unoccupied 3z2-r2 states with R 5d states. We end with some implications of our results for the nature of the 3d electrons in reduced valence nickelates.