Analysis of resonant inelastic x-ray scattering at the K edge in NiO

We analyze the resonant inelastic x-ray scattering (RIXS) spectra at the Ni K edge in an antiferromagnetic insulator NiO by applying the theory developed by the present authors. It is based on the Keldysh-Green function formalism, and treats the core-hole potential in the intermediate state within the Born approximation. We calculate the single-particle energy bands within the Hartree-Fock approximation on the basis of the multiorbital tight-binding model. Using these energy bands together with the 4p density of states from an ab initio band-structure calculation, we calculate the RIXS intensities as a function of energy loss. By taking account of electron correlation within the random-phase approximation (RPA), we obtain quantitative agreement with the experimental RIXS spectra, which consist of two prominent peaks around 5 and 8 eV, and the former shows considerable dispersion, while the latter shows no dispersion. We interpret the peaks as a result of a band-to-band transition augmented by the RPA correlation.