We analyze the resonant inelastic x-ray scattering (RIXS) spectra at the Cu and Ni K edges in La2CuO4 and La2NiO4, respectively. We make use of the Keldysh-Green-function formalism, in which the RIXS intensity is described by a product of incident-ph
oton-dependent factor and density-density correlation function in the 3d states. The former factor is calculated using the $4p$ density of states given by an ab initio band structure calculation and the latter using the wavefunctions given by a Hartree-Fock calculation of a multi-orbital tight-binding model. The initial state is described within the Hartree-Fock approximation and the electron correlations on charge excitations are treated within the random phase approximation. The calculated RIXS spectra well reproduce several characteristic features in the experiments. Although several groups have interpreted the RIXS peaks as bound excitons, our calculation indicates that they should be interpreted as band-to-band excitations augmented by electron correlations. The difference in RIXS spectra between La2CuO4 and La2NiO4 is explained from this point of view.
We analyze the resonant inelastic x-ray scattering (RIXS) spectra at the K edge of Mn in the antiferromagnetic insulating manganite LaMnO3. We make use of the Keldysh-type Green-function formalism, in which the RIXS intensity is described by a produc
t of an incident-photon-dependent factor and a density-density correlation function in the 3d states. We calculate the former factor using the 4p density of states given by an ab initio band structure calculation and the latter using a multi-orbital tight-binding model. The ground state of the model Hamiltonian is evaluated within the Hartree-Fock approximation. Correlation effects are treated within the random phase approximation (RPA). We obtain the RIXS intensity in a wide range of energy-loss 2-15 eV. The spectral shape is strongly modified by the RPA correlation, showing good agreement with the experiments. The incident-photon-energy dependence also agrees well with the experiments. The present mechanism that the RIXS spectra arise from band-to-band transitions to screen the core-hole potential is quite different from the orbiton picture previously proposed, enabling a comprehensive understanding of the RIXS spectra.
