Incident energy and polarization-dependent resonant inelastic x-ray scattering study of La₂CuO₄

We present a detailed Cu K-edge resonant inelastic x-ray scattering (RIXS) study of the Mott insulator La₂CuO₄ in the 1–7 eV energy loss range. As initially found for the high-temperature superconductor HgBa₂CuO_4+δ, the spectra exhibit a multiplet of weakly dispersive electron-hole excitations, which are revealed by utilizing the subtle dependence of the cross section on the incident photon energy. The close similarity between the fine structures for in-plane and out-of-plane polarizations is indicative of the central role played by the 1s core hole in inducing charge excitations within the CuO₂ planes. On the other hand, we observe a polarization dependence of the spectral weight, and careful analysis reveals two separate features near 2 eV that may be related to different charge-transfer processes. The polarization dependence indicates that the 4p electrons contribute significantly to the RIXS cross section. Third-order perturbation arguments and a shake-up of valence excitations are then applied to account for the final-energy resonance in the spectra. As an alternative scenario, we discuss fluorescence-like emission processes due to 1s→4p transitions into a narrow continuum 4p band.