Resonant inelastic soft X-ray scattering of insulating cuprates

We have performed high-resolution inelastic X-ray emission scattering experiments at the Cu 3p-, Cu 3s-, and O 1s-resonances of the insulating cuprates CuGeO3, CuO, La2CuO4, and SrCuO2, We introduce the novel low-energy s-edge Cu-RIXS which reveals a dd-excitation peak, which was previously unobserved due to insufficient resolution and intensity in high-energy (Cu 1s RIXS). Also, O 1s-RIXS of all cuprate sample is investigated. Surprisingly, there is a large spread in the energy loss values of the RIXS features for different compounds and we explain this by assigning the larger energy features to the occurrence of a Zhang-Rice singlet while the lower energy feature (only observed for CuGeO3) is assigned to a dd-excitation.