Resonant Inelastic X-rays Scattering (RIXS) is an energy loss spectroscopy made with x rays whose energy is tuned to a suitable absorption edge. When the instrumental resolution is good enough, RIXS spectra provide information on the energy, dispersion and symmetry of local and collective excitations, such as ligand field excitations, magnons and paramagnons, phonons, particle-hole pairs, charge density fluctuations and order. RIXS is a powerful complement of more traditional techniques like inelastic neutron scattering, Raman scattering, electron energy loss spectroscopy.
The rich physics of cuprates is very effectively captured by high resolution RIXS experiments made at Cu L3 and O K edges. This fortunate conjuncture has boosted the development of better and better instrumentation at synchrotrons and has served as one of the scientific cases for RIXS at XFELs. The field is expanding and experiments are leading to more insightful results, where the different degrees of freedom are organically studied.
After introducing the technique, I will provide a survey of results on cuprate parent compounds  and superconductors [2,3] and on infinite layer nickelates , which share several properties with high Tc superconductors.
- Martinelli, L, Betto, D., et al. Fractional spin excitations in the infinite-layer cuprate CaCuO2, Phys. Rev. X 12, 021041 (2022)
- Arpaia, R., Caprara, S., et al, Dynamical charge density fluctuations pervading the phase diagram of a Cu-based high-Tc superconductor, Science 365, 906 (2019)
- Peng, Y.Y, Martinelli, L, et al. Doping-dependence of the electron-phonon coupling in two families of bilayer superconducting cuprates. Phys. Rev. B 105, 115105 (2022).
- Krieger, G., Martinelli, L. et al. Charge and spin order dichotomy in NdNiO2 driven by SrTiO3 capping layer arXiv:2112.03341 (2021)