Resonant inelastic X-ray scattering in strongly correlated electron systems

Resonant inelastic X-ray scattering (RIXS) tuned for K and L edges of transition-metal element has recently contributed a lot to our understanding of electronic excitations in strongly correlated electron systems. We discuss a theoretical framework about K- and L-edge RIXS for cuprates and multi-orbital systems focusing on the relation to dynamical spin and charge structure factors. Present understanding of spin and charge dynamics in cuprates is also discussed based on very recent experimental and theoretical developments about electron-hole asymmetry. A small-momentum, low-energy charge mode is predicted as a challenging problem for future RIXS experiments. In multi-orbital systems, however, it is not easy to separate spin and charge components, since off-diagonal orbital contribution in dynamical response functions appears in RIXS with relatively strong intensity.