Resonant Inelastic X-ray Scattering (RIXS) is a new technique for probing elementary excitations of solids. The incoming high energy X-rays promote a core electron to an outer shell, creating a core-hole. This core-hole interacts with the valence electrons, creating excitations. When the core-hole is annihilated by the photo-excited electron, one observes the momentum and energy of the corresponding photon. A priori, the problem of calculating RIXS cross sections seems hard: at resonance, the system is strongly perturbed. However, by making use of the ultrashort lifetime of the core-hole (a small parameter in the problem), we are able to find cross sections with little effort.

I'll speak about what RIXS measures, with an emphasis on magnetic RIXS. We recently showed that RIXS can probe two-magnon excitations in the high-Tc parent compound La2CuO4. The core-hole couples to magnons by locally modifying the superexchange interaction. The two-magnon excitation has been measured recently by John Hill (arXiv:0709.3274) and more detailed measurements are being done right now...

If time permits, I'll talk about our present work on how RIXS can probe collective orbital excitations.