Resonant inelastic x-ray scattering spectra for electrons in solids

Resonant inelastic x-ray scattering (RIXS) has recently been a subject of r emarkable progress due to the advent of high-brilliance synchrotron radiati on sources. The authors present a review of both experimental and theoretic al investigations of electrons in solids using this second-order optical pr ocess, in which there is coherent absorption and emission of x rays at reso nance with electronic excitations. The review starts with some of the funda mental aspects of RIXS, after which are presented typical experimental data and their theoretical interpretation for various materials. The first clas s of materials considered is semiconductors and insulators (Si, C, and BN), which are typical systems with weak electron correlation, and the data are interpreted based on electronic states described by an energy-band model. Effects of symmetry of electronic states and electron momentum conservation ale discussed. At the opposite extreme are rare-earth systems (metals and oxides), in which the 4f electrons are almost localized with strong electro n correlation. The observations are interpreted based on the effects of int ra-atomic multiplet coupling and weak interatomic electron transfer, which are well described with an Anderson impurity model or a cluster model. In t his context a narrowing of spectral width in the excitation spectrum, polar ization dependence, and the magnetic circular dichroism in ferromagnetic ma terials are discussed. The authors then consider transition-metal compounds , materials with electron correlation strengths intermediate between semico nductors and rare-earth systems. In these interesting cases there is an int erplay of intra-atomic and interatomic electronic interactions that leads t o limitations of both the band model and the Anderson impurity model. Final ly, other topics in resonant x-ray emission studies of solids are described briefly.