Introduction by P. Glatzel, ESRF

The term electron correlation is used to describe one part of the interaction between electrons that move in the potential of nuclei. Strong electron correlations can give rise to fascinating material properties but they can also be troublesome for the theorist because the magnitudes of the interactions are difficult to calculate [1]. In fact, the underlying many body problem is impossible to solve analytically. However, understanding electron-electron interactions is the key to explaining important phenomena such as superconductivity, colossal magneto resistance and unusual phase transitions. There is thus a strong general interest in strongly correlated systems from theoreticians and, for example, from material scientists who wish to tailor the properties of materials and improve the storage capabilities of computer hard drives.

The enormous progress made over the last few decades in developing and refining theoretical models is largely driven by experiments such as those performed at the ESRF. Some of the questions that are being addressed are long-range orbital or magnetic ordering, local spin- and orbital magnetic moments, orbital hybridisation and the interplay between crystal field and electron-electron interactions. The three papers that we selected for this section are representative of the development of experimental techniques that open up new possibilities of comparing theory with experiment and allow for a detailed study of the electronic structure.

A map of the valence electron binding energy versus its momentum can be obtained by measuring the angular dependence of the valence band photoemission from a single crystal and can ultimately yield the Fermi surface. The technique is extremely surface sensitive for low incident photon energies (h ~ 20eV) and thus low kinetic energies of the photoexcited electron. Hence the question arises whether a more bulk-sensitive probe yields different results. Claesson et al. took up on this question and used a photoelectron spectrometer that is capable of detecting high kinetic energies (~ 600 eV) with good energy and momentum resolution.

An element specific probe of the electronic structure is obtained in inner-shell spectroscopy. When the photon energy is tuned to an absorption edge the resulting state can decay back to the initial state or to an excited state. When the resulting X-ray is detected, with sufficient energy resolution, we refer to resonant inelastic X-ray scattering (RIXS) spectroscopy. This has proven to be a powerful technique to study correlation effects and ligand field splittings [2]. Ghiringhelli et al. have used RIXS at the Cu L-edge to disentangle excitations with small energy transfer, i.e. electron transitions that are usually only accessible using optical spectroscopies that are not element specific. Rueff et al. investigated the - phase transition of Ce solid solutions. They treat hybridisation effects in terms of a configuration interaction approach and determine the f-orbital occupancy as a function of temperature.

Nevertheless, despite the considerable advances in experimental techniques, the nature of electron correlations and their correct theoretical treatment will continue to be subject of intense discussion for the coming years.

References
[1] P. Fulde Electron correlations in molecules and solids 3rd enl. ed.; Springer-Verlag: Berlin, New York, (1995).
[2] A. Kotani, S. Shin Rev. Mod. Phys. 73, 203-46 (2001).