Introduction by L. Paolasini and C. Detlefs

The resonant X-ray scattering technique continues to merge with other, associated techniques used to resolve complex problems associated with the electronic and magnetic properties of solids. Most notable is the extension towards soft X-ray energies, where the giant resonant effects at L2,3 edges of 3d transition metals and M4,5 edges of rare-earth metals can be accessed. However, the long-wavelength reduces the accessible reciprocal space and strongly restricts this application to the systems with a long magnetic and structural periodicity.

The intensive activity to investigate the forbidden lattice reflections, enhanced at the absorption edges, by the photon polarisation analysis (both circular and linear) and its angular dependence (azimuthal scans) have yielded a rich variety of experimental results, stimulating an intense theoretical activity to understand the change in the local symmetries at the origin of the multipole transitions involved in the resonant processes. The continuous effort to develop the experimental set-up and to push toward more extreme sample environment conditions is a consequence of the complex relationship between the long-range magnetic and electronic ordering phenomena, and the necessity to separate unambiguously the tensorial matrix elements involved in the physics of multiple ordered states. This effort concerns the main thermo-dynamical parameters involved in the Condensed Matter Physics: the development of high pressure at low temperature, the introduction of the magnetic field and the extension of the temperature range for the azimuthal setup.

The ongoing ID20 upgrade and refurbishment program, which will be completed after the summer of 2004, can be seen as having the main objective of overcoming the present difficulties.

The main goal of this program is the installation of a superconducting cryomagnet for resonant X-ray scattering experiments under a magnetic field up to 10 Tesla. It will be housed in a separate hutch, with a non-magnetic diffractometer able to support the heavy load with very precise position reproducibility.

Resonant X-ray scattering is an increasingly important probe for studies of the electronic and magnetic properties of strongly correlated electron systems. The ongoing technical developments, along with improved theories, will allow a host of novel and unique experiments in the near future. We are looking forward to this exciting period.