Introduction by L. Paolasini (ESRF)

The research field of Resonant X-ray Scattering (RXS) is relatively new, with the first experiments only fifteen years ago. Tremendous progress has been achieved since its introduction and RXS is now rapidly becoming a crucial complementary technique to neutron scattering for investigating the subtleties of microscopic magnetism. This is especially true in systems where the ground state properties reflect a delicate balance between several different correlated processes, including coupling to the lattice, selective occupation of atomic orbitals and charge order. The element and shell specificity and the extremely high resolution in reciprocal space makes RXS truly complementary to neutron diffraction, as demonstrated recently by joint experiments on the historically controversial magnetic ground state of CeFe2 [1]. The ability to study very small volume samples is a further attractive characteristic of the RXS technique, which has proved particularly useful in the study of thin films, as demonstrated in the case of U/Fe multilayers [2], and other actinide systems.

Over the last few years, polarisation and azimuthal angle analysis have developed into standard tools, opening the experimental avenue to a whole new range of studies, including the physics of ordered multipole states. The dependence of the diffracted beam polarisation on the azimuthal angle, in the framework of anisotropic tensor susceptibility formalism, gives unique information on orbital occupancy order. This is a key issue in, for example, the physics of transition metal compounds, as shown in the investigation of the interplay between orbital and magnetic order for the model system KCuF3 [3], or the direct observation of charge order at the metal-insulator transition in NdNiO3 films [4].

A second classic application, presented below, has been its contribution to unravelling the microscopic nature surrounding the mysterious heat-capacity anomaly in neptunium dioxide [5]. This work exploits not only the combined power of the polarisation and azimuthal angle analysis technique, together with a detailed symmetry analysis of the azimuthal dependence, but also the intense synchrotron flux which enables microgram quantities of transuranic elements to be examined.

As a complement to these developments in RXS techniques, further progress is now planned for the sample environment. Measurements down to dilution-refrigeration temperatures, hydrostatic pressures up to 25 kbar and magnetic fields up to 10 Tesla will be available in the near future, and one can be confident that these novel opportunities will prove to be exceedingly fertile and of great scientific value in the field of strongly correlated electron systems.

[1] L. Paolasini et al., Phys. Rev. Lett., in press (2003) and ILL Annual Report 2002.
[2] S.D. Brown, submitted to MMM conference, to be published in J. Appl. Phys. (2003).
[3] L. Paolasini et al., Phys. Rev. Lett. 88, 106403 (2002); R. Caciuffo et al., Phys. Rev. B 65, 174425 (2002).
[4] U. Staub et al., Phys. Rev. Lett. 88, 126402 (2002).
[5] J. A. Paixao, Phys. Rev. Lett. 89, 87202 (2002).