STRUCTURES AND PHASE TRANSITIONS

The study of both crystallographic and magnetic structural phase transitions is an important activity of the Laboratoire Léon Brillouin. It makes use of several neutron scattering techniques, such as diffraction or inelastic scattering; the samples are either powders or single crystals. Most of the time, such studies are performed as a function of a unique parameter, namely the temperature, but pressure has recently become another important variable. Many different physical problems are related to these activities and the present summary intends to show the main goals of the research in the field. Magnetic phase transitions are treated in a different chapter. FULLERENES AND CARBON NANOTUBES Among the materials studied at LLB, fullerenes and carbon nanotubes are fashionable materials. The study of the former is older and the recent achievement of growing single crystals of C60 of sufficiently large size allowed the precise characterisation of the intramolecular dynamics of the molecule [L. Pintschovius et al, Kernforschungszentrum, Karlsruhe; see “highlight”]. Comparison with theory is particularly rich and yields the unambiguous identification of each of the internal modes of vibration. This comparison between experimental results and theory has been done, not only at the level of the frequencies, but also at the level of the intensities, in order to access the eigenvectors. The next step of this study will concern the intermolecular modes as a function of applied pressure. The problem of the nature of these interactions (which are weaker as compared wit