The study of collective atom dynamics aims to investigate the correlated motion of atoms in materials at length scales ranging from below the typical interparticle distances up to macroscopic distances. The use of X-rays in this field represents one of the success stories of third-generation sources. This activity is still expanding, even after many years of operation. Besides the numerous scientific challenges, this is possible thanks to continuing instrumental and conceptual developments on the dedicated experimental stations. A large variety of research programmes is currently being pursued, ranging from the study of the dynamical properties of systems under high pressure to the investigation of the subtle interplay between the crystal lattice, the electronic structure and phonons in increasingly complex systems.

The present highlights try to give a flavour of the current trends and developments at the ESRF. 

An important milestone has been achieved in the study of optical phonon dispersion in high-temperature superconductors by coherent inelastic X-ray scattering (IXS), where valuable information on possible mechanisms responsible for superconductivity could be gained. For this class of materials, IXS is often the only spectroscopic tool to study phonon dispersion, since inelastic neutron scattering (INS) techniques need fairly large samples, not always available in sufficient crystalline quality.

A pilot experiment has demonstrated that absolute cross-sections of molecular vibrational excitations can be straightforwardly obtained, thus providing a complementary spectroscopic tool to the established Raman scattering and infrared absorption techniques. Other important progress has been made in the field of nuclear inelastic scattering (NIS), a technique that allows the determination of the element (Mössbauer isotope) specific phonon density of states (DOS). The method was extended to the 161Dy isotope, thus allowing studies - besides Fe, Sn and Eu - on dysprosium containing materials.

Non-inelastic techniques can also give information on dynamic properties. This is exemplified by the anomalous X-ray diffraction studies where electron density modulations, induced by a charge density wave, or externally, by a surface acoustic wave, give rise to satellite reflections, which in turn allow the quantitative characterisation of these modulations.

The selected highlights show that progress in the field of collective atom dynamics is intimately connected to a continuous development of instruments and methods, combined with an improved knowledge of the underlying physical principles, gained thanks to the fruitful interplay between different experimental techniques and theory.