Introduction by G. Ruocco, Universitá di Roma "La Sapienza" and Istituto Nazionale di Fisica della Materia, Roma (Italy)

High-resolution Inelastic X-ray Scattering techniques are spectroscopic tools particularly suited for the investigation of single particle and collective dynamics in disordered systems. A significant effort has been put into developing these technique over the last ten years at the ESRF, with the construction of state-of-the-art beamlines: ID16 and ID28 for Inelastic X-ray Scattering (IXS), ID18/22N for Nuclear Resonance Scattering (NRS) and both ID16 and ID26 for Raman Scattering. The inelastic-scattering capabilities are routinely exploited for studies on condensed and disordered matter systems ranging from quantum fluids, to biological materials, to liquid metals, to glasses, to high Tc superconductors, etc.. Thanks to these techniques, it has been possible to pose (and sometime answer) questions on the structure and dynamics of disordered materials as:

  • Are there collective excitations in liquids and glasses with wavelengths approaching the inverse interparticle separation? If so, to what extent do these modes deviate from the plane waves found in crystals? Are these deviations responsible for the anomalous thermal conductivity found in glasses at low temperatures? Similarly, how do they relate to the sound excitations observed in the long-wavelength limit?
  • With respect to the Debye behaviour of the corresponding crystal, what are the origins of the excess specific heat at low temperature and the excess density of vibrational states found in glasses?
  • What is the microscopic description of the relaxation processes in liquids and in glasses?
  • Are short-wavelength excitations still affected by relaxation processes as observed in the long-wavelength case? This last point relates to the issue of how the high-frequency dynamics may be affected by the liquid-glass transition in glass-forming liquids, and whether any critical behaviour is present in this dynamics.

The possibility of contributing to the understanding of such problems with an experimental method has created a worldwide interest in the inelastic scattering techniques, and important results have already been documented. Some examples of successful application of these techniques follow. Sergueev et al. report on the use of nuclear resonance scattering to study a typical organic glass-forming material, whereby the translational and rotational relaxation times were determined separately using both nuclear forward scattering and perturbed-angular correlation. The knowledge of these two times, and their comparison with those measured by other standard techniques, such as dielectric spectroscopy, gives us an insight into the origins of the beta-process in supercooled liquids and glasses. In another example T. Neisius reports on the use of the Raman-scattering technique for the study the intensity of the 4p-1s transition in liquid germanium and its temperature dependence. As the intensity is related to the amount of tetrahedrically-bonded atoms in the melt, the authors were able to find the existence of a transition from a four to a six nearest-neighbours atomic arrangement. On the IXS side, Scopigno et al. analysed the temperature dependence of the elastic-to-inelastic intensity ratio in the IXS spectra of several glasses. They revealed the existence of an unexpected correlation between the fragility of a liquid (an index that quantifies how fast the viscosity of a supercooled liquid increases on approaching the glass transition) and the temperature dependence of the amplitude of the collective vibrations on approaching zero temperature. This correlation, which allows us to predict the behaviour of the viscosity of a liquid from the vibrational properties of its glass is waiting for a microscopic explanation.

Many fields in disordered condensed matter physics have not yet been tackled with the inelastic scattering techniques, and for many others only the surface has only just been grazed. So there are no doubts that this area will not reach the steady state of mature spectroscopic techniques for quite some time. Rather it will continue to be a source of excitement and scientific surprises for the next few decades.