Our current knowledge of the basic properties of condensed bodies still relies heavily on the existence within such entities of periodic boundary conditions which allow the description of both the ionic and electronic subsystems in terms of relatively simple wave equations. The range of validity of such approaches is however limited to a small subset of materials of interest both in nature and also within those having applications in real life. This is best exemplified by current research on materials where their reduced dimensions (i.e. nanostructured materials) or others having significant amounts of disorder (i.e. colossal magnetoresistive materials) hamper their description in terms adequate to long-range-ordered objects such are the perfect crystals.
The description of systems such as those referred to above is conveniently made in atomistic terms by means of a hierarchy of space-time correlation functions. Such tools have been developed aiming to allow an understanding of simple liquids and glasses and are related to experimental observables which can be measured by means of radiation scattering techniques. Neutron scattering has been and still is the technique of choice for a truly quantitative measurement of such time and space-dependent properties. Its unrivalled status stems from the wide variety of frequencies (from tenths of a mirco-electron volt up to several eV) and spatial extents (from angstroms to tens of nanometers) explorable with such techniques.
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