Collective Dynamics on Intermediate
Time and Length Scale in Complex Matter
F. Mezei
Los Alamos National Laboratory
LANSCE-DO, MS H805
Los Alamos, NM 87545
(505) 667-7633
(505) 665-2676
Proposed subject for the ICAM workshop:
Fundamental properties of well ordered, crystalline materials are fully determined by what happens on the atomic length and time scale, i.e. by the structure of the elementary cell and by the elementary excitations and quasi particles. In contrast, in matter with a structure more complex than a simple periodic lattice there is a hierarchy of length and time scales, which can all have crucial roles determining macroscopic properties. The atomic scale structure and dynamics often strongly resembles to what happens in crystalline materials with similar short-range atomic arrangement. Interactions on this microscopic length scale are responsible for the primary stability and existence of the material, while a series of macroscopic properties will depend on what correlations prevail over distances of several atomic spacing. This can be due to some additional structure in a rather well ordered matter, such as stripes, or to an intermediate range order which becomes a crucial factor in the absence of long range order, e.g. in glasses or spin glasses. Correlations over an intermediate length scale between the atomic and macroscopic ones lead to dynamic properties, which tend to be characterized by time scales intermediate between that of local vibrations of atoms and macroscopic times and display a dissipative, relaxational nature in contrast to the elementary excitations in perfectly ordered model systems. By its very nature the dynamics of intermediate range correlations reflects the collective aspects in complex materials, as opposed to the local vibrations which are pretty much determined by local atomic configurations.
The experimental study of dynamics on intermediate length scale usually is an extremely difficult task. The difficulties come from the very nature of the problem: atomic scale vibrations are the dominating features, and one has to single out inherently small signals above this large "noise". Scattering techniques play a particular role here due to their unique capability of directly sorting out dynamic phenomena in space and time. Typical intermediate length (1-100 nm) and time (ps to ns) scales are best explored by high-resolution inelastic neutron scattering spectroscopy. Current developments at LANSCE (installation of powerful new cold moderators, development of new instrument concepts) open up the possibility of bringing experimental capabilities in this broad field of research to unprecedented levels within the next few years.
Recent progress in the exploration collective dynamics in matter close to the glass transition lends itself as an excellent sample case study for both illustrating the new insight information on intermediate scale dynamics can provide and the role of technical advances for making new territory accessible to experimental scrutiny.