A simple picture for structural glasses

This paper is an essay on mosaic structures in glasses, and their possible role in relaxation phenomena. (a) Near Tg the fluid is assumed to contain clusters, slightly more compact than the matrix: they cannot grow in size because of frustration effects, as noted by Kivelson and others. (Mode/mode coupling theories cannot describe this, because they do not incorporate frustration.) The size of the clusters corresponds to the Boson peak wavelength. (b) We propose that the standard free volume picture may be transposed: (i) clusters move rather than molecules, (ii) the required cavity space (‘vacancy’) for cluster motion is not empty, but filled with the low density matrix. (The old criticism against free volume based on pressure effects is thus removed.) (c) To reach one cluster, a ‘vacancy’ must hop through many ‘traps’: the distribution of hopping times ultimately leads to a stretched exponential for the relaxation, as argued by Bendler and Schlesinger. To cite this article: P.-G. de Gennes, C. R. Physique 3 (2002) 1263–1268.