The dynamics of amorphous material at low temperatures could be characterized as being a sequence of discrete activated events via which the topological network was locally reorganized. By using the activation-relaxation technique, more than 8000 events were created in order to provide an extensive database of relaxation and diffusion mechanisms. The properties of these events (size, number of atoms involved, activation energy) were found to be compatible with experimental data. A topological classification of events was applied to the study of those events which involved only 4-fold coordinated atoms. Contrary to expectations, there was found to be no significant presence of crystalline-type defects (interstitials, vacancies) in the events. Diffusion was controlled by jumps of coordination defects that could require significant reorganization of the lattice. There was no evidence of diffusion being dominated by highly stretched bonds. It was also expected that defect-mediated relaxation would be easier than fully 4-fold coordination. However, it was instead found that the distribution of activation energies was very similar to that of perfect events, with a peak at about 4eV and a tail which extended to 14eV. The total number of atoms which was involved was essentially insensitive to the class of event. This was attributed to the fact that more than 60% of conserved events were based upon one of the 3 mechanisms which dominated perfect events.

Identification of Relaxation and Diffusion Mechanisms in Amorphous Silicon G.T.Barkema, N.Mousseau: Physical Review Letters, 1998, 81[9], 1865-8