Local structural excitations in model glasses

Abstract

Structural excitations of model Lennard-Jones glass systems are investigated using the activation-relaxation technique (ART$n$), which explores the potential energy landscape around a local minimum energy configuration by converging to a nearby saddle-point configuration. Performing ART$n$ results in a distribution of barrier energies that is single-peaked for well-relaxed samples. The present work characterizes such atomic-scale excitations in terms of their local structure and environment. It is found that, at zero applied stress, many of the identified events consist of chainlike excitations that can either be extended or ringlike in their geometry. The location and barrier energy of these saddle-point structures are found to correlate with the type of atom involved, and with spatial regions that have low Kelvin eigenshear moduli and are close to the excess free volume within the configuration. Such correlations are, however, weak and more generally the identified local structural excitations are seen to exist throughout the model glass sample. The work concludes with a discussion within the framework of $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ relaxation processes that are known to occur in the undercooled liquid regime.