Hop dynamics in glasses

Although glassy materials are widely used, we still know little about the underlying physics of the glass state. One key challenge is the non-equilibrium nature, which manifests in time-dependent material properties. A link between this ``aging'' and structural changes is still missing. Elementary structural relaxation events, called ``hops,'' have been identified as particles leaving the shell of their immediate neighbors. We present first results of molecular dynamics simulations with a new algorithm that enables us to track these hops throughout the system. Using a standard polymer glass model, we show a complete ``map'' of the hop-dynamics in the bulk. This data allows us to explore the correlations between relaxation events. Our map of hop-events is also useful in the study of dynamical heterogeneities (DH), a concept that arose from the discovery of ``faster'' and ``slower'' moving groups of particles in glasses. Current research aims at connecting their growth to the glass transition, but their observation and study has proven to be challenging. The hop-detection gives us a coarse-grained picture of the dynamics and we can identify DH as spatio-temporal accumulations of hops. This makes a direct study of DH possible and we present preliminary results based on a cluster analysis.