Dynamic correlations and heterogeneity in the primary and secondary relaxations of a model molecular liquid

Molecular dynamics simulations were carried out on a series of Lennard-Jones binary mixtures of rigid, asymmetric, dumbbell-shaped molecules. Below an onset temperature, the rotational and translational dynamics split into the slow structural α relaxation and a higher-frequency Johari-Goldstein β relaxation. Both processes are dynamically heterogeneous, having broad distributions of relaxation times. However, only the α relaxation shows strong dynamic correlations; correlations at the β time scale are weak, in particular for molecules having shorter bonds. Despite the close connection between the two processes, we find no correlation between the α and β relaxation times of individual molecules; that is, a molecule exhibiting slow β motion does not necessarily undergo slow α dynamics and likewise for fast molecules. However, the single-molecule α relaxation times do correlate with both the α and β relaxation strengths.