Takeshi Kawasaki, Akira Onuki
We perform molecular dynamics simulation on glassy particle systems in three dimensions using the soft-core potential. We investigate the configuration changes caused by string-like jump motions on long timescales, the thermal vibrational motions on short timescales, and their close correlations. First, with lowering the temperature $T$, the motions of the particles composing strings become larger in sizes and displacements, while those of the particles surrounding strings become smaller. Thus, the contribution of the latter to time-correlation functions tends to be long-lived as $T$ is lowered. Due to this crossover, the relaxation time $\tau_\alpha$ and the viscosity $\eta$ grow more steeply than the inverse diffusion constant $D^{-1}$ at low $T$, leading to breakdown of the Stokes-Einstein relation. Second, we examine the thermal vibrational motions introducing a vibration length $S_i(t)$ for each particle $i$. We show that particles with larger $S_i(t)$ tend to trigger jump motions more frequently at later times. The structure factor for the inhomogeneity of $S_i(t)^2$ is also calculated. Third, the diffusion at low $T$ may well be described by short-time analysis of rare jump motions with broken bonds and large displacements. We also find some characteristic features of the van Hove self-correlation function at low $T$, which arise from escape jumps over high potential barriers.
View original:
http://arxiv.org/abs/1210.0369
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