Slow relaxations and stringlike jump motions in fragile glass-forming liquids: Breakdown of the Stokes-Einstein relation

TL;DR

This study uses molecular dynamics simulations to explore how stringlike jump motions in fragile glass-forming liquids lead to a breakdown of the Stokes-Einstein relation at low temperatures, highlighting the role of activated diffusion processes.

Contribution

It reveals the temperature-dependent crossover of particle motions and connects stringlike jumps to the violation of the Stokes-Einstein relation in glass-forming liquids.

Findings

Stringlike jump motions grow larger and more displacements increase with decreasing temperature.

The relaxation time and viscosity grow faster than inverse diffusion constant at low T.

Diffusion at low T is dominated by activated jumps over high potential barriers.

Abstract

We perform molecular dynamics simulation on a glass-forming liquid binary mixture with the soft-core potential in three dimensions. We investigate crossover of the configuration changes caused by stringlike jump motions. 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. Then, the contribution of the latter to time-correlation functions tends to be long-lived as $T$ is lowered. As a result, 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. At low $T$, the diffusion occurs as activation processes and may well be described by short-time analysis of rare jump motions with broken bonds and large displacements. Some characteristic features of the van Hove self-correlation function arise from escape jumps over high potential barriers.