Single Particle Jumps in a Binary Lennard-Jones System Below The Glass Transition

TL;DR

This study uses molecular dynamics simulations to analyze particle jumps in a binary Lennard-Jones glass, revealing temperature-dependent jump behaviors, reversible and irreversible jumps, and insights into microscopic relaxation mechanisms below the glass transition.

Contribution

It introduces a detailed classification of particle jumps and their temperature dependence in a glassy system, highlighting the role of reversible and irreversible jumps in relaxation.

Findings

Both reversible and irreversible jumps occur at all temperatures.

Jump lengths and energy changes increase with temperature.

Waiting times between jumps are temperature-independent.

Abstract

We study a binary Lennard-Jones system below the glass transition with molecular dynamics simulations. To investigate the dynamics we focus on events ("jumps") where a particle escapes the cage formed by its neighbors. Using single particle trajectories we define a jump by comparing for each particle its fluctuations with its changes in average position. We find two kinds of jumps: "reversible jumps," where a particle jumps back and forth between two or more average positions, and "irreversible jumps," where a particle does not return to any of its former average positions. For all investigated temperatures both kinds of particles jump and both irreversible and reversible jumps occur. With increasing temperature relaxation is enhanced by an increasing number of jumps, and growing jump lengths in position and potential energy. However, the waiting time between two successive jumps is independent of temperature. This temperature independence might be due to aging, which is present in our system. The ratio of irreversible to reversible jumps is also increasing with increasing temperature, which we interpret as a consequence of the increased likelihood of changes in the cages, i.e. a blocking of the "entrance" back into the previous cage. A comparison of the fluctuations of jumping particles and non-jumping particles indicates that jumping particles are more mobile even when not jumping. The jumps in energy normalized by their fluctuations are decreasing with increasing temperature, which is consistent with relaxation being increasingly driven by thermal fluctuations. In accordance with subdiffusive behavior are the distributions of waiting times and jump lengths in position.