Unveiling the complex glassy dynamics of square shoulder systems: simulations and theory

TL;DR

This study combines simulations and theory to explore the complex glassy dynamics of square shoulder systems, revealing glass-glass transitions, higher-order singularities, and anomalous diffusion behaviors.

Contribution

It provides the first combined simulation and Mode Coupling Theory analysis of binary square shoulder systems, uncovering complex dynamical phenomena including glass-glass transitions and higher-order singularities.

Findings

Discovered non-monotonic diffusion behavior upon cooling.

Identified a disconnected glass-glass transition line with higher-order singularities.

Confirmed simulation results with novel MCT calculations using MD-derived structure factors.

Abstract

We performed extensive molecular dynamics (MD) simulations, supplemented by Mode Coupling Theory (MCT) calculations, for the Square Shoulder (SS) model, a purely repulsive potential where the hard-core is complemented by a finite shoulder. For the one-component version of this model, MCT predicted [Sperl {\it et al.} Phys. Rev. Lett. {\bf 104}, 145701 (2010)] the presence of diffusion anomalies both upon cooling and upon compression and the occurrence of glass-glass transitions. In the simulations, we focus on a non-crystallising binary mixture, which, at the investigated shoulder width, shows a non-monotonic behaviour of the diffusion upon cooling but not upon isothermal compression. In addition, we find the presence of a disconnected glass-glass line in the phase diagram, ending in two higher-order singularities. These points generate a logarithmic dependence of the density correlators as well as a subdiffusive behaviour of the mean squared displacement, although with the interference of the nearby liquid-glass transition. We also perform novel MCT calculations using as input the partial structure factors obtained within MD, confirming the simulation results. The presence of two hard sphere glasses, differing only in their hard core length, is revealed, showing that the simple competition between the two is sufficient for creating a rather complex dynamical behaviour.