Dynamic and thermodynamic crossover scenarios in the Kob-Andersen mixture: Insights from multi-CPU and multi-GPU simulations

TL;DR

This study uses advanced multi-CPU and multi-GPU simulations to explore the thermodynamic and dynamic crossovers in the Kob-Andersen mixture, revealing insights into glass transition phenomena and the limitations of current simulation techniques.

Contribution

It introduces efficient parallel tempering protocols to investigate equilibrium properties below the mode-coupling temperature, clarifying the nature of thermodynamic anomalies and dynamic crossovers.

Findings

Specific heat grows regularly down to T=0.38

Crystallization hampers analysis below T=0.4

Hints of dynamic crossover from four-point susceptibility analysis

Abstract

The physical behavior of glass-forming liquids presents complex features of both dynamic and thermodynamic nature. Some studies indicate the presence of thermodynamic anomalies and of crossovers in the dynamic properties, but their origin and degree of universality is difficult to assess. Moreover, conventional simulations are barely able to cover the range of temperatures at which these crossovers usually occur. To address these issues, we simulate the Kob-Andersen Lennard-Jones mixture using efficient protocols based on multi-CPU and multi-GPU parallel tempering. Our setup enables us to probe the thermodynamics and dynamics of the liquid at equilibrium well below the critical temperature of mode-coupling theory, $T_{\rm MCT} = 0.435$. We find that below $T=0.4$ the analysis is hampered by partial crystallization of the metastable liquid, which nucleates extended regions populated by large particles arranged in an fcc structure. By filtering out crystalline samples, we reveal that the specific heat grows in a regular manner down to $T=0.38$. Possible thermodynamic anomalies suggested by previous studies can thus occur only in a region of the phase diagram where the system is highly metastable. Using the equilibrium configurations obtained from the parallel tempering simulations, we perform molecular dynamics and Monte Carlo simulations to probe the equilibrium dynamics down to $T=0.4$. A temperature-derivative analysis of the relaxation time and diffusion data allows us to assess different dynamic scenarios around $T_{\rm MCT}$. Hints of a dynamic crossover come from analysis of the four-point dynamic susceptibility. Finally, we discuss possible future numerical strategies to clarify the nature of crossover phenomena in glass-forming liquids.