Structure and dynamics of coupled viscous liquids

TL;DR

This study uses Monte-Carlo simulations to explore the structure and dynamics of coupled viscous liquids, revealing precursors to a phase transition and decoupling phenomena at low temperatures.

Contribution

It introduces a novel simulation approach to analyze coupled viscous liquids and identifies signatures of an equilibrium phase transition in this system.

Findings

Identification of a Widom line where susceptibilities peak

Decoupling of diffusion and structural relaxation in high-overlap regime

Evidence of precursors to a first-order phase transition

Abstract

We perform Monte-Carlo simulations to analyse the structure and microscopic dynamics of a viscous Lennard-Jones liquid coupled to a quenched reference configuration of the same liquid. The coupling between the two replicas is introduced via a field epsilon conjugate to the overlap Q between the two particle configurations. This allows us to study the evolution of various static and dynamic correlation functions across the (epsilon, T) equilibrium phase diagram. As the temperature is decreased, we identify increasingly marked precursors of a first-order phase transition between a low-Q and a high-Q phase induced by the field epsilon. We show in particular that both static and dynamic susceptibilities have a maximum at a temperature-dependent value of the coupling field, which defines a `Widom line'. We also show that, in the high-overlap regime, diffusion and structural relaxation are strongly decoupled because single particle motion mostly occurs via discrete hopping on the sites defined by the reference configuration. These results, obtained using conventional numerical tools, provide encouraging signs that an equilibrium phase transition exists in coupled viscous liquids, but also demonstrate that important numerical challenges must be overcome to obtain more conclusive numerical evidence.