A Theory of Localized Excitations in Supercooled Liquids

TL;DR

This paper develops a structure-based field theory linking localized excitations in supercooled liquids to elastic properties and energy landscape features, enhancing understanding of glassy dynamics.

Contribution

It introduces a novel framework connecting excitation events to the potential energy landscape and elastic fluctuations, providing a quantitative formula for excitation energy barriers.

Findings

Good agreement between the derived energy barrier and DF theory predictions.

The framework links structural and elastic properties to localized excitations.

Strengthens the role of structure and elasticity in glassy dynamics.

Abstract

A new connection between structure and dynamics in glass-forming liquids is presented. We show how the origin of spatially localized excitations, as defined by dynamical facilitation (DF) theory, can be understood from a structure-based framework. This framework is constructed by associating excitation events in DF theory to hopping events between energy minima in the potential energy landscape (PEL). By reducing the PEL to an equal energy well picture and applying a harmonic approximation, we develop a field theory to describe elastic fluctuations about inherent states, which are energy minimizing configurations of the PEL. We model an excitation as a shear transformation zone (STZ) inducing a localized pure shear deformation onto an inherent state. We connect STZs to T1 transition events that break the elastic bonds holding the local structure of an inherent state. A formula for the excitation energy barrier, denoted as $J_\sigma$, is obtained as a function of inherent-state elastic moduli and radial distribution function. The energy barrier from the current theory is compared to one predicted by the DF theory where good agreement is found in various two-dimensional continuous poly-disperse atomistic models of glass formers. These results strengthen the role of structure and elasticity in driving glassy dynamics through the creation and relaxation of localized excitations.