A universal origin for secondary relaxations in supercooled liquids and structural glasses

TL;DR

This paper proposes a unified theoretical framework explaining secondary relaxations in supercooled liquids and glasses as a natural consequence of fluctuations and structural diversity, bridging different dynamical regimes.

Contribution

It introduces a model that accounts for secondary relaxations within the random first order transition theory, linking them to a low free energy tail in activation barriers.

Findings

Secondary relaxations arise from a low free energy tail in activation barriers.

Secondary relaxations become dominant near the crossover temperature $T_c$.

The model unifies primary and secondary relaxations within a single theoretical framework.

Abstract

Nearly all glass forming liquids display secondary relaxations, dynamical modes seemingly distinct from the primary alpha relaxations. We show that accounting for driving force fluctuations and the diversity of reconfiguring shapes in the random first order transition theory yields a low free energy tail on the activation barrier distribution which shares many of the features ascribed to secondary relaxations. While primary relaxation takes place through activated events involving compact regions, secondary relaxation corresponding to the tail is governed by more ramified, string-like, or percolation-like clusters of particles. These secondary relaxations merge with the primary relaxation peak becoming dominant near the dynamical crossover temperature $T_c$, where they smooth the transition between continuous dynamics described by mode-coupling theory and activated events.