Excess wings and asymmetric relaxation spectra in a facilitated trap model

TL;DR

This paper introduces a facilitated trap model that combines spatial heterogeneity and kinetic facilitation to explain excess wings and asymmetric relaxation spectra observed in supercooled liquids, supported by numerical and analytical analysis.

Contribution

It develops a minimal empirical facilitated trap model that captures the physics of excess wings and asymmetric spectra in supercooled liquids, linking microscopic facilitation to macroscopic relaxation features.

Findings

Facilitation accelerates relaxation of deep traps.

Spectra become asymmetric with excess high-frequency signals.

Model reproduces experimental spectra of organic supercooled liquids.

Abstract

In a recent computer study, we have shown that the combination of spatially heterogeneous dynamics and kinetic facilitation provides a microscopic explanation for the emergence of excess wings in deeply supercooled liquids. Motivated by these findings, we construct a minimal empirical model to describe this physics and introduce dynamic facilitation in the trap model, which was initially developed to capture the thermally-activated dynamics of glassy systems. We fully characterise the relaxation dynamics of this facilitated trap model varying the functional form of energy distributions and the strength of dynamic facilitation, combining numerical results and analytic arguments. Dynamic facilitation generically accelerates the relaxation of the deepest traps, thus making relaxation spectra strongly asymmetric, with an apparent "excess" signal at high frequencies. For well-chosen values of the parameters, the obtained spectra mimic experimental results for organic liquids displaying an excess wing. Overall, our results identify the minimal physical ingredients needed to describe excess processes in relaxation spectra of supercooled liquids.