Microscopic origin of excess wings in relaxation spectra of supercooled liquids

TL;DR

This study uncovers the microscopic origins of asymmetric relaxation spectra in supercooled liquids near the glass transition, revealing that rare localized relaxations and dynamic facilitation produce the observed power law wings.

Contribution

We introduce a novel computational method to simulate supercooled liquids and identify the microscopic mechanisms behind the asymmetric relaxation spectra near $T_g$, linking heterogeneity and facilitation.

Findings

Power law wings emerge from localized relaxation regions.

Heterogeneous activated dynamics and facilitation explain spectral asymmetry.

Simulation results match experimental observations of supercooled liquids.

Abstract

Glass formation is encountered in diverse materials. Experiments have revealed that dynamic relaxation spectra of supercooled liquids generically become asymmetric near the glass transition temperature, $T_g$, where an extended power law emerges at high frequencies. The microscopic origin of this "wing" remains unknown, and was so far inaccessible to simulations. Here, we develop a novel computational approach and study the equilibrium dynamics of model supercooled liquids near $T_g$. We demonstrate the emergence of a power law wing in numerical spectra, which originates from relaxation at rare, localised regions over broadly-distributed timescales. We rationalise the asymmetric shape of relaxation spectra by constructing an empirical model associating heterogeneous activated dynamics with dynamic facilitation, which are the two minimal physical ingredients revealed by our simulations. Our work offers a glimpse of the molecular motion responsible for glass formation at relevant experimental conditions.