On the location of the excess wing relative to the a-loss peak in the susceptibility spectra from simulations with the swap Monte Carlo algorithm

TL;DR

This paper identifies the excess wing in susceptibility spectra from swap Monte Carlo simulations as the unresolved Johari-Goldstein b-relaxation, using the Coupling Model to accurately locate it relative to the alpha-loss peak, aligning simulation results with experimental data.

Contribution

It applies the Coupling Model to simulation data to determine the position of the excess wing as the JG b-relaxation, bridging the gap between simulations and experimental observations.

Findings

Excess wing corresponds to unresolved JG b-relaxation.

Coupling Model accurately predicts the location of the excess wing.

Simulation results align with experimental data of molecular glass-formers.

Abstract

An advance was made by Guiselin et al. [arXiv:2103.01569 (2021)] in molecular dynamics simulations of the equilibrium dynamics of supercooled liquids near the experimental glass transition by utilizing the giant equilibration speedup provided by the swap Monte Carlo algorithm. The found emergence of a power law in relaxation spectra at lower temperatures on the high frequency flank of the alpha-loss peak in analogy to the excess wings observed experimentally in molecular glass-formers. Their remarkable finding leads to the question of where the excess wing is located relative to the alpha-loss peak in the susceptibility spectrum. I provide an answer by identifying the excess wing as the unresolved Johari-Goldstein (JG) b-relaxation and using the reciprocal of its relaxation time tau_JG to assess the location of the excess wing. The Coupling Model (CM) has a history of being successful in determining approximately the values of tau_JG(T) in molecular liquids whether the JG beta-relaxation is resolved or not (i.e., excess wing). It is applied to the simulation data and the results of tau_JG(T) successfully account for the locations of the excess wings in the simulation spectra at different temperatures. The time evolution of the dynamics of the distribution of processes composing the JG b-relaxation suggested by the CM based on experimental data in molecular glass-formers are in agreement with that given by Guiselin et al. from simulations of a size-polydisperse mixture of N soft spheres interacting with a repulsive power law pair potential. The agreement brings their results closer to experimental data of real molecular glass-formers.