Microscopic origin of excess wings in the relaxation spectra of deeply supercooled liquids*

The dielectric loss spectrum of many glass-forming liquids exhibits an excess contribution in the high-frequency part of the α-relaxation peak, referred to as an ``excess wing’’. The physical origin of the excess wing has been long debated, but a microscopic understanding remains elusive.

By combining the swap Monte Carlo algorithm to long molecular dynamics simulations, we provide a clear microscopic picture for the equilibrium relaxation dynamics of a model atomic supercooled liquid, down to the experimental glass transition at T_g. Near T_g, structural relaxation is initiated by dilute clusters of mobile particles. The distribution of appearance time of these clusters displays a power law at times much shorter than the structural relaxation time, which is responsible for the emergence of an excess wing in the relaxation spectrum. We show that large dynamically correlated regions at τ_α originate from the small initial clusters forming the excess wing, via dynamic facilitation.

We devise a simple model based on these observations, and successfully reproduce the qualitative shape of dielectric loss spectra, including the excess wing.