Structure and dynamics in supercooled liquids: A theory of localized excitations

The microscopic motion of glass-forming liquids dramatically slows down with decreasing temperature. This slowdown is accompanied by dynamical heterogeneity where localized regions of particle mobility and extended immobile regions emerge from the liquid. There are two competing perspectives to explain these phenomena. One perspective proposes that structural/static properties of the liquid may be used to explain the slowdown. Another perspective, the dynamical facilitation (DF) theory, proposes that dynamics are driven by emergent excitations whose origins are believed to be independent of liquid structure, which then facilitate the creation and relaxation of nearby excitations. DF theory predicts many key properties of dynamics including, relaxation behaviors of single- and multi-component systems, temperature-dependence of heat capacities, breakdown of Stokes-Einstein diffusion, and competing dynamics of crystallization and vitrification.

However, two important and fundamental questions remain to be answered in DF theory: (1) what is the origin of excitations? and (2) why do excitations facilitate the relaxation and creation of nearby excitations? Here, we focus on answering the first question, where our work shows that the origin of excitations lies, in fact, within the structure. We do so by constructing a theory where excitations correspond to localized regions of pure shear emerging from the liquid. Inside the localized zone, a bond-breaking event occurs in the form of a T1 transition which then re-organizes the local structure. The energy barrier to form such excitations can be quantitatively predicted from the knowledge of structure and elasticity inherent within the liquid. We compare the predictions of our theory to that of the DF theory, where good quantitative agreement is found across six model glass formers with continuous poly-dispersity. These results may provide a new connection between the competing perspectives of supercooled liquid dynamics.