Enhanced cooperativity below the caging temperature of o-terphenyl

TL;DR

This paper demonstrates how a cooperative length scale describes glass-forming liquid dynamics below the caging temperature, using simulations and experimental data to quantify the growth of slow regions as temperature decreases.

Contribution

It introduces a method to extract the size of slow regions in glass-forming liquids from probe measurements, validated against 4-D NMR data, and clarifies their relation to cooperative events.

Findings

Cooperative length scale grows as temperature decreases below T_A.

Probe rotation/diffusion measurements can quantify slow region sizes.

The size of slow regions matches the largest cooperative events.

Abstract

The utility of a cooperative length scale for describing the dynamics of glass-forming liquids is shown using literature data on o-terphenyl. Molecular dynamics and Monte Carlo simulations reveal a distribution of cooperative fractal events below the caging temperature T_A. Guided by these results, we show how to extract the size of slow regions in any glass-forming liquid from probe rotation/diffusion measurements, which agrees quantitatively with 4-D NMR and grows steadily as temperature is lowered below T_A. We clarify why this length must also be the size of the largest cooperative events.