Jump-precursor state emerges below the crossover temperature in supercooled o-terphenyl

Area of Science:

• Condensed matter physics
• Chemical physics

Background:

• Supercooled liquids exhibit distinct high-temperature diffusive and low-temperature activated dynamics separated by a crossover temperature (Tc).
• Understanding the molecular origins of this dynamic crossover is crucial for predicting material properties.

Purpose of the Study:

• To investigate the molecular mechanisms and emergent phenomena associated with the dynamic crossover in supercooled liquids.
• To analyze molecular dynamics simulations to identify features preceding large-scale molecular rearrangements.

Main Methods:

• All-atom, flexible o-terphenyl molecular dynamics simulations were performed.
• Advanced statistical methods were employed to analyze simulation data, extending to 14 μs relaxation times at 272.5 K.

Main Results:

• A distinct "jump-precursor" state emerges below Tc, characterized by a looser molecular cage and preceding orientational jumps.
• Rate heterogeneity becomes the dominant cause of stretched relaxation at Tc (290 K).
• The exchange time within the distribution of relaxation rates slows relative to alpha relaxation as temperature decreases.

Conclusions:

• The dynamic crossover is a broad transition zone, not a sharp boundary, involving multiple interconnected phenomena.
• Emerging features include increased heterogeneity, larger jump sizes, a precursor state, and slower rate exchange.
• These findings align with experimental observations near the glass transition, suggesting a consistent picture of dynamic crossover behavior.