Scaling between relaxation, transport and caged dynamics in a binary mixture on a per-component basis

Area of Science:

• Condensed Matter Physics
• Materials Science
• Computational Chemistry

Background:

• A universal scaling relationship exists between slow dynamics (relaxation/transport) and fast picosecond rattling motion within atomic cages.
• This scaling has been observed in various simulations and experiments, but its fundamental basis and applicability across different material states remain areas of investigation.

Purpose of the Study:

• To demonstrate that the universal scaling law is independent of the glass transition region and relies on a characteristic length scale.
• To introduce and investigate a novel reduced rattling amplitude metric using molecular dynamics simulations.
• To explore the correlation between transport properties, structural relaxation, and rattling dynamics in atomic binary mixtures.

Main Methods:

• Extensive molecular-dynamics simulations were performed on an atomic binary mixture.
• Simulations covered diverse states varying in potential, density, and temperature.
• Analysis focused on incoherent van Hove functions, diffusivity, structural relaxation, and reduced rattling amplitude.

Main Results:

• The characteristic length scale a(2)(1/2) determines the scaling, independent of the glass transition region.
• A novel reduced rattling amplitude (1/2) was defined and investigated.
• Correlations between diffusivity, structural relaxation, and reduced rattling amplitude collapse onto a single master curve for both components of the binary mixture.
• The breakdown of the Stokes-Einstein law is predicted at a specific reduced rattling amplitude.
• Thermodynamic scaling (temperature/density) extends to the picosecond rattling motion, linking fast and slow dynamics.

Conclusions:

• The study establishes a robust scaling law between fast rattling motion and slow dynamics, applicable across various material states.
• The reduced rattling amplitude serves as a key parameter predicting transport and relaxation behaviors, including Stokes-Einstein law deviations.
• A direct link is demonstrated between fast picosecond dynamics and the thermodynamic scaling of slow dynamics, unifying different aspects of material behavior.