Asymmetric Energy Landscapes Control Diffusion in Glasses

TL;DR

This paper reveals that in glasses, large diffusion activation energies are mainly due to correlated back-and-forth atomic motions caused by asymmetric energy barriers, not local rearrangement barriers.

Contribution

It introduces a quantitative framework linking energy landscape asymmetry to diffusion dynamics in glasses, applicable across different disordered materials.

Findings

Back-and-forth correlations dominate activation energy.

Asymmetry between forward and reverse barriers is key.

Framework applies to metallic glasses, silica, and Lennard-Jones systems.

Abstract

While diffusion in crystalline solids is quantitatively understood through defect-mediated atomic hops, no comparable quantitative framework exists for glasses. In these systems, the origin of large diffusion activation energies remains puzzling, despite local rearrangements involving low barriers. Using molecular dynamics simulations of metallic glasses, we decompose diffusion into random-walk and correlation contributions and find that back-and-forth correlated motion, not local rearrangement barriers, dominates the activation energy, resolving how low-barrier rearrangements yield large macroscopic activation energies. These correlations arise from asymmetry between forward and reverse barriers, a generic feature of disordered energy landscapes. We find that the correlation-driven mechanism is active beyond metallic glass alloys, including SiO2 and a single-component Lennard-Jones glass. The latter demonstrates that the correlation originates from structural disorder rather than chemical complexity. The framework also explains accelerated surface diffusion, where reduced activation energies arise primarily from weaker correlations rather than changes in local rearrangement barriers. Our results establish a direct, quantitative link between atomic-scale dynamics and macroscopic transport, providing a predictive basis for kinetics in disordered materials.