The Structural Origin of Enhanced Dynamics at the Surface of a Glassy Alloy

TL;DR

This study reveals that the enhanced surface mobility in glassy alloys is structurally rooted in increased Debye-Waller factors, with simulations showing a correlation between surface softness and diffusion behavior.

Contribution

It introduces a combined simulation and normal mode analysis approach to link surface mobility enhancement to structural features in glassy alloys.

Findings

Surface Debye-Waller factor peaks at the surface.

Bulk and surface diffusion relate similarly to Debye-Waller factors.

Surface constraint decrease explains mobility enhancement.

Abstract

The enhancement of mobility at the surface of an amorphous alloy is studied using a combination of molecular dynamic simulations and normal mode analysis of the non-uniform distribution of Debye-Waller factors. The increased mobility at the surface is found to be associated with the appearance of Arrhenius temperature dependence. We show that the transverse Debye-Waller factor exhibits a peak at the surface. Over the accessible temperature range, we find that the bulk and surface diffusion coefficients obey the same empirical relationship with the respective Debye-Waller factors. Extrapolating this relationship to lower T, we argue that the observed decrease in the constraint at the surface is sufficient to account for the experimentally observed surface enhancement of mobility.