Direct Visualization of Two-State Dynamics on Metallic Glass Surfaces Well Below Tg

TL;DR

This study directly visualizes surface dynamics of metallic glasses deep below their glass transition temperature, revealing two-state hopping behavior with heterogeneous spatial and temporal characteristics.

Contribution

It provides the first direct atomically resolved observation of two-state surface dynamics in metallic glasses deep in the glassy regime.

Findings

Surface clusters exhibit two-state hopping with ~6% probability of three-state events.

Clusters are compact, 2-8 atomic spacings wide, and show heterogeneous dynamics.

Average activation free energy for surface clusters is estimated at 14 kBT.

Abstract

Direct atomically resolved observation of dynamics deep in the glassy regime has proved elusive for atomic and molecular glasses. Studies below the glass transition temperature Tg are especially rare due to long waiting times required to observe dynamics. Here we directly visualize surface glass dynamics deep in the glassy regime. We analyze scanning tunneling microscopy movies of the surface of metallic glasses with time resolution as fast as 1 minute and extending up to 1,000 minutes. Rearrangements of surface cluster occur almost exclusively by two-state hopping (P(3-state) ~ 0.06). All clusters are compact structures with a width of 2-8 atomic spacings along the surface plane. The two-state dynamics is both spatially and temporally heterogeneous. We estimate an average activation free energy of 14 kBT for surface clusters.