Direct visualization of two-state dynamics on metallic glass surfaces well below T _g

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 T _g 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 min and extending up to 1000 min. Rearrangements of surface clusters 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 k _B T for surface clusters.