  We report nano-scale mechanical heterogeneity of a metallic glass
characterized by dynamic atomic force microscopy. Apparent energy dissipation
with the variation of ~12%, originating from non-uniform distribution of local
viscoelasticity, was characterized. The correlation length of heterogeneous
viscoelasticity was measured to be ~2.5{\pm}0.3 nm, which is well consistent
with the dimension of shear transformation zones for plastic flow. This study
provides the first experimental observation on the nano-scale mechanical
heterogeneity in a metallic glass, and may fill the gap between atomic models
and the macroscopic properties of metallic glasses.
