  Upon cooling, glass-forming liquids experience a two-step relaxation
associated to the cage rattling and the escape from the cage, and the following
decoupling between the \b{eta}- and the {\alpha}-relaxations. The found
decoupling behaviors have greatly changed the face of glassy physics and
materials studies. Here we report a novel dynamic decoupling that the
relaxation function changes gradually from a single-step to a two-step form as
temperature declines through the stress relaxation of various metallic glasses
in a broad time and temperature range below glass transition temperature (Tg).
Such a two-step relaxation is unexpected in glassy state and reveals a
decoupling of dynamic modes arising from two different mechanisms: a faster one
exhibiting ballistic-like feature, and a slower one associated with a broader
distribution of relaxation times typical of subdiffusive atomic motion. This
first observation of two-step dynamics in metallic glassy state points to a far
richer-than-expected scenario for glass relaxation.
