  Modern detector technology and highly brilliant fourth-generation
synchrotrons allow to improve the temporal resolution in time-resolved
diffraction studies. Profiting from this, we applied temperature scanning X-ray
photon correlation spectroscopy (XPCS) to probe the dynamics of a Pt-based
metallic glass former in the glass, glass transition region, and supercooled
liquid, covering up to six orders of magnitude in time scales. Our data
demonstrates that the structural alpha-relaxation process is still observable
in the glass, although it is partially masked by a faster source of
decorrelation observed at atomic scale. We present an approach that interprets
these findings as the superposition of heterogeneous liquid-like and
stress-driven ballistic-like atomic motions. This work not only extends the
dynamical range probed by standard isothermal XPCS, but also clarifies the fate
of the alpha-relaxation across the glass transition and provides a new
perception on the anomalous, compressed temporal decay of the density-density
correlation functions observed in metallic glasses and many out-of-equilibrium
soft materials.
