  The dynamic properties of liquid phase-change materials (PCMs), such as
viscosity $\eta$ and atomic self-diffusion coefficients D, play an essential
role in ultrafast phase switching behavior of novel non-volatile phase-change
memory applications, as they are intimately related to crystallization kinetics
and phase stabilities. To connect $\eta$ to D, the Stokes-Einstein relation
(SER) is commonly assumed to be valid at high temperatures near or above the
melting temperature $T_{m}$ and is frequently employed for assessing liquid
fragility (or crystal growth velocity) of technologically important PCM
compositions. However, using quasi-elastic neutron scattering (QENS), we give
here experimental evidence for a breakdown of the SER even at temperatures
above $T_{m}$ in the high-atomic-mobility state of a typical PCM,
Ge$_{1}$Sb$_{2}$Te$_{4}$, where the decay of density correlation functions
still remains exponential. The origin of the breakdown is thus unlikely the
result of dynamical heterogeneities, as is usually postulated for viscous
liquids. Rather, we discuss its possible connections to a metal-semiconductor
and fragile-strong transition hidden below $T_{m}$.
