  The non-equilibrium relaxational properties of a three dimensional Coulomb
glass model are investigated by kinetic Monte Carlo simulations. Our results
suggest a transition from stationary to non-stationary dynamics at the
equilibrium glass transition temperature of the system. Below the transition
the dynamic correlation functions loose time translation invariance and
electron diffusion is anomalous. Two groups of carriers can be identified at
each time scale, electrons whose motion is diffusive within a selected time
window and electrons that during the same time interval remain confined in
small regions in space. During the relaxation that follows a temperature quench
an exchange of electrons between these two groups takes place and the
non-equilibrium excess of diffusive electrons initially present decreases
logarithmically with time as the system relaxes. This bimodal dynamical
heterogeneity persists at higher temperatures when time translation invariance
is restored and electron diffusion is normal. The occupancy of the two
dynamical modes is then stationary and its temperature dependence reflects a
crossover between a low-temperature regime with a high concentration of
electrons forming fluctuating dipoles and a high-temperature regime in which
the concentration of diffusive electrons is high.
