  Using nonequilibrium dynamical mean-field theory, we compute the time
evolution of the current in a Mott insulator after a strong electric field is
turned on. We observe the formation of a quasistationary state in which the
current is almost time-independent although the system is constantly excited.
At moderately strong fields this state is stable for quite long times. The
stationary current exhibits a threshold behavior as a function of the field, in
which the threshold increases with the Coulomb interaction and vanishes as the
metal-insulator transition is approached.
