  Using nonequilibrium dynamical mean-field theory, we study the isolated
Hubbard model in a static electric field in the limit of weak interactions.
Linear response behavior is established at long times, but only if the
interaction exceeds a critical value, below which the system exhibits an
AC-type response with Bloch oscillations. The transition from AC to DC response
is defined in terms of the universal long-time behavior of the system, which
does not depend on the initial condition.
