  Using dynamical mean-field theory and the non-crossing approximation as
impurity solver, we study the response of a Mott insulator to strong dc
electric fields. The breakdown of the Mott insulating state is triggered by
field-induced creation of doublon-hole pairs. In a previous investigation, Ref.
[1], it was found that the system approaches a long-lived quasi-steady state in
which the current is time-independent although the number of carriers
constantly increases. Here we investigate and clarify the nature of this state,
which exists only because thermalization is slow in the Hubbard model at strong
coupling. The current is time-independent because doublons and holes have an
infinite temperature distribution. Evidence for this fact is obtained from
spectral functions and by comparing the electric current with the field-induced
doublon-hole creation rate. Implications to real experiments, in systems with
energy dissipation, are discussed.
