  The many-body formalism for dynamical mean-field theory is extended to treat
nonequilibrium problems. We illustrate how the formalism works by examining the
transient decay of the oscillating current that is driven by a large electric
field turned on at time t=0. We show how the Bloch oscillations are quenched by
the electron-electron interactions, and how their character changes
dramatically for a Mott insulator.
