Bloch particle in the presence of dc and ac fields

In this Letter we study the metastable states of a Bloch particle in the pr esence of external ac and de fields. For the resonance condition between th e period of the driving frequency and the Bloch period, the complex quasi-e nergies are numerically calculated for two qualitatively different regimes (quasi-regular and chaotic) of the system dynamics. For the chaotic regime an effect of quantum stabilization, which suppresses the classical decay me chanism, is found. This effect is demonstrated to be a kind of quantum phen omenon sensitive to the resonance condition. (C) 1998 Elsevier Science B.V.