Bloch particle in the presence of dc and ac fields

In this Letter we study the metastable states of a Bloch particle in the presence of external ac and de fields. For the resonance condition between the period of the driving frequency and the Bloch period, the complex quasi-energies 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 mechanism, is found. This effect is demonstrated to be a kind of quantum phenomenon sensitive to the resonance condition. (C) 1998 Elsevier Science B.V.