QUANTUM COHERENCE OF A 2-LEVEL SYSTEM INTERACTING WITH PHONONS IN A STRONG TIME-DEPENDENT ELECTRIC-FIELD

A master equation for population in a two-level system interacted with bosons in a time-dependent electric field is derived in a noninteracting-blip approximation. For a cw field the asymptotic solution of this equation is determined to all orders of the perturbation expansion with respect of the small parameter (DELTA/omega). It is found that the temperature for the transition from a coherent motion of a particle to a decay depends on the parameters of a field. At high temperatures and strong electric fields the decay rate increases exponentially with respect to the amplitude of the field when the field frequency is less than the relaxation energy of a lattice. At low temperatures a coherent motion is observed. The frequency of the coherent oscillations and the decay rate depends on the ratio of the amplitude to the frequency of the field. The transition from coherent to incoherent motion is governed by the field parameters. If the frequency of the field is larger than the relaxation energy one obtains a localization of a particle in a metastable well at low temperatures. A decay rate vanishes faster than a coherent frequency even if the coupling with phonons is strong. The estimations for quantum wells in GaAs/AlGaAs heterojunctions is made. (C) 1994 Academic Press, Inc.