COOPERATIVE EFFECTS IN THE NONLINEARLY DRIVEN SPIN-BOSON SYSTEM

We study the quantum dynamics of a periodically driven biased two-stat e system which additionally is subject to either Ohmic or to frequency -dependent damping. Within the noninteracting-blip approximation for t he stochastic forces, the solution is given in terms of a series which is defined by recursion relations. Avoiding rotating wave and Markovi an approximation, we deduce the solution in closed form for the import ant cases of high- and low-frequency driving. For low-frequency drivin g, the dynamics is governed by a rate equation with a time-dependent r ate and with a time-dependent adiabatic equilibrium state. In the high -frequency case, we find novel cooperative effects, such as resonances in the spectral distribution near fractional values of the static det uning energy.