Spin dynamics and violation of the fluctuation dissipation theorem in a nonequilibrium ohmic spin-boson model

We present results for the dynamics of an impurity spin coupled to a magnetic field and to two ohmic baths that are out of equilibrium due to the application of a bias voltage. Both the nonequilibrium steady state and the rate constants describing the approach to the steady state are found to depend sensitively on the relative strengths of a magnetic field and a voltage dependent decoherence rate. Computation of physical quantities, including the frequency dependent ratio of the response to the correlation functions and the probabilities of the two spin states, allows the extraction of voltage dependent effective temperatures. The temperatures extracted from different quantities differ from one another in magnitude and in their dependence on parameters, and, in general, are nonmonotonic.