a Transport Theory for Polarons.

A general theory is proposed for the transport of polarons in strong external fields of arbitrary time -dependence. The theory consists of a set of two transport equations, containing the one- and two-particle correlation functions as dynamical degrees of freedom. The equations are derived by expressing the generating functional of particle correlation functions as a complex-time path integral and by approximating two of the identities this generating functional implies. The transport equations are shown to be consistent with energy and momentum conservation, Kubo's linear response formula, and the fluctuation-dissipation theorem. The solution of the equations for the weakly -coupled optical polaron in a constant electric field is given and compared with other theories and with experimental observations in InSb. Finally, the qualitative behavior of solutions for strong coupling is discussed.