NONLINEAR TRANSPORT OF POLARONS

A method is presented to calculate the nonlinear mobility of a Frohlic h polaron in high electric fields for arbitrary strength of the electr on-phonon coupling. It is based on an eikonal expansion of the exact F eynman influence functional for the nonequilibrium Wigner distribution function. The method becomes exact both in linear response and for la rge transport velocities. The nonlinear mobility is obtained from a mo mentum balance equation containing a systematic frictional force and t he field-dependent fluctuations due to the eliminated phonon degrees o f freedom. For weak coupling we recover the standard results of Stratt on and Conwell. For strong coupling, however, the current-voltage char acteristics are quite different from those obtained by Thornber and Fe ynman. Our results are in good agreement with measurements of high-fie ld transport in InSb and AgCl.