Nonlinear transport of polarons

A method is presented to calculate the nonlinear mobility of a Fröhlich polaron in high electric fields for arbitrary strength of the electron-phonon coupling. It is based on an eikonal expansion of the exact Feynman influence functional for the nonequilibrium Wigner distribution function. The method becomes exact both in linear response and for large transport velocities. The nonlinear mobility is obtained from a momentum balance equation containing a systematic frictional force and the field-dependent fluctuations due to the eliminated phonon degrees of freedom. For weak coupling we recover the standard results of Stratton and Conwell. For strong coupling, however, the current-voltage characteristics are quite different from those obtained by Thornber and Feynman. Our results are in good agreement with measurements of high-field transport in InSb and AgCl.