ANOMALOUS FREQUENCY DEPENDENCE OF DIFFERENTIAL MOBILITY IN SUPERLATTICE MINIBAND TRANSPORT

We examine the frequency-dependence of carrier drift-velocity in superlattice miniband conduction for a small signal ac electric field superposed on a dc bias E0. Our determination of the differential mobility is based on an accurate microscopic treatment of phonon and impurity scatterings. At finite E0, the differential mobility exhibits anomalous frequency-dependence: its real part exhibits a broad hump before finally approaching zero at high frequency, and its imaginary part has a marked dip to negative values before going through the conventional maximum. A simple dual-parameter empirical formula adapted from velocity overshoot studies provides excellent fits to this unusual behavior.