Impurity effect on the Fröhlich conductivity in NbSe3

The effect of Ta and Ti impurities on the anomalous transport properties (non-Ohmic dc conductivity and excess microwave absorption) of NbSe3 has been studied. Data from 22 samples show that the characteristic field E0 and the threshold field ET are very sensitive to doping. Both E0 and ET obey a c2 behavior where c is the Ta concentration. The anomalous microwave response is also suppressed by the presence of impurities. As in the dc case Ti is more effective than Ta in suppressing the anomalous behavior. These impurity studies lend strong support to the Fröhlich model in which the dc non-Ohmicity is ascribed to a sliding mode of the charge-density-wave condensate and the excess microwave absorption is ascribed to resonant oscillations of the condensate about pinning centers. The presence of impurities suppresses both the sliding and oscillatory modes. We also briefly describe the effect of the dynamic response of the massive condensate to very short pulses and its effect on non-Ohmic measurements.