Investigation of Elastic Effects of Sliding Charge - Waves

The elastic softening associated with the sliding of a charge-density wave (CDW), driven by an electric field, is investigated in o-TaS_3 by use of a vibrating-reed technique. Experiments performed with a dc sliding CDW find the magnitude of the change in Young's modulus, Y, to be proportional to the square of the CDW order parameter and to be independent of pinning strength, in agreement with most theories regarding the phenomenon. The fractional portion of the softening realized at any given field is found to be controlled by the current transported by the CDW. Analysis is presented showing the softening for dc sliding to be consistent with relaxation involving a broad distribution of characteristic times. This distribution is correlated to that for the dielectric function of the stationary state, providing evidence that the pinning forces are not fundamentally different between the stationary and sliding states. Experiments where the CDW was driven by an ac electric field, switched at a frequency f _{rm I} from 1 Hz to MHz, showed a non-monotonic dependence of the degree of softening on f_{rm I} near 10 kHz accompanied by a peak in internal friction, and recovery of Y towards its value for a stationary CDW at higher frequency. Simultaneously, the amplitude of the overshoot phenomenon was seen to decrease with f _{rm I}. Such findings indicate the presence of slow dynamical processes which are though to involve relaxations of the CDW phase configuration. The non-monotonic behavior of Y at intermediate frequency is speculated to be due to availability of new relaxational modes not accessible to the dc sliding CDW.