Dislocations and the motion of weakly pinned charge-density waves: Experiments on niobium triselenide containing mobile indium impurities

The threshold field E(T) for conduction by the charge-density wave (CDW) which forms in NbSe3 at 144 K has been studied in specimens containing about 1% of intercalated In. The In increases E(T) by an amount delta E(T), which is greatest when the CDW is at rest, and can be reduced almost to zero by its continuous motion. The time taken for delta E(T) to adjust to a change in the state of the CDW (of the order of Is at 115 K) varies as exp(theta/T), where T is the temperature and theta approximate to 3200 K. In a specimen which exhibits ''mode locking'' between an applied alternating field and the frequency nu of the quasiperiodic component of the current I-C carried by the CDW, the development of delta E(T) is accompanied by a decrease in the ratio I-C/nu. The pinning giving rise to delta E(T) is attributed to the minimization of free energy through the thermally activated diffusion of In in a potential exerted by the CDW. The greatest delta E(T) develops in the steady potential exerted by the CDW at rest; when the CDW is in motion the potential varies periodically at a rate fast compared with the diffusion, and delta E(T), which responds to its mean value, is reduced accordingly. Various couplings between the in and CDW which might provide the potential are examined, and it is concluded that the only coupling consistent with the observed time dependence of delta E(T) is that between the In and the strain field of dislocations in the CDW. To account for the behavior of delta E(T), it is necessary to assume that the dislocations are on the boundaries of localized regions of the CDW which remain pinned when the rest is in motion. The behavior of I-C/nu, shows that such immobile regions expand as delta E(T) develops, and thus confirms their existence in a CDW which otherwise moves coherently. The coexistence of stationary and moving regions clearly conflicts with the model of Fukuyama, Lee, and Rice (FLR), which treats the CDW as an elastic medium, and is widely accepted as applying to NbS3 except when macroscopic crystal defects are present. The evidence of stationary regions in every specimen examined, even when perfect enough to exhibit complete mode locking, suggests that they are intrinsic to the depinning of a weakly pinned CDW from randomly distributed impurities. A modification of the FLR model is proposed, in which immobile regions appear as a result of the thermal nucleation of dislocation loops where large stresses arise from the nonuniformity of the pinning.