Effects of charge-density-wave depinning on the elastic properties of NbSe3

Detailed measurements on the dc-voltage dependence of the Youngs modulus E, shear modulus G, and their associated internal friction, in NbSe3 below both charge-density-wave (CDW) transitions, are reported. As for TaS3 [Phys. Rev. B 36, 2969 (1987)], the moduli decrease when the CDW becomes depinned, with the relative change in G an order of magnitude greater than that in E. However, the changes, which are typically 2 orders of magnitude smaller for NbSe3, are much sharper functions of voltage. Most surprisingly, the total magnitudes of the modulus changes are correlated with the threshold depinning field, and the internal friction associated with G (and with E in the lower CDW state) is less when the CDW is depinned than pinned. ac-induced softening of G, but not E, for voltages well below threshold is also observed. The implications of these results on various models of CDW depinning are discussed.