Effects of charge-density-wave depinning on the elastic properties ofNbSe3

Detailed measurements on the dc-voltage dependence of the Young’s modulus E, shear modulus G, and their associated internal friction, in ${\mathrm{NbSe}}_3$ below both charge-density-wave (CDW) transitions, are reported. As for ${\mathrm{TaS}}_3$ [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 ${\mathrm{NbSe}}_3$, 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.