Elastic properties of charge-density-wave conductors in applied electric fields.

Abstract

The elastic properties of the charge-density-wave (CDW) conductors ${\mathrm{TaS}}_{3}$ and ${\mathrm{NbSe}}_{3}$ have been measured in the presence of ac, dc, and combined ac and dc longitudinal electric fields. For both ${\mathrm{TaS}}_{3}$ and ${\mathrm{NbSe}}_{3}$, application of ac fields ${E}_{\mathrm{ac}}$ in the MHz frequency range softens the crystal lattice (as indicated by a decrease in Young's modulus Y), even for values of ${E}_{\mathrm{ac}}$ well below the dc threshold field ${E}_{T}$ for CDW depinning. For applied dc fields ${E}_{\mathrm{dc}}$, lattice softening is observed only for ${E}_{\mathrm{dc}>{E}_{T}}$, consistent with previous studies. For combined ac and dc fields, anomalies in the electronic response (such as Shapiro step structure) are reflected also in Y and the internal friction \ensuremath{\delta}. On mode-locked steps, the elastic constants tend toward their pinned CDW values, despite the finite CDW drift velocity. It is suggested that these electroelastic experiments probe directly the internal degrees of freedom of the CDW, and our results are discussed in terms of applicable theoretical models.