Thermal fluctuations and charge-density-wave depinning inNbSe3: Evidence for phase creep

We have studied the temperature dependence of the threshold electric field ${\mathit{E}}_{\mathit{T}}$ for charge-density-wave (CDW) depinning in ${\mathrm{NbSe}}_3$. At temperatures well below the Peierls transition ${\mathit{T}}_{\mathit{P}}$, the fractional increase in ${\mathit{E}}_{\mathit{T}}$ with decreasing temperature is independent of crystal thickness. In very thin crystals near ${\mathit{T}}_{\mathit{P}}$, the sharp threshold vanishes and nonlinear conduction occurs at arbitrarily small fields. A weak-pinning analysis suggests (1) that ${\mathit{E}}_{\mathit{T}}$’s low temperature increase may arise from a temperature dependence of the effective condensate density, and (2) that ${\mathit{E}}_{\mathit{T}}$’s vanishing in thin crystals is due to thermally assisted depinning of the CDW.