CDW dynamics in NbSe$_3$ probed by $^{93}$Nb NMR

Using $^{93}$Nb NMR spin echoes applied on a single crystal of NbSe$_3$, we have probed electric-field induced displacements of the CDW forming below T$_2$=144K. In our experiments, good S/N was achieved by aligning the chain axis of a single large crystal (cross-section O(500$\mu m^2$) with the coil symmetry axis. Evidence for CDW motion throughout the sample for $E>E_T$ was observed in motional narrowing experiments. For $E\le 0.9E_T$, we find a wide distribution of displacements less than one CDW wavelength, for both unipolar and bipolar electric field pulse excitations. At $E\sim 0.9E_T$, the mean displacement is approximately 6-7 degrees, and the width of the distribution is about twice the mean displacement. We discuss the results in the context of the Fukuyama-Lee phase Hamiltonian, and describe the constraints imposed by these experiments on the proposal that CDW depinning is an example of a dynamic critical phenomenon.