Space- and time-resolved X-ray diffraction from pinned and sliding charge-density-waves in NbSe3

We have determined the spatial distribution of the local charge-density-wave (CDW) strain in the sliding state of NbSe3. The strain is measured by monitoring the spatially-varying shift q(x) of the CDW satellite wave vector between current contacts. Experiments were carried out at T = 90 K in the upper CDW state using high spatial resolution (30-50 µm) X-ray diffraction. Applying direct currents about twice the threshold value, we observe a steep exponential decrease of the shift within a few hundred microns from the contact followed by a linear variation of q in the central section of the sample. This latter regime is attributed to transverse pinning of the CDW dislocation loops (DL), while the exponential regime is controlled by the finite DL nucleation rate. Additional to these data in the stationary state of the sliding CDW, we investigated the relaxation of the CDW strain q(t) upon switching off the current (T = 75 K). Using time-resolved high-spatial resolution X-ray diffraction, we observe at 800 µm from the electrode a decay law of the stretched exponential type : q(t) = qoexp(-(t/τ)µ) , with τ = 283 ms and µ = 0.37