Pinning and depinning process of an incommensurate CDW as revealed by coherent X-ray diffraction

Experiments on the coherent X-ray diffraction, and their modeling, have been performed on the Charge Density Wave (CDW) system NbSe3. The 2kF2kF satellite reflection associated with the CDW has been measured with respect to external dc currents. Below the threshold current, reflection displays several fringes in the transverse direction which disappear when the threshold current is exceeded. In the sliding state, the transverse satellite profile has a form of two nonsymmetric peaks, one of them being centered at the same position as below the threshold and another one being shifted. The shift of the longitudinal peak position below the threshold current and the nonsymmetric peak in the transverse direction above the threshold one is interpreted as the influence of strong linear defect like a crystal step present on the sample surface, combined with induced arrays of dislocations. Coherent X-rays provide a new access to processes in a CDW driven by an external force in a random pinning potential.