Imaging Shear in Sliding Charge-Density Waves by X-Ray Diffraction Topography

We have imaged the electric-field-dependent structure of charge-density waves (CDWs) in ribbonlike NbSe₃ single crystals by monochromatic x-ray topography. Below the depinning threshold ET the CDW is well correlated. Just above ET, the CDW shears along longitudinal steps in crystal thickness associated with small-angle grain boundaries, and at high fields transverse correlations recover. These results demonstrate x-ray topography as an effective probe of superlattice structure and with earlier transport measurements establish that extrinsic sources dominate CDW plasticity observed in NbSe₃.