Imaging shear in sliding charge-density waves by X-ray diffraction topography

We have imaged the electric-field-dependent structure of charge-density wav es (CDWs) in ribbonlike NbSe3 single crystals by monochromatic x-ray topogr aphy. Below the depinning threshold E-T the CDW is well correlated. Just ab ove E-T, the CDW shears along longitudinal steps in crystal thickness assoc iated with small-angle grain boundaries, and at high fields transverse corr elations recover. These results demonstrate x-ray topography as an effectiv e probe of superlattice structure and with earlier transport measurements e stablish that extrinsic sources dominate CDW plasticity observed in NbSe3.