We have imaged the electric-field-dependent structure of charge-density waves (CDWs) in ribbonlike NbSe 3 single crystals by monochromatic x-ray topography. Below the depinning threshold E T the CDW is well correlated. Just above E T , 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 NbSe 3 . PACS numbers: 71.45.Lr, 61.72.Ff Charge-density waves (CDWs) in quasi-one-dimensional conductors provide one of the most important systems in condensed matter for studying collective dynamics in the presence of disorder. Above a threshold electric field E T , the CDW depins from impurities and slides relative to the host lattice, producing a nonlinear current-field relation, coherent voltage/current oscillations whose fundamental frequency n is proportional to the CDW velocity, and a rich variety of other phenomena [1]. Related phenomena are observed in the flux lattices of type-II superconductors [2] and in Wigner crystals [3].The spatiotemporal dynamics that underlies these phenomena is complex. Early theoretical efforts modeled the CDW as a perfectly elastic periodic medium interacting with random impurities and predicted a sliding state with long-range temporal order [4]. Experimentally, CDWs can show abundant evidence of plasticity especially at fields just above E T , including broad and complex oscillation spectra and enormous f 2a noise. More recent calculations [5] predict field-dependent plasticity that degrades temporal and spatial CDW order in the sliding state. Evaluating these predictions (and related predictions for flux lattices) is problematic because it is often not obvious whether observed plasticity is intrinsic to the dynamics of the randomly pinned periodic medium or instead arises from such extrinsic sources as boundary conditions and chemical or structural inhomogeneities.We have directly observed field-dependent CDW structure in NbSe 3 using x-ray diffraction topography. Above E T the CDW shears along the longitudinal steps in crystal thickness characteristic of this and many related CDW conductors, and transverse CDW correlations recover somewhat at high fields. Our results demonstrate the potential of x-ray topography as a structural probe of CDWs and other superlattices and establish an important extrinsic source of CDW plasticity.A CDW consists of a modulation of the conduction electron density of wave vector Q 2k F coupled to a lattice distortion u͑r, t͒ u 0 sin͓Q ? r 1 f͑r, t͔͒, where f is the phase of the CDW order parameter. X-ray scattering from the distortion produces superlattice peaks displaced from host lattice Bragg peaks by 6Q. In a macroscopically homogeneous system the line shape of these peaks is related to the CDW's phase-phase correlation function, and the inverse half widt...

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Using high-resolution x-ray scattering techniques, we have measured the transverse static structure factor of the sliding charge-density wave ͑CDW͒ in NbSe 3. For temperatures between 70 and 120 K and for applied currents up to 40 times the threshold current for sliding, the scattering peak for the sliding CDW is significantly broader than that for the pinned CDW, indicating that the sliding state is less correlated than the pinned state. Using scaling analysis, we show that the CDW phase roughness exponent ␣ rises from 0.60Ϯ0.01 in the pinned state to 0.80Ϯ0.01 in the sliding state, indicating that the phase fronts of the sliding CDW are significantly rougher than those of the pinned CDW.

Using time-resolved, high-resolution x-ray scattering techniques, we have measured the evolution of the transverse structure of the NbSe 3 Q 1 charge-density wave as it relaxes from the sliding state to the pinned state. Measurements were made at temperatures between 70 and 120 K and at electric field strengths between 23 and 103 the threshold for sliding. These time-dependent data are accurately described by dynamic scaling theory. [S0031-9007(99)