CHARGE-DENSITY-WAVE STRUCTURE IN NBSE3

We have studied the structure of the Q(1) charge-density wave (CDW) in pure and Ta-doped NbSe3 as a function of temperature using high-resol ution x-ray scattering. The CDW's phase-phase correlation lengths are much greater than the impurity spacing, and in pure crystals can be co mparable to typical sample thicknesses. The correlations decay exponen tially in all directions. In a heavily Ta-doped crystal, transverse co rrelation lengths vary with temperature as the square of the CDW gap. These experimental results agree with the weak-pinning model of Fukuya ma, Lee, and Rice, and with the weak-pinning interpretation of previou s transport measurements.