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-resolution x-ray scattering. The CDW's phase-phase correlation lengths are much greater than the impurity spacing, and in pure crystals can be comparable to typical sample thicknesses. The correlations decay exponentially in all directions. In a heavily Ta-doped crystal, transverse correlation lengths vary with temperature as the square of the CDW gap. These experimental results agree with the weak-pinning model of Fukuyama, Lee, and Rice, and with the weak-pinning interpretation of previous transport measurements.