Q1 CHARGE-DENSITY WAVE IN NBSE3

Using synchrotron x-ray-scattering measurements on NbSe3, we show that the diffuse scattering associated with the q1 charge-density-wave fluctiations are rods perpendicular to the (a,b) planes reflecting a 2D regime of fluctuations with an in-plane anisotropy b/a=3.5+0.5. The plane of the 2D fluctuations is likely to be formed by the chains with the shortest Se-Se pairing. The 3D coupling sets in within 2 K above the transition temperature T1 150 K with an out-of-plane anisotropy b/c=27+2. $q sub 1e" has a significant temperature variation from about 0.2445(1) at T1 to 0.2411(1) at about 80 K and then remians constant to the lowest temperatures. Upon cooling, the incommensurate wave vector is driven far away from the commensurate value 1/4. No anomaly in either the amplitude or the phase of q1 is observed at the second charge-density-wave transition. © 1990 The American Physical Society.