New aspect of the q2-CDW phase transition of NbSe3

We present an x-ray study of the q2-pretransitional structural fluctuations of the low temperature T2=59 K Peierls transition of the trichalcogenure NbSe3. These fluctuations are less anisotropic than the q1-ones of the high temperature Peierls transition. We present evidence that close to T2 the q2-fluctuations exhibit two length scales. The longer one appears 3 K above T2 and diverges at T2 while the shorter one is not critical. These features recall those observed in magnetic systems and perovskites.