Large length-scale fluctuations at the spin-Peierls transition in CuGeO3

We report a detailed synchrotron x-ray-scattering study of the structural fluctuations near the spin-Peierls transition in CuGeO3. Below the transition temperature T-SP, the superlattice peak profile is resolution limited and the peak intensity, which is proportional to the order parameter squared, is well described by a simple power law, (1-T/T-SP)(2 beta). The fitted value of beta, 0.33(3), agrees quantitatively with previously reported results from neutron-diffraction measurements. Above the transition temperature T-SP, pretransitional lattice fluctuations are observed within about 1 K above T-SP The length scale of these fluctuations is about an order of magnitude larger than that characterizing the bulk critical fluctuations. The line shape of these large length-scale scale fluctuations is consistent with a Lorentzian-squared form. The measured critical exponents associated with the large length-scale fluctuations are nu=0.56(9), and gamma=2.0(3). Similar large length-scale fluctuations have been observed at the structural transitions in some perovskites and the magnetic transitions in holmium and terbium. We suggest that in CuGeO3 the large length-scale fluctuations reflect the disconnected susceptibility originating from random field Ising-type local defects.