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

Abstract

We report a detailed synchrotron x-ray-scattering study of the structural fluctuations near the spin-Peierls transition in CuGe${\mathrm{O}}_{3}$. Below the transition temperature ${T}_{\mathrm{SP}}$, 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, ${(1\ensuremath{-}\frac{T}{{T}_{\mathrm{SP}}})}^{2\ensuremath{\beta}}$. The fitted value of $\ensuremath{\beta}$, 0.33(3), agrees quantitatively with previously reported results from neutron-diffraction measurements. Above the transition temperature ${T}_{\mathrm{SP}}$, pretransitional lattice fluctuations are observed within about 1 K above ${T}_{\mathrm{SP}}$. 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 fluctuations is consistent with a Lorentzian-squared form. The measured critical exponents associated with the large length-scale fluctuations are $\ensuremath{\nu}=0.56(9)$, and $\overline{\ensuremath{\gamma}}=2.0(3)$. 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 CuGe${\mathrm{O}}_{3}$ the large length-scale fluctuations reflect the disconnected susceptibility originating from random field Ising-type local defects.