Characterization of the structural and magnetic fluctuations near the spin-Peierls transition in CuGeO3.

Abstract

Extensive neutron-scattering experiments have been performed on ${\mathrm{CuGeO}}_{3}$ single crystals to study the structural fluctuations in the neighborhood of the spin-Peierls transition at temperature ${\mathit{T}}_{\mathrm{SP}}$. We have succeeded in measuring the critical fluctuations up to a few K above ${\mathit{T}}_{\mathrm{SP}}$. The inverse correlation lengths \ensuremath{\kappa} associated with the structural fluctuations obtained along the [1 0 1] and [ 0 1 0] directions are consistent with those reported in the low-resolution diffuse x-ray-scattering measurements of Pouget et al. This agreement implies that the energy-integrated scattering function S(Q)=${\mathrm{\ensuremath{\int}}}_{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\infty}}}^{\mathrm{\ensuremath{\infty}}}$S(Q,\ensuremath{\omega})d\ensuremath{\omega} measured by diffuse x-ray scattering is dominated by structural fluctuations around \ensuremath{\omega}=0, not by the softening of a zone-boundary phonon as expected in ordinary spin-Peierls systems. In order to examine the relationship between magnetic and structural fluctuations, further information on the magnetic excitations was collected. There remains considerable intensity above ${\mathit{T}}_{\mathrm{SP}}$ in the spin-excitation spectrum at (1/2 1 1/2), slowly decaying over a wide energy range (\ensuremath{\sim}10 meV) and persisting up to about 50 K. We measured the magnetic dynamical structure factor S(Q,\ensuremath{\omega}) over a wide range of energy and momentum space at temperatures of 4 K (\ensuremath{\ll}${\mathit{T}}_{\mathrm{SP}}$) and 16 K (\ensuremath{\gtrsim}${\mathit{T}}_{\mathrm{SP}}$). The wave-vector dependent susceptibility \ensuremath{\chi}(Q) deduced from the S(Q,\ensuremath{\omega}) contour map at 16 K yields inverse correlation lengths consistent with those which we determined for the structural fluctuations. This indicates a strong symbiotic coupling between the magnetic and structural fluctuations near ${\mathit{T}}_{\mathrm{SP}}$ as expected. We speculate that the Cu ions are surrounded by strongly correlated spins and that the motions are heavily overdamped. The structural fluctuations towards dimerization are accompanied by magnetic correlations leading to a spin-singlet state.