X-ray-diffraction study of critical phenomena at the spin-Peierls transition

We have performed x-ray-diffraction measurements of the critical phenomena associated with the spin-Peierls phase transition in (Formula presented) Measurements of the order parameter indicate a value of the critical exponent β of (Formula presented) consistent with conventional three-dimensional (3D) behavior and in closest agreement with 3D (Formula presented) universality, as expected on theoretical grounds. This work also indicates a rather narrow asymptotic critical region which explains much of the inconsistency reported in previous critical exponent estimates. Measurements of the relative changes in lattice constants have also been carried out to allow for detailed comparison between the spontaneous strains present below the transition temperature and the order parameter. The order parameter and lattice constant measurements were performed in identical thermal environments on the same sample to allow for direct comparison with no adjustment of transition temperature. This comparison suggests good agreement between the two except near the transition temperature where fluctuation effects are observed in the spontaneous strain which are less evident in the order parameter.