Thermal contraction at the spin-Peierls transition inCuGeO3

Abstract

We report the results of an x-ray-scattering study of the b-axis lattice constant of the quasi-one-dimensional spin-1/2 antiferromagnetic chain system ${\mathrm{CuGeO}}_3$ as a function of temperature and magnetic field. A spontaneous thermal contraction Δb along the b axis perpendicular to the chain direction is observed below the spin-Peierls transition temperature ${\mathit{T}}_{\mathit{c}}$ near 14 K. The contraction, Δb, is well described by a simple power law, Δb∝(1-T/${\mathit{T}}_{\mathit{c}}$ ${)}^{\mathit{x}}$, where the exponent x is found to be close to 0.5. Below the transition temperature, Δb scales with the intensity of the superlattice reflections with indices (h/2,k,l/2)(h,l: odd, k: nonzero integer) measured by neutron scattering. The shift of the transition temperature, Δ${\mathit{T}}_{\mathit{c}}$≡${\mathit{T}}_{\mathit{c}}$(0)-${\mathit{T}}_{\mathit{c}}$(H), is found to scale as ${\mathit{H}}^2$ in quantitative agreement with the results of magnetic susceptibility measurements and with theory. A small increase in the a-axis lattice constant is observed below ${\mathit{T}}_{\mathit{c}}$.