Thermal contraction at the spin-Peierls transition in CuGeO3

We report the results of an x-ray-scattering study of the [ital b]-axis lattice constant of the quasi-one-dimensional spin-1/2 antiferromagnetic chain system CuGeO[sub 3] as a function of temperature and magnetic field. A spontaneous thermal contraction [Delta][ital b] along the [ital b] axis perpendicular to the chain direction is observed below the spin-Peierls transition temperature [ital T][sub [ital c]] near 14 K. The contraction, [Delta][ital b], is well described by a simple power law, [Delta][ital b][proportional to](1[minus][ital T]/[ital T][sub [ital c]])[sup [ital x]], where the exponent [ital x] is found to be close to 0.5. Below the transition temperature, [Delta][ital b] scales with the intensity of the superlattice reflections with indices ([ital h]/2,[ital k],[ital l]/2)([ital h],[ital l]: odd, [ital k]: nonzero integer) measured by neutron scattering. The shift of the transition temperature, [Delta][ital T][sub [ital c]][equivalent to][ital T][sub [ital c]](0)[minus][ital T][sub [ital c]]([ital H]), is found to scale as [ital H][sup 2] in quantitative agreement with the results of magnetic susceptibility measurements and with theory. A small increase in the [ital a]-axis lattice constant is observed below [ital T][sub [ital c]].