Giant anomalies of the thermal expansion at the spin-Peierls transition inCuGeO3

The thermal-expansion coefficients α of the spin-Peierls cuprate ${\mathrm{CuGeO}}_3$ have been measured along the three orthorhombic directions using a high-resolution capacitance dilatometer. We observe giant, strongly anisotropic anomalies at the spin-Peierls transition temperature (${\mathit{T}}_{\mathrm{SP}}$) and spontaneous strains along all three crystal axes below ${\mathit{T}}_{\mathrm{SP}}$. From the jumps of α and the specific heat we estimate large uniaxial pressure dependencies of ${\mathit{T}}_{\mathrm{SP}}$, in particular for pressure applied perpendicular to the one-dimensional ${\mathrm{Cu}}^{2+}$ chains. In addition, we observe an anomalous temperature dependence of the thermal expansion above the transition showing a striking similarity to that of the magnetic susceptibility.