Magnetoelastic coupling and order parameter in the spin-Peierls systemCuGeO3

The anomalous elastic character of the dimerization axis in the spin-Peierls system ${\mathrm{CuGeO}}_3$ is established above and below the transition temperature ${\mathit{T}}_{\mathrm{sp}}$ by studying, in a magnetic field, the propagation of longitudinal ultrasonic waves along the three crystalline directions. At high temperatures an important anomaly on the elastic constant ${\mathit{C}}_{33}$ is identified and discussed in relation with lattice fluctuations recently observed below 100 K in an electron diffraction experiment and attributed to vibrations of ${\mathrm{CuO}}_2$ ribbons along the chain. Stiffening anomalies are observed along the three crystalline directions below ${\mathit{T}}_{\mathrm{sp}}$; a softening of the velocity is seen only along the chain axis above the transition. The temperature and magnetic-field dependences of these anomalies are studied with a Landau free-energy model for the magnetoelastic coupling which gives access to the order parameter, the magnetic gap.