MAGNETOELASTIC COUPLING AND ORDER-PARAMETER IN THE SPIN-PEIERLS SYSTEM CUGEO3

The anomalous elastic character of the dimerization axis in the spin-P eierls system CuGeO3 is established above and below the transition tem perature T-sp by studying, in a magnetic field, the propagation of lon gitudinal ultrasonic waves along the three crystalline directions. At high temperatures an important anomaly on the elastic constant C-33 is identified and discussed in relation with lattice fluctuations recent ly observed below 100 K in an electron diffraction experiment and attr ibuted to vibrations of CuO2 ribbons along the chain. Stiffening anoma lies are observed along the three crystalline directions below T-sp; a softening of the velocity is seen only along the chain axis above the transition. The temperature acid magnetic-field dependences of these anomalies are studied with a Landau free-energy model for the magnetoe lastic coupling which gives access to the order parameter, the magneti c gap.