Ultrasonic Study of Magnetoelastic Effects in the Spin-Peierls State of CuGeO₃

Abstract
The quasi-one-dimensional CuGeO₃ compound is the first inorganic material to exhibit a spin-Peierls transition driven by the magnetoelastic interaction between antiferromagnetic chains and the phonon field which opens a finite energy gap in the spin excitation spectrum by the dimerization of the lattice. The anomalous elastic character of the spin-Peierls system is established by studying in a high magnetic field the propagation of longitudinal ultrasonic waves along the three crystalline axes. We have measured the longitudinal elastic constants C₁₁, C₂₂ and C₃₃ of CuGeO₃ in magnetic fields up to 20 Teslas. The phase boundaries between the uniform (U), modulated (M) and dimerized (D) phases are associated to large anisotropic variations of the elastic constants. The temperature dependence of the three elastic constants exhibits a minimum at the U-D and U -M phase boundaries. The field dependences of C₂₂ and C₃₃ are reported.