PHONON-SOLITON INTERACTION IN THE INCOMMENSURATE PHASE OF THE SPIN-PEIERLS COMPOUND CUGEO3

Measurements of the elastic constants C-22 and C-33 Of the spin-Peierl s system CuGeO3 in high magnetic fields up to 27 T are reported. C-33 presents a large magnetic-field dependence for fields above the commen surate-incommensurate phase transition which occurs at 12-13 T. We rel ate this effect to phonon-soliton interactions. Anisotropic and hyster etic behaviors are reported. We deduce from the behavior of C-33 an av erage soliton density.