Phonon-soliton interaction in the incommensurate phase of the spin-Peierls compoundCuGeO3

Measurements of the elastic constants ${\mathrm{C}}_{22}$ and ${\mathrm{C}}_{33}$ of the spin-Peierls system ${\mathrm{CuGeO}}_3$ in high magnetic fields up to 27 T are reported. ${\mathrm{C}}_{33}$ presents a large magnetic-field dependence for fields above the commensurate-incommensurate phase transition which occurs at 12–13 T. We relate this effect to phonon-soliton interactions. Anisotropic and hysteretic behaviors are reported. We deduce from the behavior of ${\mathrm{C}}_{33}$ an average soliton density.