An experimental and theoretical study of the field dependence of the structural modulation in the incommensurate high field phase of the spin-Peierls cuprate ${\mathrm{CuGeO}}_3$ is presented. The averaged structural order parameter is determined from magnetostriction and thermal expansion experiments in magnetic fields up to 28 T. The results are compared to density matrix renormalization group calculations using a self-consistent one-dimensional approach. The experimental and numerical data agree very well with each other and clearly show a continuous change from a solitonlike lattice close to the dimerized incommensurate transition to a sinusoidal distortion at higher fields.

• Received 17 December 1997

DOI:https://doi.org/10.1103/PhysRevLett.81.148