The influence of a secondary order parameter on the incommensurate-commensurate transition

Abstract

Based on a critical view of existing theories, the influence of a secondary order parameter eta (elastic strain, polarisation) on the incommensurate-commensurate transition is reinvestigated. The necessity to regard the intrinsic nature of eta in the variational treatment, which yields the Landau free energy, is emphasised. Restricting themselves to a one-dimensionally modulated structure, the authors prove the statement of Bruce et al. (1978), that the coupling to the strain makes the transition discontinuous. On the contrary, for the coupling to the ferroelectric polarisation P, they did not find change of the transition both for P parallel and perpendicular to the modulation. The electric susceptibility chi exhibits a Curie-Weiss law. They argue that thermal fluctuations do not change these conclusions, but lead to a weaker singularity of chi .