New results in incommensurate crystals: Kink diffusion and long‐period lock‐in phases

Abstract

Two new results for incommensurate phases of crystals are discussed: the experimental observation of a q2 dependence of the inverse relaxation time observed in the hypersonic regime via near-forward Brillouin scattering experiments, tentatively interpreted as kink diffusion, and the theoretical demonstration that long-period lock-in phases observed in crystals which are simultaneously ferroelectric in their incommensurate phases are stabilized by the spontaneous polarization field. This theory explains the observed limits of incommensurate wave vector in BaMnF, as a function of temperature: 2a*/5 = 0.399a* -0.389a* = 5a*/13, where the 2/5 value is the low-temperature limit and the 5/13 value is the high-temperature limit. It also explains one mechanism for stabilizing the 2/5 phase at lower temperatures than are permitted in the ANNNI (axial next-nearest neighbour Ising) model of Yamada and Hamaya in the absence of a spontaneous polarization field. ~~~