Light Scattering from Incommensurate Insulators; Mainly BaMnF4
The light scattering work on pyroelectric BaMnF4 is reassessed here in view of new structural information on the incommensurate phase. The dispersions in the acoustic phonon velocities Vcb and Vcc are ascribed to coupling to the phason and amplitudon, respectively; the failure to observe a true soft mode in the Raman spectrum for T < Ti ≃ 247 K is attributed to the overdamped nature of the amplitudon even for T ≃ 85 K; the static central peak is associated with symmetry breaking frozen defects; and the dynamic central peak previously thought to be due to the phason is now assigned to LA-phonon — amplitudon coupling. Thus the light scattering experiments have provided no direct evidence for phasons or amplitudons. It is suggested that Brillouin scattering experiments should be performed on incommensurate ThBr4, which is a very simple system with small phason and amplitudon damping, to search for the overdamped phason and predicted amplitudon — acoustic-phonon coupling. Finally, it is demonstrated that quasiperiodic superlattices provide exciting possibilities for detailed comparisons between theory and experiment.
KeywordsCentral Peak Soft Mode Incommensurate Phase Rochelle Salt Transverse Acoustic
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