Light scattering in displacive incommensurate structures: a Landau theory approach

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Abstract
This paper is concerned with the derivation of selection rules for light scattering by soft modes in displacive incommensurate structures, when the latter result from the condensation of a single pair of non-degenerate modes at q0 and -q0, the high-temperature phase being centrosymmetric. The analysis is based on a direct second-order high-temperature Raman process involving the soft modes which condense at the transition temperature Ti. Special attention is given to the case of the pseudo-acoustic phason. It is shown that its integrated intensity is proportional to (Ti-T) near Ti, and that its selection rules are similar to those obtained in Brillouin scattering. It is further shown that the latter are somewhat relaxed by the acoustic phonon-phason coupling. The method is finally extended to light scattering by hard phonons at q0 and -q0.