Phason dispersion in deuterated thiourea by inelastic neutron scattering

The phason origin of the quasielastic diffuse scattering in the incommensurate phase of thiourea is demonstrated by inelastic neutron scattering on a deuterated crystal. The analysis of its frequency spectrum allows one to draw the dispersion of the phason relaxational frequency along the modulation direction. Results indicate that the lowest-frequency mode seen in submillimeter infrared spectra is an overdamped inhomogeneous phason mode corresponding to the antiphase motion of the nearest-neighbor nodes of the modulation profile.