Brillouin-scattering studies of the longitudinal-acoustic mode propagating along the c* axis of K2SeO4 were performed in the vicinity of the incommensurate phase transition in both 90°and 180°scattering geometries. A theoretical derivation of the complex elastic constant C33() was carried out within the mean-field approximation including both bilinear (Landau-Khalatnikov) coupling to the amplitude mode, and anharmonic fluctuation contributions from the 2 soft mode above TI and from pairs of amplitude modes and phase modes below TI. Comparison between theory and the Brillouin data led to an excellent fit using free-energy parameters close to values deduced from previous static and dynamical experiments. Our measurements also indicate that the phason gap (0) is at least 100 GHz.
ASJC Scopus subject areas
- Condensed Matter Physics