| Popis: |
In ${\mathrm{Rb}}_{2}$${\mathrm{ZnBr}}_{4}$ and ${\mathrm{Rb}}_{2}$${\mathrm{ZnCl}}_{4}$, Brillouin scattering of longitudinal-acoustic phonons is performed as a function of the temperature. Small anomalies, arising through a coupling with the soft-going modes, are observed in both the phonon velocity and damping near the transition from the orthorhombic to the modulated phase. The anomalies are quantitatively accounted for with a comprehensive analysis based on an adaptation of Levanyuk's Landau theory for sound absorption near second-order phase transitions. In the incommensurate regime, two mechanisms appear to contribute by about equal weights: an effective coupling bilinear in the strain and amplitude fluctuations, and a coupling linear in the strain, but quadratic in the amplitude fluctuations. Above the transition, the soft-mode counterpart of the latter coupling remains. Estimates of these contributions derived from the theory with parameter values from other sources are found to be in conformity with the experiment. The analysis further yields the amplitude-mode and soft-mode relaxation times as well as the transition temperatures. Additionally, the anomalous phonon velocity and damping observed in ${\mathrm{Rb}}_{2}$${\mathrm{ZnBr}}_{4}$ near a transition at 115 K are analyzed. Finally, the elastic constants of ${\mathrm{Rb}}_{2}$${\mathrm{ZnBr}}_{4}$ and ${\mathrm{Rb}}_{2}$${\mathrm{ZnCl}}_{4}$ are determined at 300 K. |
