Temperature dependent low-frequency modes in the incommensurate phase of quartz

Abstract

Simultaneous measurements of Raman and Brillouin scattering spectra were performed on the incommensurate (IC) phase of quartz near the alpha - beta transition at Tc=572 degrees C. In the low-frequency region there are two strongly temperature dependent modes. One is the 7 cm-1 mode observed in Raman spectra with considerable intensity only in the IC phase between Ti(=Tc+1.8K) and Tc. The other mode is the Rayleigh (elastic) component in Brillouin spectra which shows a sharp intensity maximum at about 1.0K above Tc, that is in the middle of the IC phase. This mode, coupled to the longitudinal acoustic mode, seems to separate the IC phase into two temperature regions. Raman activities of the amplitude and phase modes in the IC phase of quartz are studied and compared with the experimental results. In particular, possible differences between the '1-q state', for which the phonon condensation occurs in a single q-direction, and the '3-q state' are discussed. Then, assuming the existence of the '1-q state' in the IC phase, it is shown that the temperature dependences of the 7 cm-1 mode and the Rayleigh peak qualitatively agree with those of the A symmetry amplitude and A symmetry phase mode, respectively.