ACOUSTIC-PHONON DISPERSION IN THE INCOMMENSURATE PHASE OF SIO2 (QUARTZ)

Ultrasonic and Brillouin scattering measurements were performed near t he incommensurate (INC) transition of SiO2 (quartz). The decrease of t he LA sound velocity in the INC phase was found to be much less for 10 MHz than for 27 GHz, being remarkably different from other crystals w hich undergo INC phase transitions. The dispersion in the INC phase ca n be well characterized by a simple Landau-Khalatnikov type relaxation with a relaxation time tau(T) = (2.6 +/- 0.5) x 10(-12) (T-i - T)(-1) s. A small but critical decrease of the TA velocity as T --> T-c(= T- i - 1.4 K) was also detected by the ultrasonic measurement.