Low-temperature ultrasonic attenuation by strongly dispersive transverse-acoustic phonons in α-quartz

Measurements of the ultrasonic absorption in synthetic and natural quartz crystals below 20 K at frequencies less than 1 GHz show that the temperature dependence of the intrinsic attenuation coefficient departs from the power law predicted by theory. The deviations are explained in terms of anharmonic interactions of the ultrasonic waves with superthermal phonons of the strongly dispersive transverse-acoustic mode in quartz whose lifetimes are limited by point-defect scattering. The attenuation is independent of frequency and its magnitude falls below 2×10-5 dB/s at about 8 K.