Hypersonic Absorption in Quartz at Temperatures Below 30°K

Abstract

We have extended earlier absorption measurements between 500 and 10 000 Mc/sec to temperatures below 30\ifmmode^\circ\else\textdegree\fi{}K where the mean free path of thermal phonons is likely to be larger than the ultrasonic wave-length. The absorption of transverse waves follows in magnitude, as well as in the dependence on temperature and frequency, as expected for a three-phonon process. However, the measured absorption per cm for longitudinal waves depends only slightly on frequency in contrast to the expectations for a four-phonon process; instead it supports a different absorption mechanism discussed here in detail. Finally, experiments with imperfect and neutron-irradiated quartz are reported in which strong additional absorption appears, which may be related to the absorption in fused quartz.