PHONON-PHONON INTERACTIONS IN SILICON

At low temperatures the acoustic properties of glasses differ from tho se of crystals. The anomalies can be explained by the existence of loc alized low-energy excitations with a broad distribution of energy and relaxation times. The dynamics of the excitations, also described as t unneling states (TS) is, on a phenomenological level, well understood for silica-like glasses. The presence of TS in overconstrained amorpho us solids like pure bulk Si is however not so well established. We sta rted a low-temperature ultrasonic study of Si partly amorphised by neu tron irradiation. We measured the ultrasonic attenuation (for 320 and 425 MHz) as a function of temperature (0.3 to 80 K) before irradiation . The data show that the anharmonic phonon-phonon interactions dominat e the behavior.