Ultrasonic attenuation in a quasicrystal studied by picosecond acoustics as a function of temperature and frequency

We report on the acoustic attenuation in an AlPdMn icosahedral quasicrystal between 15 and 300 K, in the tens of GHz range. High frequency coherent longitudinal acoustic waves are excited and detected by an optical ``pump and probe'' technique, in an interferometric configuration. Our results reveal that, at high temperature (T>100K), the acoustic attenuation is rather small, close to the attenuation in crystals and much smaller than in disordered solids. We stress that Akhieser processes (phonon-phonon interactions) must be active in quasicrystals and give the correct behavior and order of magnitude of the acoustic attenuation, at high temperature. At low temperature, departure from the standard behavior arising from phonon-phonon interaction is not understood.