Ultrasonic Measurements on Quench—Condensed Noble Gas Films

Quench–condensed noble gas films are a simple model system for studying the elastic properties of polycrystalline or amorphous solids at low temperatures. We have investigated the temperature dependence of the sound velocity δv/v and attenuation α of quench–condensed polycrystalline Ar, Ne, and HD films using surface acoustic waves at 800 MHz. Up to 0.7 K the sound velocity of all samples increases logarithmically in temperature and decreases above 1 K. This behavior is similar to what is found in glasses and is indicative of the presence of tunneling systems with a wide distribution of energies. Despite the qualitative similarities between the samples, remarkable quantitative differences were observed: Compared to argon the changes of sound velocity and of attenuation in neon are reduced by almost one order of magnitude and in HD even by approximately two orders of magnitude.