ACOUSTIC PROPERTIES OF AMORPHOUS SIO2 AND PDSICU, AND OF CRYSTALLINE AG, NBTI AND TA AT VERY LOW-TEMPERATURES

With the vibrating reed and vibrating wire techniques we have investigated the acoustic properties of vitreous silica (SiO2, Suprasil I) and of amorphous PdSiCu as well as of polycrystalline Ag, NbTi and Ta at frequencies of 100 Hz less-than-or-equal-to omega/2-pi < 6 kHz and at temperatures of 0.1 mK less-than-or-equal-to T less-than-or-equal-to 1 K. The relative change of sound velocity DELTA-upsilon/upsilon of SiO2 shows saturation effects, strain amplitude dependence, as well as an unexpected temperature dependence below its maximum at T < 50 mK. For PdSiCu we observe that below a certain temperature, which depends on the applied strain, the temperature dependence of the sound velocity upsilon deviates from the logarithmic behavior observed at higher temperatures and reaches an almost constant value at T < 1 mK. In the same temperature range Q-1 does not remain constant but steadily decreases. The acoustic properties of the two amorphous materials at finite strain show substantial deviations from the standard tunneling model. Some of the observed anomalies can be explained taking into account the change of population of the tunneling systems energy states and a nonlinear relaxation absorption. For polycrystalline Ag we find DELTA-upsilon/upsilon is-proportional-to 1n T and Q-1 is-proportional-to T1/3 over three decades in T at T < 100 mK; it shows low-temperature acoustic properties which are strikingly similar to those of amorphous materials. The temperature and strain dependencies of the acoustic properties of polycrystalline super-conducting NbTi and Ta resemble those obtained for SiO2. These results indicate that there are basically no differences in the low-temperature acoustic properties of polycrystals and amorphous materials.