| I describe experiments on the low-temperature behavior of mechanical oscillators at frequencies between 50 Hz and 200 kHz and temperatures between 15 mK and 300 K. These include simple vibrating reeds and more complex resonators fabricated from wafers, as well as piezoelectrically driven compound oscillators.;Finally, an analysis of the data on all three classes of material demonstrates an empirical correlation between the elastic properties below 1 K and those in the 2.5 K to 50 K range, while elastic properties above 50 K do not correlate with those at lower temperatures. I conclude that the wide range of solids in which glasslike behavior is found shows that a systematic understanding in terms of soft modes and phase transformations is more fruitful than the identification of microscopic tunneling entities.;First I examine in detail a high-purity silica, Suprasil W, to find out how well the tunneling model of glasses describes the data in this frequency and temperature range. I find no gross deviations from the predicted frequency and temperature variations of internal friction and sound velocity, while measuring small but significant departures from the model for these quantities. I then examine the elastic properties of quasicrystals, which are ordered but aperiodic, to see to what extent this model still applies in a system sharing the features of both crystals and glasses. I find that quasicrystals are essentially crystalline, yet can be fabricated to continuously cover the range between fully crystalline and fully glasslike elastic properties. Third, I examine a series of polycrystalline alloys with glass-like thermal properties to determine whether the elastic properties of glasses are also present. I find that those alloys which have diffusionless phase transitions have elastic properties of the same magnitude and functional form as those of silica. Alloys with diffusive phase transitions do not have these elastic properties. |
