Similarities and differences between the low-temperature acoustic properties of crystalline materials and glasses

We report on measurements of the acoustic properties of single crystals of alkali halides and aluminum, and polycrystalline copper at kHz frequencies and temperatures down to 8 mK. The goal of this study was to examine in several crystals with a small number of defects the presence and influence of two-level tunneling systems which commonly govern the low-temperature behavior of amorphous solids. The samples were investigated using vibrating reed and torsional oscillator techniques. In contrast to recent reports on glasslike behavior for a large variety of pure polycrystalline metals, our results confirm the ''traditional'' point of view that the low-temperature acoustic properties of pure monatomic crystals can be well distinguished from those of amorphous solids.