SQUID-based technique to study the elastic properties of solids at very low temperatures

Mechanical oscillators are widely used for measurements of the elastic properties of solids at low temperatures. In most of such experiments mechanical displacements are detected via capacitive changes. In order to improve the sensitivity we have developed a novel detection technique relying on the high sensitivity of a commercial dc SQUID. To demonstrate the performance of the new setup we have measured the variation of the sound velocity and the internal friction of vitreous silica and of the borosilicate glass BK7 in the temperature range from 5 mK to 1 K. Our results agree favorably with former experiments where the displacement was detected capacitively. Furthermore, our results confirm that deviations of the elastic behavior of amorphous solids from the predictions of the standard tunneling model exist at very low temperatures.