Abstract
The sound speed δV/V0 of amorphous silica between 1 and 30 mK was measured using a double paddle oscillator, cut with a laser from a 0.4 mm thick plate of vitreous silica. The oscillator was operated at four of its resonant modes between 610 Hz and 14 kHz. Above 3 mK and up to the maximum of δV/V0, the sound speed exhibits the logarithmic temperature dependence predicted by the tunneling model with the slope C = 2.4 × 10−4, varying by a few percent between the various modes. Below 3 mK, the sound speed departs from the logarithmic dependence and levels off. This can be explained either by a large heat input (>10−3 nW/g) of unknown origin or by a low energy cut off, Emin, in the density of tunneling states of 1.5 mK.
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