Acoustic absorption in amorphous silicate down to 1K

The acoustic absorption in amorphous silica was measured between 1 K and 20 K and at frequencies between 15 and 65 MHz. It was shown that in the range (3÷5) K there exists an absorption peak whose position is temperature dependent, thus indicating a thermally activated relaxation process. The experimental results were discussed in terms of recent theories based on the Anderson model. The absorption peak observed represents a new feature that cannot be fitted into the current theories. It was suggested that a more refined explanation could be found in terms of stochastic resonance, whose existence has been postulated elsewhere in order to describe the behaviour of the main absorption peak observed at higher temperatures.