Low Temperature Anomalies of Vitreous Silica and the Tunneling Model

The thermal conductivity, the relative change of sound velocity, the heat capacity and the heat release of Suprasil have been measured at low temperatures. The spectral density was determined in the range of relaxation time between 10^-10 and 10^5 s. The spectral density is nearly constant for relaxation times shorter 1 s and longer 500 s. However, the absolute value at short time is roughly a factor four larger than in the long-time range. The obtained dependence of the spectral density on the relaxation time disagrees with the Standard Tunneling Model as well as with the Soft Potential Model.