Relaxation due to Incoherent Tunnelling in Dielectric Glasses
  A new relaxation mechanism is shown to arise from overdamped two-level systems above a critical temperature $T^*\approx 5$ K, thus yielding an explanation for experimental observations in dielectric glasses in the temperature range between $T^*$ and the relaxation peak at 50 K. Using the distribution function of the tunnelling model for the parameters of the two-level systems, both the linear decrease of the sound velocity and the linear increase of the absorptionup to the relaxation maximum, are quantitatively accounted for by our theory. 
Peter Neu and Alois W\"urger