Dielectric long-time relaxation of amorphous materials at low temperatures

We have measured the dielectric long-time relaxation of various glasses at temperatures of 1.5--75 K. The data were taken in the time range 2--7000 s after a step excitation. Below 4 K, the relaxation is logarithmically dependent on time and independent of temperature as predicted by the tunneling model. At higher temperatures, the prefactor of the logarithmic relaxation law increases. We discuss several modifications of the original tunneling model in this context. For the case of a glassy ferroelectric we have performed additional experiments, which indicate that in this substance massive and possibly correlated relaxing systems determine the behavior above 10 K.