Beyond the tunneling model: quantum phenomena in ultracold glasses

The low-temperature properties of amorphous solids are governed by atomic tunneling systems, the parameters of which are widely distributed due to the irregular structure of these materials. The standard tunneling model (STM) approximates such tunneling systems by particles moving in a double-well potential. Furthermore it assumes a constant distribution of the relevant parameters and in this way allows an understanding of many low-temperature properties of glasses on a phenomenological basis. However, a number of recent experiments show that the STM in its present form fails to explain some essential aspects of the physics of glasses at temperatures below about 100 mK. As examples we mention the rapid decay of spontaneous polarization echoes, the temperature dependence of the elastic and dielectric properties, the observation of various memory effects and the occurrence of an unexpected magnetic field dependence of the dielectric properties. A possible cause for the shortcomings of the STM might be the interaction between tunneling systems via their elastic and electric moments, which have not been considered in the original formulation of the STM. We will summarize the new findings and will discuss their implications. (C) 2002 Elsevier Science B.V. All rights reserved.