Anomalous Behavior of Insulating Glasses at Ultra-low Temperatures

Abstract

The low-temperature properties of amorphous solids differ considerably from those of their crystalline counterparts. These differences are caused by the presence of atomic tunneling systems in the irregular structure of glasses. Although the microscopic nature of these tunneling states has not been identified, the so-called tunneling model provides a satisfying description of many properties of amorphous solids below 1 K on a phenomenological basis. For certain properties below about 100 mK, however, serious discrepancies between the predictions of the tunneling model and the experimental results exist. It appears that these discrepancies become more and more pronounced with decreasing temperature. In addition new intriguing phenomena occur that are unexpected on the basis of the tunneling model. These findings might be taken as an indication that elastic and electric interaction between the tunneling systems become important at low temperatures, because in the original formulation of the tunneling model such a mutual interaction was not taken into account. We will discuss a few examples of the shortcoming of the tunneling model.