Contribution of Small Clusters of Tunneling Centers to Some Static Properties of Orientational Glasses

Abstract

The low temperature properties of glasses, amorphous solids, and of certain impurities in alkali halides (orientational glasses) all arise, according to the tunneling center model, from tunneling states [1,2]. It is therefore important to have some knowledge about the low-lying energy excitations from tunneling units. The tunneling center model, although capable to account for the main anomalous low-temperature properties of these materials, does not give information about the microscopic structure of the tunneling entities. The question about the nature of the tunneling states, however, does not arise in orientational glasses — crystalline solids containing certain impurities capable of tunneling motion — where the microscopic structure of the impurities is well-known [3]. In order to account for the experimentally observed results on the specific heat, dielectric constant and thermal conductivity, a constant density of states of the low-lying energy excitations is needed. In orientational glasses, it seems that the broad distribution in energy spectrum results from a distribution of interactions between tunneling units. It is possible to deduce the relationship between the probability density of the coupling parameters between tunneling units and the probability density of the excitation energies, wherefrom the main static properties of the material may be obtained [4,5].