Intrinsic low-temperature thermal properties of glasses

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Abstract
The well-known specific-heat anomaly in amorphous dielectrics and their universal thermal conductivity have been carefully studied for the same wide variety of glasses as reported previously. The present studies have shown that the thermal conductivity is entirely insensitive to variations of the chemical purity or the thermal history of the samples. In contrast, the excess specific heat is sample dependent. In some materials, it is reduced by a factor of 2 by careful preparation. However, it has not been found to scale with any known impurity and is still present in the most carefully prepared and purest samples. There is a correlation between the remaining excess heat capacity and the thermal conductivity, but assuming them to be caused by the same centers requires a very strong scatterer compared to previously observed defect centers in crystals. These observations indicate that the connection between the specific-heat anomaly and the thermal conductivity, i.e., the phonon scattering, is more complex than has previously been assumed. Preliminary measurements on a chemically complex, crystallized glass ceramic, pyroceram, were also preformed. It showed the same thermal characteristics as the homogenous glasses. Hence, it appears that the disorder required to produce these "glassy" properties is not incompatible with a substantial amount of atomic order.