Low-temperature internal friction of glass ceramics

Low-temperature internal-friction measurements have been used to study the universal low-energy excitations in glasses before and after crystallization in two glass ceramics, one based on MgO-Al2O3-SiO2 (Corning Code 9606) and one based on Li2O-Al2O3-SiO2 (Corning Code 9623). In the Code 9606 sample, the number density of excitations is greatly reduced, while in the Code 9623 sample, their number density remains practically unaltered in the crystallized state. These measurements confirm the conclusions reached by Cahill et al. (preceding paper), which were based on thermal measurements up to room temperature. These measurements also demonstrate the usefulness of internal friction as a tool for the study of these low-energy excitations, since internal friction is less sensitive to defects common to glass ceramics, like magnetic impurities and grain boundaries, which tend to dominate low-temperature specific heat and thermal conductivity, respectively.