The velocity of 1-GHz longitudinal acoustic waves has been measured in single-crystal Na β-alumina at low temperature. The temperature dependence is logarithmic in the range 0.1-5 K. This behavior, previously observed only in amorphous materials, is explained in terms of the two-level systems with a broad distribution of energy-level splittings. A value for the two-level-system acoustic-phonon coupling constant is deduced from the data.
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Doussineau et al. (1980) studied this question.
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