Abstract: The mm- and submillimeter wave spectrum of a glass at low temperatures is often dominated by the dynamics of two level systems (TLS). These E and M properties are reviewed here. Next the low-temperature far-IR absorption measurements on a number of alkaline earth: rare earth fluoride mixed crystals with the fluorite structure are described, compared to the glass results, and found to display a similar frequency distribution of TLS. At higher far IR frequencies a characteristic temperature dependence associated with excited state transitions of the TLS is observed for both mixed crystals and glasses, suggesting a fundamental connection between the TLS in these very different systems. !24
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