2-LEVEL SYSTEMS AND EXCITED-STATE TRANSITIONS IN FLUORITE MIXED-CRYSTALS AND SILICA GLASS

Low-temperature far-infrared absorption measurements have been made on a number of alkaline earth: lanthanum fluoride mixed crystals with the fluorite structure. In the mm wave region a continuous distribution of two-level glassy states is identified as being very similar to that previously found for glasses. At shorter wavelengths a characteristic temperature dependence associated with excited state transitions is observed in these mixed crystals. When silica glass is examined with the same precision, similar excited state transitions are found. © 1994 The American Physical Society.