Two-level systems in fluorite mixed crystals - a far-infrared study
Low-temperature far-infrared absorption measurements have been made on six different alkaline earth: rare earth fluoride mixed crystal systems at a number of concentrations. All of these chemically disordered crystals display a glass-like continuous distribution of two-level systems (TLS) throughout the millimetre wave spectral region. Much of this study has focused on LaF3 doping where the effective density of states of the TLS initially increases with dopant concentration, reaching a maximum saturated value at a concentration of about 5 mol% for a CaF2 host, 20% for SrF2 and at least 35% for BaF2. The magnitude of the saturated TLS optical density of states is system dependent, varying from roughly one-fifth to five times that found in a standard soda-lime-silica glass. The optical strength indicates that only a small fraction, less than one in a thousand, of the dopant ions contribute to the TLS optical absorption. At submillimetre wavelengths it is found that both the fluorite mixed crystals and a standard glass exhibit a characteristic temperature-dependent absorption associated with transitions to excited levels above the ground-state TLS manifold. It is proposed that resonant phonon scattering by these excited state transitions is the cause of the plateau observed in the temperature dependence of the thermal conductivity of glasses.
FitzGerald, S.A., A.J. Sievers, and J.A. Campbell. 2001. "Two-level systems in fluorite mixed crystals - a far-infrared study." Journal of Physics: Condensed Matter 13(10): 2177-2200.
Journal of Physics: Condensed Matter
Physics and Astronomy
Condensed matter: electrical, magnetic and optical, Condensed matter: structural, mechanical and thermal