Two-level systems in fluorite mixed crystals - a far-infrared study

Abstract

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.

Ionized donor pairs and microwave and far-infrared absorption in semiconductingCdF2

“…The conventional ''ion'' TLS's are produced by atoms or groups of atoms tunneling in amorphous dielectrics or in glass-like features of the crystals with extended ''porous'' defects. In particular noteworthy are the studies of the ion TLS's in dielectric crystals with the fluorite-type structure, [50][51][52] in which a broadband resonant microwave and FIR absorption was also found. All the experimental facts discussed here are clear evidence of the electronic nature of the observed TLS.…”

Glasslike Two-Level Systems in Minimally Disordered Mixed Crystals

“…The two-level sensitive to the precise sample concentrations. The maximum measured ODOS for Ba 1ÿx Ca x F 2 is about one tenth of that in saturated mixed crystals and glasses [9], and the ODOS in Pb 1ÿx Ca x F 2 is about 100th of the saturated values. Studies with doped alkali halide crystals have already demonstrated that for a given lattice containing a smaller size dopant cation that although single ions may have insufficient volume to tunnel, pairs of such ions can [16].…”

“…So far TLS behavior in crystals has been demonstrated only in systems for which disorder is produced either by a low symmetry component with orientational degrees of freedom, or by chemical disorder. Extensive work on fluorite mixed crystals shows broad TLS-like behavior for aliovalent dopants in the range of 1-45 mol% [1,9]. There the substitutional dopant cation introduces charge compensating interstitial anions and clustering while at the same time producing local strain and electric fields due to the different components.…”

“…is the optical TLS density-of-states (ODOS). Its frequency dependence is due to a cutoff in the high frequency tunneling energy and the onset of excited state transitions [9]. Taking the limit P!…”

Vibrational linewidths and dephasing of H− local modes in disordered calcium fluoride

“…Previous studies on disordered mixed fluorite (CaF 2 ) crystals include far-infrared absorption [4,5], relaxation of non-equilibrium phonons under carbon dioxide laser excitation [6], Raman scattering from glasslike features [7] and a review of glasslike excitations in chemically disordered crystals [8].…”