J.A. Campbell

Glass-like properties observed in low-frequency Raman scattering of mixed fluorite crystals

Abstract The low-frequency Raman spectra of (CaF 2 ) l− x (LaF 3 ) x mixed crystals in the concentration range from 0.05 ≤ x ≤ 0.4 have been measured as a function of temperature from 4.2 K to 295 K. Concentration dependent quasi-elastic Raman scattering is found for frequencies below 10 cm −1 for A 1g + E g scattering but not for T 2g symmetry scattering. The temperature dependence of Raman intensity suggests that this effect is dominated by scattering from a distribution of two-level systems between 4.2 K and 20 K. The lowest impurity-induced vibrational Boson scattering peak at 82 cm −1 , which shifts to higher frequency with increasing dopant concentration, also only appears in A 1g + E g symmetry, indicating that the two different kinds of glass-like excitations in these mixed crystals may have a common origin.

The defect structure of CaF2:U3+

Neutron diffraction on two single crystals of CaF 2 doped with 6 and 12 mole% UF 3 shows anionic vacancies on the normal F ion site and excess anions on an interstitial site F. The ratio of F sites to anionic vacancies and to U 3+ dopant cation indicates that the cluster is of the 1:0:n (n=2, 3) type found for homologous CaF 2 :LaF 3 solid solution. Electron Paramagnetic Resonance measurements on isolated U 3+ ions (<0.05%) in CaF 2 crystals show they are in a tetragonal site, in which the trivalent U 3+ ion substitutes for a divalent Ca 2+ ion with the charge compensation F - ion along the (100) axis. The local modes of vibration of H - ions in crystals of CaF 2 :UF 3 double-doped with H ions show clusters for U 3+ of intermediate dopant concentrations, typically 0.1-10 mole%, and hence have promise to bridge the gap between the concentration ranges for which Electron Paramagnetic Resonance and neutron diffraction are respectively suitable. At very low concentrations, the electron-phonon splitting (0.6 cm -1 ) of the xy vibration of the H - ion is evident.

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.

Bayesian Methods and Universal Darwinism

Bayesian methods since the time of Laplace have been understood by their practitioners as closely aligned to the scientific method. Indeed a recent Champion of Bayesian methods, E. T. Jaynes, titled his textbook on the subject Probability Theory: the Logic of Science. Many philosophers of science including Karl Popper and Donald Campbell have interpreted the evolution of Science as a Darwinian process consisting of a ‘copy with selective retention’ algorithm abstracted from Darwin’s theory of Natural Selection. Arguments are presented for an isomorphism between Bayesian Methods and Darwinian processes. Universal Darwinism, as the term has been developed by Richard Dawkins, Daniel Dennett and Susan Blackmore, is the collection of scientific theories which explain the creation and evolution of their subject matter as due to the Operation of Darwinian processes. These subject matters span the fields of atomic physics, chemistry, biology and the social sciences. The principle of Maximum Entropy states that Sy...

Far-infrared properties of resonant modes and tunnelling states in rare-earth-doped calcium fluoride

The low-temperature observation of a broad distribution of two-level systems in chemically disordered fluorite mixed crystals has heightened interest in the spectroscopic properties of the corresponding lightly doped crystals. The low-temperature far-ir absorption of calcium fluoride crystals doped with Y3+, La3+ or Lu3+ in the range less than 1 mol% shows three completely different types of defect-induced spectra and the temperature and concentration dependence are found to vary widely. In addition, the far-ir properties of CaF2 doped with Y3+ or Lu3+ can be dramatically altered by heat treatment, indicating that clustering plays an important role. By comparing the spectroscopic signatures with those previously observed for other defect systems, the lowest-lying excitations for these doped CaF2 crystals have been assigned as follows: tunnelling transitions in two different elastic configurations for Y3+, a variety of ground state tunnelling transitions for the La3+ doping and an anharmonic resonant mode for the Lu3+ doped system. The surprising result is that a broad distribution of two-level systems is clearly evident in two of the three systems and the optical activity of this distribution appears to be controlled by the optical activity of the dominant defect dynamics.

Glasslike two-level systems in minimally disordered mixed crystals.

THz spectroscopy is used to identify a broad distribution of two-level systems, characteristic of glasses, in the substitutional monatomic mixed crystal systems, Ba(1-x)Ca(x)F(2) and Pb(1-x)Ca(x)F(2). In these minimally disordered systems, two-level behavior, which was not previously known to occur, begins at a specific CaF(2) concentration. The concentration dependence, successfully modeled using the statistics of the impurity distribution in the lattice, points to a collective dopant tunneling mechanism.

mm-Wave absorption and Raman scattering from glass-like features in fluorite mixed crystals

Abstract One way to obtain new information about the ubiquitous low-lying two-level systems found in glasses is to identify them in appropriately modified crystals and then vary the crystal properties. Here our low-temperature mm-wave absorption and Raman scattering measurements on a number of mixed fluorite crystals are reviewed. It is shown that these single-crystal materials possess a broad distribution of two-level systems similar to what is found in the amorphous state.

Optical spectra and photoconductivity of uranium-doped CaF2 crystals

Using infrared and optical absorption spectroscopy in combination with photoconductivity measurements on uranium-doped calcium fluorite crystals, we clarify a controversy in the assignment of U2+ and U4+ ion valences in CaF2 crystals which exhibit different coloration. The energy thresholds of donor-like processes (photoionization) follow the sequence of ionization potentials for uranium ions of different valences and indicate the green and yellow colors to be due to U4+ of different charge compensation.

Multi-phonon relaxation of the H- local mode in CaF2

Abstract We have investigated the non-radiative relaxation of the substitutional H − local mode ( v = 965 cm −1 ) in CaF 2 . This system is attractive due to its simplicity, the only possible decay mechanism, aside from radiative relaxation, is non-radiative multiphonon decay into host phonons. Saturation experiments were performed at the Stanford Picosecond Free Electron Laser Center, using a pump-probe technique. We find an energy relaxation time T 1 of 45 ps at low temperature. We compare this result with the relaxation of the local mode of an interstitial H − ion next to a Lanthanum impurity ion, and find a much more rapid relaxation time T 1 of 6 ps.

Far-infrared evidence for glass-like properties, exhibited by crystals of CaF2 doped with YF3

Abstract Low temperature far-infrared transmission measurements between 3 cm−1 and 18 cm−1 have been performed on crystals of CaF2 doped with YF3 for molar concentrations ranging from 1% to 22%. These single crystal materials posses a broad distribution of two-level systems typical of amorphous materials. The distribution is similar in magnitude and frequency dependence to that of a standard soda-lime silica glass. In contrast to other fluorite mixed crystal systems the magnitude of the two-level system distribution for YF3 doped samples can be reversibly altered by high temperature cycling between 600 and 900°C.