Electron paramagnetic resonance evidence for Jahn-Teller glasses

Single crystal E.P.R. studies of copper as a dopant in lithium potassium sulphate, lithium ammonium sulphate and lithium sodium sulphate have been carried out from room temperature down to 77K. The three Jahn-Teller (JT) systems behave very similarly to one another. The room temperature dynamic JT spectra with giso = 2·19 ± 0·01 and Aiso = ±(33 ± 4) times 10-4 cm-1 transform around 247 K to spectra characterized by randomly frozen-in axial strains with g‖ = 2·4307 ± 0·0005, g⊥ = 2·083 ± 0·001, A‖ = ±(116 ± 2) times 10-4 cm-1 and A⊥ = ∓(14 ± 4) times 10-4 cm-1. We proposed that the low temperature phase (below 247 K) of each of these systems provides an example of a Jahn-Teller glass.

ESR and optical studies of Mo5+ and W5+ ions in phosphomolybdate and phosphotungstate glasses

ESR and optical studies of phosphomolybdate and phosphotungstate glasses are discussed. Both the ESR and optical results indicate that molybdenum or tungsten ions are present in distorted octahedral environments in these glasses. In addition, ESR spectra of Mo5+ and W5+ ions show that the d electrons are localized on molybdenum and tungsten sites respectively. The variation of gperpendicular and gshort parallel values has been examined using appropriate structural models of these glasses.

Gauge theory of a group of diffeomorphisms. III. The fiber bundle description

A new fiber bundle approach to the gauge theory of a group G that involves space‐time symmetries as well as internal symmetries is presented. The ungauged group G is regarded as the group of left translations on a fiber bundle G(G/H,H), where H is a closed subgroup and G/H is space‐time. The Yang–Mills potential is the pullback of the Maurer–Cartan form and the Yang–Mills fields are zero. More general diffeomorphisms on the bundle space are then identified as the appropriate gauged generalizations of the left translations, and the Yang–Mills potential is identified as the pullback of the dual of a certain kind of vielbein on the group manifold. The Yang–Mills fields include a torsion on space‐time.

High-pressure studies on Ge-Te glasses. Evidence for a critical composition in IV-VI chalcogenide glassy systems

The electrical resistivity of bulk GexTe100-x glasses has been measured as a function of temperature and pressure. Under high pressure, all the glasses were found to undergo sharp discontinuous transitions from glassy semiconductors to crystalline metal. Several of the observed properties such as the transition pressure, conductivity activation energy and pre-exponential factor, exhibit anomalous trends at a composition x = 20. These results suggest that the x = 20 composition in the Ge-Te system should possess salient structural features. A model based on the unusual stability of structural units is proposed for explaining the anomaly at 20 at.% Ge concentration.

Transport properties of phosphomolybdate and phosphotungstate glasses

The d.c. conductivity of phosphomolybdate and phosphotungstate glasses is discussed. The conductivity of these glasses is due to the hopping of electrons between two valence states (Mo5+ to Mo6+ or W5+ W6+). In some of the glasses, the activation energy itself is found to be a function of temperature. This appears to be due to thermally activated and variable-range hopping mechanisms operating in different temperature regimes. The relation between conductivity and the [M5+]/[Mtotal](M ≡ Mo, W) ratio does not show any systematic variation. This anomaly can be understood using the structural models of these glasses. In contrast, Mott's theory and the Triberis and Friedman model have been used to obtain conductivity parameters such as the percolation distance Rij and 2agrRij (agr is the tunnelling probability). The conductivity parameter 2agrRij is quite useful to resolve the controversy regarding the tunnelling term exp(2agrRij) existing in the literature. For low values of 2agrRij, it is shown that the exp (2agrRij) term is very significant.

Elastic properties of PbS-As2S3 glasses

The temperature dependence of the longitudinal and shear ultrasound wave velocities in (As2S3)1-x(PbS)x glasses has been determined from 77 to 300K using a pulse echo interferometer. Elastic constants of the prepared glasses at room temperature have been computed from the experimental data. Both longitudinal and shear ultrasound wave velocities in these glasses show a linear temperature dependence with a negative temperature coefficient.

Mechanical and Chemical Thresholds in IV-VI Chalcogenide Glasses

It has been possible to identify two critical compositions in the IV-VI chalcogenide glassy system GexSe100-x by the anomalous variations of the high-pressure electrical resistivity behavior. The first critical composition, the chemical threshold, refers to the stoichiometric composition. The second critical composition, identified recently as the mechanical percolation threshold, is connected with the structural rigidity of the material.