High-temperature superconductivity: The roles of soft-phonon driven structural phase transitions and biphonon-electron coupling

For the first time, fully self-consistent soft phonon data, associated with the tetragonal-orthorhombic structural transition in the high-temperature superconductor La_2–x Sr _x CuO₄, are used to solve Eliashberg equations. It is found that the electron-phonon coupling constant is stronglyx- and temperature-dependent. The superconducting transition temperatureT _c follows the experimentalx-dependence but does not exceed 28 K. The higherT _c values observed are attributed to nonlinear electronphonon interactions, due to high oxygen ion polarizability, which producebiphonon resonances and bound states and significantly increaseT _c .     

Disorder-induced local magnetic moments in weakly correlated metallic systems

We consider the formation and the Kondo effect of local magnetic moments in the Anderson-Hubbard model with off-diagonal disorder. The existence of moments at sites weakly coupled to the environment is deduced in effective medium approximation. The distribution of moments is calculated both deep in the metallic phase and near the metal-insulator transition. We discuss the Kondo quenching of the moments and derive a distribution of local Kondo temperatures.     

Dynamics of dipolar glasses: elastic and dielectric properties

We have formulated a model for the coupled dynamics of the local polarization and the elastic deformations of dipolar glasses. Randomly interacting dipoles undergo a phase transition to a glassy state, which is signalled above T _c by a divergence of the relaxation time of the dipolar autocorrelation. Dipolar dynamics is directly observable in the dynamic dielectric susceptibility, for which we predict a power law dependence on frequency at and close to T _c. The slowing down of dipolar degrees of freedom also gives rise to dynamic critical anomalies in the elastic and structural properties, like the appearance of a central peak and divergence of the sound damping. All these phenomena are a consequence of dynamic scaling of the dipolar autocorrelation and hence interrelated. We discuss two scaling functions in detail: One is calculated within mean field theory and the other one is taken from numerical simulations of short range Ising spin glasses. The predictions of the model are in accordance with most experimental findings for dipolar glasses.     

Sensitive methods for the determination of microbial activities in water samples using fluorigenic substrates

Summary Sensitive methods for the determination of several catabolic enzyme activities in the natural water cycle as well as in water treatment plants have been developed. The tests are based on the fluorimetric measurement of dyes which are released by enzymatic cleavage from fluorigenic substrates added to the water sample. On the basis of a linear reaction the turnover rate can be calculated. Compared to photometric tests the detection limits are lower by a factor of 20 to 500, which now allows a precise study of the catabolic activity in samples with low biomass density. In addition, the use of fluorigenic substrates for the determination of microbial enzyme activities provides a higher precision in shorter reaction times compared to photometric tests. In detail, instructions for the determination of esterase, phosphatase, aminopeptidase and glucosidase activities are presented.

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Empfindliche Methoden zur Bestimmung der mikrobiellen Aktivitäten in Wasser mit Hilfe fluorigener Substrate

Zusammenfassung Es werden empfindliche Methoden zur Bestimmung verschiedener mikrobieller Enzymaktivitäten im natürlichen Wasserkreislauf und in Wasseraufbereitungsanlagen vorgestellt. Die Tests basieren auf der Verwendung von fluorigenen Substraten, die der Probe direkt zugesetzt werden. Die durch enzymatische Aktivität abgespaltenen Farbstoffe werden fluoreszenzspektroskopisch gemessen. Aus der Konzentration der freigesetzten Farbstoffe läßt sich bei linearem Reaktionsverlauf der Substratumsatz in der Probe berechnen. Im Vergleich zu photometrischen Tests sind die Nachweisgrenzen der fluorimetrischen Tests um den Faktor 20 bis 500 niedriger, so daß nunmehr auch eine differenzierte Untersuchung von mikrobiell gering besiedelten Proben möglich ist. Zudem wird eine höhere Genauigkeit bei kürzerer Reaktionszeit erreicht. Im Einzelnen werden Testvorschriften zur Bestimmung der Esterasen-, Phosphatasen-, Aminopeptidasen- und Glucosidasen-Aktivität beschrieben.

     

Development of Passive and Active Barrier Coatings on the Basis of Inorganic–Organic Polymers

Summary. With a new kind of barrier coating material, namely inorganic–organic polymers, it is possible to obtain excellent barrier properties against oxygen, water vapor, and flavor permeation. These hybrid polymers can be synthesized by the sol–gel technique. If extremely low permeation values are needed, the combination of hybrid polymer coatings with thin inorganic oxidic layers (SiO_x, AlO_x) is very effective and leads to permeation values for oxygen and water vapor below 10⁻³ cm³/m² · d · bar or g/m² · d. These passive barrier layers can be further improved by the combination with active oxygen barrier layers which have been developed for the food packaging industry. This approach makes these multilayer laminates promising candidates for special applications in the food packaging industry as well as for sophisticated applications in technical areas: the encapsulation of sensitive organic devices like solar cells, organic light emitting diodes, or polymer electronic systems.     

Spectres de vibration de composes organiques des elements de la colonne IVB : VIII. Derives organometalliques a liaison germanium—azote: Trialkylgermyl-et bis(trialkylgermyl)-amines

We have examined in the 3600–100 cm^?1 region the IR and Raman spectra of three bis(trialkylgermyl)amines (R₃Ge)₂NR′ (R = Me, Et; R′ = H, C₆H₅), of three dialkyl(trialkylgermyl)amines R₃GeNR′₂ (R = Me, Et; R′ = Me, n-Bu) and of two triethylgermylpyrazoles:

Assignments are discussed for the characteristic vibrations of these derivatives.