Coherent motion of a hole in a two-dimensional quantum antiferromagnet

The dispersion relation for the coherent propagation of a hole moving in a two-dimensional quantum antiferromagnet is discussed. The system is described by two model Hamiltonians, thet-J model and thet-t-J model, which have been used frequently to discuss strong electron-correlation effects present in high-T _c superconductors. The calculations are based on the introduction of a new wave function which is constructed by use of equations derived by Shraiman and Siggia. The different mechanisms for the coherent propagation, which are due to the spin fluctuation and the hopping terms of the Hamiltonian, are treated on the same footing. As a result of the inclusion of an effective hopping mechanism along spiral paths-first discussed by Trugman-the minimum of the band is somewhat changed compared to results recently obtained in the literature. For large values of the ratiot/J an inversion of the whole dispersion relation occurs. The overall shapes of the dispersion within both models are found to agree quite well, though for small values oft/J the bandwidth within thet-J model becomes significantly smaller than that of thet-t-J model.     

Synthesen von Heterocyclen, 111. Mitt.: Zur Umsetzung von Benzoylaceton und seinen Derivaten mit Malonylchlorid

Zusammenfassung Der Vorgang der thermischen Umlagerung des 4-Hydroxy-5-acetyl-6-phenyl-pyrons-(2) (1) zum 4-Hydroxy-5-benzoyl-6-methyl-pyron-(2) (4) wird an Hand weiterer Beispiele studiert und ferner der Einfluß der Substituenten am C-5 bzw. C-6 des Lactonringes untersucht.

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An investigation of more examples of the thermal rearrangement of 5-acetyl-4-hydroxy-6-phenyl-pyrone-(2) to 5-benzoyl-4-hydroxy-6-methyl-pyrone-(2) was carried out, it showed the influence of substituents at C-5 and C-6 of the lactone ring.

     

Theory of shear-induced crystallization of polymer melts

In the presented model elements of polymer melt rheology and polymer crystallization kinetics are combined. In particular, the proneness of the melt to the special type of crystallization which is characteristic for shear treatment is supposed to emerge only gradually during shear flow. Following Avrami's early ideas on crystal growth, an induction time is introduced. In principle, the model can be applied to any flow and temperature history. The special case of isothermal flow at constant shear rate is covered in greater detail: A favorable comparison is made with experimental results, as published by Lagasse and Maxwell [10].Dedicated to Prof. J. Meissner on the occasion for his 60th birthday.     

Determination of copper in medicinal plants used as dietary supplements by atomic absorption spectrometry with direct flame solid analysis

An alternative device for the direct solid analysis (DSA) for copper determination by flame atomic absorption spectrometry (FAAS) is proposed. Copper was directly determined in commercial medicinal plants used as dietary supplements. The determination of copper in solid samples by DSA–FAAS was made by using a conventional air–acetylene flame. Between 0.05 and 1.5 mg of each test, sample was weighed directly into a small polyethylene vial connected to the device used for solid introduction into the flame. Test samples were introduced into the flame as a dry aerosol using a T-quartz cell set between the burner and the optical path. The T-quartz cell has a slit in the superior part by which the solid aerosol passes to the flame. A transient signal, evaluated as integrated absorbance, is produced and it is totally integrated in 2 s. Background signals always presented absorbance values less than 0.1. It was found a characteristic mass of 0.8 ng Cu and absolute limit of detection of 1.2 ng (3 s), or 1.2 μg g⁻¹ if a sample mass of 1 mg was used. Optimized conditions for air flow rate, flame stoichiometry, and so on were established as well. No excessive grinding of the samples was needed and samples with particle of size less than 80 μm were used throughout. No statistical difference between the results from the proposed system and those obtained by sample digestion and determination by conventional FAAS was observed. With the proposed procedure, more than 50 test samples can be analyzed in 1 h and it can be easily adapted to conventional spectrometers for FAAS.     

Hydroselenation of alkynes using NaBH4/BMIMBF4: easy access to vinyl selenides

A general and easy method for the synthesis of several vinyl selenides using NaBH₄ and BMIMBF₄ as a recyclable solvent is described. This efficient and improved method furnishes the corresponding vinyl chalcogenides preferentially with Z configuration. We also observed that when the same protocol was applied to phenyl acetylene, (E)-bis-phenylseleno styrene was obtained in good yield and with high selectivity.