Aristotelinone and serratoline: New indole alkaloids from Aristotelia serrata W.R.B. Oliver

Aristotelinone and serratoline, two new Aristotelia alkaloids, are shown to have indole and indolenine structures respectively from their spectra and reduction products.     

The influence of small amounts of crystallinity on the transport properties of metallic glasses

Small amounts of crystallinity may be present in “amorphous” alloy specimens in the form of “cast-in” crystallites or partially crystallized surfaces. in this paper the possible effects of such low-level crystallinity upon the measured electrical transport properties are considered. It is found that the observed ageing effects in the transport properties of a number of amorphous alloys may be explained as due to surface crystallization driven by selective oxidation. It is further argued that for the ferromagnetic Metglasses cast-in crystallites could have a significant effect particularly upon the thermopower, and that this is probably part of the explanation of the observed discrepancies between the different measured values. It is shown that even very small levels of crystallinity can have significant effects upon the measured transport properties and the many disagreements to be found in the published values are discussed in the light of this.     

Microscopic analysis of the valence band and impurity band theories of (Ga,Mn)As

We analyze microscopically the valence and impurity band models of ferromagnetic (Ga,Mn)As. We find that the tight-binding Anderson approach with conventional parametrization and the full potential local-density approximation+U calculations give a very similar band structure whose microscopic spectral character is consistent with the physical premise of the k·p kinetic-exchange model. On the other hand, the various models with a band structure comprising an impurity band detached from the valence band assume mutually incompatible microscopic spectral character. By adapting the tight-binding Anderson calculations individually to each of the impurity band pictures in the single Mn impurity limit and then by exploring the entire doping range, we find that a detached impurity band does not persist in any of these models in ferromagnetic (Ga,Mn)As.     

Approaches to Modeling Bed Drag in Pine Forest Litter for Wildland Fire Applications

Transport in Porous Media - Modeling flow in vegetative fuel beds is a key component in any detailed physics-based tool for simulating wildland fire dynamics. Current approaches for drag modeling,...     

Some applications of thermomagnetometry to the study of chromindur alloys

Thermomagnetic experiments are shown to be a useful and rapid technique for studying phase changes involving magnetic materials. Particular examples are chosen from Chromindur II (Fe28Cr10.5Co) alloys having different thermal histories. The Curie temperature of the single phase alloy is found to be ～650°C and the metastable spinodal decomposition boundary is at essentially the same temperature. The Cr-rich near equilibrium decomposition product is less magnetic and has a lower T_c, while the Cr-deficient product is more magnetic, with a T_c ? 720°C.Comparisons are made at different heating and cooling rates between temperatures measured using magnetic standards and those measured by a thermocouple in close proximity to the sample.     

Molecular beam epitaxy growth and properties of GaAs/(AlGa)As p-type heterostructures on (100), (011), (111)B, (211)B, (311)B, and (311)A oriented GaAs

We report on a series of Be-doped GaAs/AlGaAs two-dimensional hole gas (2DHG) structures grown on (110), (111)B, (211)B and (311)B oriented substrates and compare their properties with high-mobility samples grown on (311)A using Si doping. The samples were prepared and grown under the same conditions so as to render them comparable. They are found to have mobilities which are strongly anisotropic within the plane. The highest mobility is found on the (110) surface with 100,000 cm² V⁻¹ s⁻¹, while the (211) surface gave the lowest values 10,000 cm² V⁻¹ s⁻¹. However, the later samples are found to have quantum Hall effect critical currents of >70 μA: an exceptionally high value for a hole gas which makes them suitable for metrology. All the samples show strong low-field positive magnetoresistance with resistance increases of up to 30% at magnetic fields of only 0.1 T. The presence of this feature on all the different planes shows that it does not depend upon the details of the band structure. It is identified with the lifting of the degeneracy of the spin sub-bands by the asymmetrical potential giving rise to a classical two-band magnetoreresistance.     

Synthesis, stoichiometry and thermal stability of Zn3N2 powders prepared by ammonolysis reactions

Zn₃N₂ powders were prepared by ammonolysis reactions at 600 °C and examined by thermogravimetric analysis, powder X-ray and neutron diffraction. The powders obtained in this way are unstable in an oxygen atmosphere above 450 °C. In an argon atmosphere, the powders are stable up to their decomposition point at around 700 °C. Structural models obtained from Rietveld refinements against the powder neutron diffraction data indicate that the Zn₃N₂ powders so-prepared have the anti-bixbyite structure and are almost certainly stoichiometric with no compelling evidence of nitrogen vacancies. Further, no evidence was found for aliovalent oxygen substitution at the nitrogen sites. The calculated bond valence sums imply that Zn₃N₂ cannot be described as a 100% ionic compound. The structural findings are supported by photoluminescence measurements that reveal a band gap of approximately 0.9 eV.     

Variable temperature H NMR studies on Grubbs catalysts

Variable temperature ¹H NMR studies were conducted to investigate whether steric congestion is influencing the structural rigidity of (IMesH₂)(PCy₃)(Cl)₂RuCHPh (IMesH₂ = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) in solution. It was shown that both mesityl ligands rotate at about the same rate around the N-Mesityl bonds in the IMesH₂ ligand and that changing the solvent does not significantly alter this rotation. It was found that the increased steric congestion in (IMesH₂)(PCy₃)(Cl)₂RuCHPh compared to (PCy₃)₂(Cl)₂RuCHPh does affect the rates of rotation around the C_alkylidene-Ph bonds. Unusual chemical shift positions were also observed in the low temperature ¹H NMR spectrum for the aromatic proton signals for (IMesH₂)(PCy₃)(Cl)₂RuCHPh and (PCy₃)₂(Cl)₂RuCHPh.