Effect of Cryogenic Temperature Deposition of Various Metal Contacts on Bulk Single-Crystal <Emphasis Type="Italic">n</Emphasis>-Type ZnO

The effect of cryogenic temperatures during metal deposition on the contact properties of Pd, Pt, Ti, and Ni on bulk single-crystal n-type ZnO has been investigated. Deposition at both room and low temperature produced contacts with Ohmic characteristics for Ti and Ni metallizations. By sharp contrast, both Pd and Pt contacts showed rectifying characteristics after deposition with barrier heights between 0.37 eV and 0.69 eV. Changes in contact behavior were measured on Pd to anneal temperatures of ∼300 °C, showing an increase in barrier height along with a decrease in ideality factor with increasing annealing temperature. This difference with annealing temperature is in sharp contrast to previous results for Au contacts to ZnO. There were no differences in near-surface stoichiometry for the different deposition temperatures; however, low temperature contacts demonstrated some peeling/cracking for Pt and Pd.     

Mitigating the risk of high power damage to the BT network

Optical powers used in optical telecommunications networks have increased over time with the advent of technologies such as erbium-doped fibre amplifiers, dense WDM, and fibre Raman amplifiers. Erbium fibre and Raman amplifiers with output powers up to 1 W and 2 W respectively are now available for deployment giving optical power densities of ≈ 14 000 MW/m² per Watt launched into a single mode fibre. This compares with 74 MW/m² for the surface of the sun. It is thus essential to ensure that the risk of optical damage to the fibre is understood and steps are taken to mitigate it where necessary. Research carried out by BT over the last few years has shown that fibres carrying relatively modest optical powers, as low as 200 mW for the most sensitive fibres, can be catastrophically damaged at tight fibre bends. Damage is caused by some of the optical power lost at the bend being absorbed by the fibre coating that may over time lead to a sudden increase in temperature to over 700 °C causing either the silica to kink mimicking a fibre break, or the coating to burn off at the bend leading to fibre failure on subsequent handling. This paper describes factors that increase the risk of short-term catastrophic damage at fibre bends, presents experimental results to show the variation in sensitivity between different fibres, and discusses new important theoretical results modelling the temperature rise in the fibre.     

A parallel computational method for simulating two‐phase gel dynamics on a staggered grid

We develop a parallel computational algorithm for simulating models of gel dynamics where the gel is described by two phases, a networked polymer and a fluid solvent. The models consist of transport equations for the two phases, two coupled momentum equations, and a volume‐averaged incompressibility constraint. Multigrid with Vanka‐type box‐relaxation scheme is used as preconditioner for the Krylov subspace solver (GMRES) to solve the momentum and incompressibility equations. Through numerical experiments of a model problem, the efficiency, robustness and scalability of the algorithm are illustrated. Copyright © 2008 John Wiley & Sons, Ltd.     

Observation of complex magnetic behavior in the perovskite rare earth copper oxide systems, R2CuO4

EPR, microwave absorption, and dc magnetization measurements were made on single crystals of the form R₂CuO₄, which are the host compounds for the newly discovered series of electron cuprate superconductors. These measurements reveal two characteristic transition temperatures associated with a novel complex magnetic behavior, including weak ferromagnetism, two sharp peaks in the low field DC magnetization, an unusual anisotropy in the EPR resonance field for R = Gd, and two additional anisotropic microwave absorption modes. The higher characteristic transition temperature, at ≈270K, is attributed to AF ordering of the Cu moments, and the lower, at ≤20K, to a spontaneous canted spin reorientation. An understanding of this magnetic behavior is important in order to ascertain its relationship to possible mechanisms of high temperature superconductivity.     

Can density functional theory (DFT) be used as an aid to a deeper understanding of tandem mass spectrometric fragmentation pathways?

Prediction of tandem mass spectrometric (MS/MS) fragmentation for non‐peptidic molecules based on structure is of immense interest to the mass spectrometrist. If a reliable approach to MS/MS prediction could be achieved its impact within the pharmaceutical industry could be immense. Many publications have stressed that the fragmentation of a molecular ion or protonated molecule is a complex process that depends on many parameters, making prediction difficult. Commercial prediction software relies on a collection of general heuristic rules of fragmentation, which involve cleaving every bond in the structure to produce a list of ‘expected’ masses which can be compared with the experimental data. These approaches do not take into account the thermodynamic or molecular orbital effects that impact on the molecule at the point of protonation which could influence the potential sites of bond cleavage based on the structural motif. A series of compounds have been studied by examining the experimentally derived high‐resolution MS/MS data and comparing it with the in silico modelling of the neutral and protonated structures. The effect that protonation at specific sites can have on the bond lengths has also been determined. We have calculated the thermodynamically most stable protonated species and have observed how that information can help predict the cleavage site for that ion. The data have shown that this use of in silico techniques could be a possible way to predict MS/MS spectra. Copyright © 2009 John Wiley & Sons, Ltd.     

On the origin of compositional fluctuations in ZnSe1- x S x alloys grown by metal-organic vapour-phase epitaxy

Periodic vertical modulations of the sulphur concentration have been observed in epitaxial layers of ZnSe_{1? x} S _x grown by metal-organic vapour-phase epitaxy. High-spatial-resolution microanalysis has shown that the sulphur variation is of the order of 1 at.% with a period of about 44 ± 0.1 nm. A detailed analysis of the possible causes of this modulation is presented which shows that periodic temperature fluctuations of the substrate are the most likely source.     

A Survey of Calculator Usage in High Schools

This survey investigated the current status of calculator use in classrooms and schools. The results indicated that the prevailing policy in the sample of high schools is to allow the use of calculators during classroom learning activities and tests. Scientific calculators are more frequently used than graphing calculators in algebra I and geometry; whereas, graphing calculators are more frequently used in algebra II and precalculus/trigonometry. At the time of this survey, school policies regarding the use of graphing calculators with symbolic algebra capabilities were still not determined.