Anomalous thermopower in the metalliclike phase of a 2D hole system

We report very low temperature ( T) thermopower and resistivity ( rho) measurements on variable-density, two-dimensional hole systems confined to GaAs quantum wells. As the hole density is lowered from 1.49x10(11) cm(-2) to 0.14x10(11) cm(-2), the system crosses from an insulating ( drho / dT less, similar0) to a metallic regime ( drho / dT>0) and finally displays insulating behavior ( drho / dT<0). Diffusion thermopower shows a striking sign reversal in a narrow range of density in the metallic regime, suggesting a qualitative change in the conduction or the scattering mechanism.     

Limits on the high-energy gamma and neutrino fluxes from the SGR 1806-20 giant flare of 27 December 2004 with the AMANDA-II detector

On 27 December 2004, a giant gamma flare from the Soft Gamma-Ray Repeater 1806-20 saturated many satellite gamma-ray detectors, being the brightest transient event ever observed in the Galaxy. AMANDA-II was used to search for down-going muons indicative of high-energy gammas and/or neutrinos from this object. The data revealed no significant signal, so upper limits (at 90% C.L.) on the normalization constant were set: 0.05(0.5) TeV-1 m;{-2} s;{-1} for gamma=-1.47 (-2) in the gamma flux and 0.4(6.1) TeV-1 m;{-2} s;{-1} for gamma=-1.47 (-2) in the high-energy neutrino flux.     

The <Emphasis Type="SmallCaps">Mcf</Emphasis>-separator: detecting and exploiting multi-commodity flow structures in MIPs

Given a general mixed integer program, we automatically detect block structures in the constraint matrix together with the coupling by capacity constraints arising from multi-commodity flow formulations. We identify the underlying graph and generate cutting planes based on cuts in the detected network. Our implementation adds a separator to the branch-and-cut libraries of Scip and Cplex. We make use of the complemented mixed integer rounding framework but provide a special purpose aggregation heuristic that exploits the network structure. Our separation scheme speeds-up the computation for a large set of mixed integer programs coming from network design problems by a factor two on average. We show that almost 10% of the instances in general testsets contain consistent embedded networks. For these instances the computation time is decreased by 18% on average.     

A monopole mass filter with a hyperbolic V-shaped electrode

In this article we compare the classical monopole mass filter of von Zahn and the monopole mass filter with a hyperbolic V-shaped electrode. The experimental results and those of computer simulation for both mass spectrometers are presented. We show that the replacement of a conventional 90 degrees V-shaped electrode by an electrode with a hyperbolic profile substantially improves the peak shape of any given mass, and increases the mass resolution by a factor of 3-4 and the abundance sensitivity by a factor of 100. The potential of high analytical performance combined with electroforming techniques for electrode manufacture indicate future practical uses of such instruments. Copyright 1999 John Wiley & Sons, Ltd.     

Shape resonances in slow electron scattering by aromatic molecules. I. Anthraquinone derivatives

A series of anthraquinone (C(14)O(2)H(8)) derivatives has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), photoelectron spectroscopy (PES), and AM1 quantum chemical calculations. Mean lifetimes of molecular negative ions M(-.) (MNI) have been measured. The mechanism of long-lived MNI formation in the epithermal energy region of incident electrons has been investigated. A simple model of a molecule (a spherical potential well with the repulsive centrifugal term) has been applied for the analysis of the energy dependence of cross sections at the first stage of the electron capture process. It has been shown that a temporary resonance of MNI at the energy approximately 0.5 eV corresponds to a shape resonance with lifetime 1-2.10(-13) s in the f-partial wave (l = 3) of the incident electron. The next resonant state of MNI at the energy approximately 1.7 eV has been associated with the electron excited Feshbach resonance (whose parent state is a triplet npi* transition). In all cases the initial electron state of the MNI relaxes into the ground state by means of a radiationless transition, and the final state of the MNI is a nuclear excited resonance with a lifetime measurable on the mass spectrometry timescale. Copyright 1999 John Wiley & Sons, Ltd.     

Domain decomposition algorithms for indefinite weakly singular integral equations: the h and p versions

We extend the approach of Cai and Widlund (Domain decompositionalgorithms for indefinite elliptic problems, SIAM J. Sci. Stat.Comput. 13 (1992), 243-258), which was designed for finite elementdiscretizations, to boundary element discretizations of indefiniteweakly singular integral equations. Both the h and p versionsof the Galerkin approximation are considered. We prove thatthe additive Schwarz method suggested by Cai and Widlund canbe used for this equation as an efficient preconditioner forGMRES, an iterative method of conjugate gradient type. For bothversions, the rates of convergence of this iterative methodare shown to approach 1 only logarithmically as the degreesof freedom tend to infinity.     

Development of a colorimetric microfluidic pH sensor for autonomous seawater measurements

High quality carbonate chemistry measurements are required in order to fully understand the dynamics of the oceanic carbonate system. Seawater pH data with good spatial and temporal coverage are particularly critical to apprehend ocean acidification phenomena and their consequences. There is a growing need for autonomous in situ instruments that measure pH on remote platforms. Our aim is to develop an accurate and precise autonomous in situ pH sensor for long term deployment on remote platforms. The widely used spectrophotometric pH technique is capable of the required high-quality measurements. We report a key step towards the miniaturization of a colorimetric pH sensor with the successful implementation of a simple microfluidic design with low reagent consumption. The system is particularly adapted to shipboard deployment: high quality data was obtained over a period of more than a month during a shipboard deployment in northwest European shelf waters, and less than 30 mL of indicator was consumed. The system featured a short term precision of 0.001 pH (n = 20) and an accuracy within the range of a certified Tris buffer (0.004 pH). The quality of the pH system measurements have been checked using various approaches: measurements of certified Tris buffer, measurement of certified seawater for DIC and TA, comparison of measured pH against calculated pH from pCO₂, DIC and TA during the cruise in northwest European shelf waters. All showed that our measurements were of high quality. The measurements were made close to in situ temperature (+0.2 ?C) in a sampling chamber which had a continuous flow of the ship’s underway seawater supply. The optical set up was robust and relatively small due to the use of an USB mini-spectrometer, a custom made polymeric flow cell and an LED light source. The use of a three wavelength LED with detection that integrated power across the whole of each LED output spectrum indicated that low wavelength resolution detectors can be used instead of the current USB mini spectrophotometer. Artefacts due to the polychromatic light source and inhomogeneity in the absorption cell are shown to have a negligible impact on the data quality. The next step in the miniaturization of the sensor will be the incorporation of a photodiode as detector to replace the spectrophotometer.     

Characterisation and deployment of an immobilised pH sensor spot towards surface ocean pH measurements

The oceans are a major sink for anthropogenic atmospheric carbon dioxide, and the uptake causes changes to the marine carbonate system and has wide ranging effects on flora and fauna. It is crucial to develop analytical systems that allow us to follow the increase in oceanic pCO₂ and corresponding reduction in pH. Miniaturised sensor systems using immobilised fluorescence indicator spots are attractive for this purpose because of their simple design and low power requirements. The technology is increasingly used for oceanic dissolved oxygen measurements.     

Branching rules revisited

We present a new generalization called reliability branching of today's state-of-the-art strong branching and pseudocost branching strategies for linear programming based branch-and-bound algorithms. After reviewing commonly used branching strategies and performing extensive computational studies we compare different parameter settings and show the superiority of our proposed new strategy.