Growth of large single crystals of Rare Earth Hexaborides

Single crystal growth of several rare earth hexaborides has been carried out by the floating zone technique. A high power Xenon arc lamp image furnace was used for the crystal growth. Large high quality crystals of LaB$_{6}$, CeB$_{6}$, PrB$_{6}$ and NdB$_{6}$, about 1 cc in volume have been obtained. Crystals of all these compounds have also been grown using enriched $^{11}$B isotope for use in neutron scattering experiments.Comment: 3 pages, 3 figures, in press (J. Crystal Growth

The oxygen isotope evolution of parent body aqueous solutions as recorded by multiple carbonate generations in the Lonewolf Nunataks 94101 CM2 carbonaceous chondrite

The CM2 carbonaceous chondrite LON 94101 contains aragonite and two generations of calcite that provide snapshots of the chemical and isotopic evolution of aqueous solutions during parent body alteration. Aragonite was the first carbonate to crystallize. It is rare, heterogeneously distributed within the meteorite matrix, and its mean oxygen isotope values are δ18O 39.9±0.6‰, Δ17O -0.3±1.0‰ (1σ). Calcite precipitated very soon afterwards, and following a fall in solution Mg/Ca ratios, to produce small equant grains with a mean oxygen isotope value of δ18O 37.5±0.7‰, Δ17O 1.4±1.1‰ (1σ). These grains were partially or completely replaced by serpentine and tochilinite prior to precipitation of the second generation of calcite, which occluded an open fracture to form a millimeter-sized vein, and replaced anhydrous silicates within chondrules and the matrix. The vein calcite has a mean composition of δ18O 18.4±0.3‰, Δ17O -0.5±0.5‰ (1σ). Petrographic and isotopic results therefore reveal two discrete episodes of mineralization that produced Ca-carbonates with contrasting δ18O, but whose Δ17O values are indistinguishable within error. The aragonite and equant calcite crystallized over a relatively brief period early in the aqueous alteration history of the parent body, and from static fluids that were evolving chemically in response to mineral dissolution and precipitation. The second calcite generation crystallized from solutions of a lower Δ17O, and a lower δ18O and/or higher temperature, which entered LON 9410 via a fracture network. As two generations of calcite whose petrographic characteristics and oxygen isotopic compositions are similar to those in LON 94101 occur in at least one other CM2, multiphase carbonate mineralization could be the typical outcome of the sequence of chemical reactions during parent body aqueous alteration. It is equally possible however that the second generation of calcite in formed in response to an event such as impact fracturing and concomitant fluid mobilisation that affected a large region of the common parent body of several CM2 meteorites. These findings show that integrated petrographic, chemical and isotopic studies can provide new insights into the mechanisms of parent body alteration including the spatial and temporal dynamics of the aqueous system

QCD Down Under: Building Bridges

The strong coupling regime of QCD is responsible for 99% of hadronic phenomena. Though considerable progress has been made in solving QCD in this non-perturbative region, we nevertheless have to rely on a disparate range of models and approximations. If we are to gain an understanding of the underlying physics and not just have numerical answers from computing `` black'' boxes, we must build bridges between the parameter space where models and approximations are valid to the regime describing experiment, and between the different modellings of strong dynamics. We describe here how the Schwinger-Dyson/Bethe-Salpeter approach provides just such a bridge, linking physics, the lattice and experiment.Comment: 8 pages, 10 figures. Opening talk at Workshop on QCD Down Under, March 2004, Barossa Valley and Adelaide (to be published in the Proceedings

Juvenile root traits show limited correlation with grain yield, yield components and grain mineral composition traits in Indian wheat under hostile soils

Correlations between juvenile wheat root traits, and grain yield and yield component traits under optimal field conditions have previously been reported in some conditions. The aim of this study was to test the hypothesis that juvenile wheat root traits correlate with yield, yield components and grain mineral composition traits under a range of soil environments in India. A diverse panel of 36 Indian wheat genotypes were grown for ten days in ‘pouch and wick’ high-throughput phenotyping (HTP) system (20 replicates). Correlations between juvenile root architecture traits, including primary and lateral root length, and grain yield, yield components and grain mineral composition traits were determined, using field data from previously published experiments at six sites in India. Only a limited number of juvenile root traits correlated with grain yield (GYD), yield components, and grain mineral composition traits. A narrow root angle, potentially representing a ‘steep’ phenotype, was associated with increased GYD and harvest index (HI) averaged across sites and years. Length related root traits were not correlated with GYD or HI at most sites, however, the total length of lateral roots and lateral root number correlated with GYD at a sodic site of pH 9.5. The total length of lateral roots (TLLR) correlated with grain zinc (Zn) concentration at one site. A wider root angle, representing a shallow root system, correlated with grain iron (Fe) concentration at most sites. The total length of all roots (TLAR) and total length of primary roots (TLPR) correlated with grain S concentration at most sites. Narrow root angle in juvenile plants could be a useful proxy trait for screening germplasm for improved grain yield. Lateral root and shallow root traits could potentially be used to improve grain mineral concentrations. The use of juvenile root traits should be explored further in wheat breeding for diverse environments

Fluid evolution in CM carbonaceous chondrites tracked through the oxygen isotopic compositions of carbonates

The oxygen isotopic compositions of calcite grains in four CM carbonaceous chondrites have been determined by NanoSIMS, and results reveal that aqueous solutions evolved in a similar manner between parent body regions with different intensities of aqueous alteration. Two types of calcite were identified in Murchison, Mighei, Cold Bokkeveld and LaPaz Icefield 031166 by differences in their petrographic properties and oxygen isotope values. Type 1 calcite occurs as small equant grains that formed by filling of pore spaces in meteorite matrices during the earliest stages of alteration. On average, the type 1 grains have a δ18O of ∼32–36‰ (VSMOW), and Δ17O of between ∼2‰ and −1‰. Most grains of type 2 calcite precipitated after type 1. They contain micropores and inclusions, and have replaced ferromagnesian silicate minerals. Type 2 calcite has an average δ18O of ∼21–24‰ (VSMOW) and a Δ17O of between ∼−1‰ and −3‰. Such consistent isotopic differences between the two calcite types show that they formed in discrete episodes and from solutions whose δ18O and δ17O values had changed by reaction with parent body silicates, as predicted by the closed-system model for aqueous alteration. Temperatures are likely to have increased over the timespan of calcite precipitation, possibly owing to exothermic serpentinisation. The most highly altered CM chondrites commonly contain dolomite in addition to calcite. Dolomite grains in two previously studied CM chondrites have a narrow range in δ18O (∼25–29‰ VSMOW), with Δ17O ∼−1‰ to −3‰. These grains are likely to have precipitated between types 1 and 2 calcite, and in response to a transient heating event and/or a brief increase in fluid magnesium/calcium ratios. In spite of this evidence for localised excursions in temperature and/or solution chemistry, the carbonate oxygen isotope record shows that fluid evolution was comparable between many parent body regions. The CM carbonaceous chondrites studied here therefore sample either several parent bodies with a very similar initial composition and evolution or, more probably, a single C-type asteroid

Anomalous NMR Spin-Lattice Relaxation in SrB_{6} and Ca_{1-x}La_{x}B_{6}

We report the results of {11}B nuclear magnetic resonance (NMR) measurements of SrB_{6} and Ca_{0.995}La_{0.05}B_{6} below room temperature. Although the electrical resistivities of these two materials differ substantially, their {11}B-NMR responses exhibit some strikingly common features. Both materials exhibit ferromagnetic order, but their {11}B-NMR spectra reveal very small hyperfine fields at the Boron sites. The spin lattice relaxation T_{1}^{-1} varies considerably with external field but changes with temperature only below a few K. We discuss these unusual results by considering various different scenarios for the electronic structure of these materials.Comment: Accepted for publication in Phys. Rev. B Rapid communication, 4 pages, 3 figures. This manuscript replaces an earlier version and includes some minor changes in the text and in Fig.

Iodine source apportionment in the Malawian diet

The aim of this study was to characterise nutritional-I status in Malawi. Dietary-I intakes were assessed using new datasets of crop, fish, salt and water-I concentrations, while I status was assessed for 60 women living on each of calcareous and non-calcareous soils as defined by urinary iodine concentration (UIC). Iodine concentration in staple foods was low, with median concentrations of 0.01 mg kg−1 in maize grain, 0.008 mg kg−1 in roots and tubers, but 0.155 mg kg−1 in leafy vegetables. Freshwater fish is a good source of dietary-I with a median concentration of 0.51 mg kg−1. Mean Malawian dietary-Iodine intake from food, excluding salt, was just 7.8 μg d−1 compared to an adult requirement of 150 μg d−1. Despite low dietary-I intake from food, median UICs were 203 μg L−1 with only 12% defined as I deficient whilst 21% exhibited excessive I intake. Iodised salt is likely to be the main source of dietary I intake in Malawi; thus, I nutrition mainly depends on the usage and concentration of I in iodised salt. Drinking water could be a significant source of I in some areas, providing up to 108 μg d−1 based on consumption of 2 L d−1

Incremental Medians via Online Bidding

In the k-median problem we are given sets of facilities and customers, and distances between them. For a given set F of facilities, the cost of serving a customer u is the minimum distance between u and a facility in F. The goal is to find a set F of k facilities that minimizes the sum, over all customers, of their service costs. Following Mettu and Plaxton, we study the incremental medians problem, where k is not known in advance, and the algorithm produces a nested sequence of facility sets where the kth set has size k. The algorithm is c-cost-competitive if the cost of each set is at most c times the cost of the optimum set of size k. We give improved incremental algorithms for the metric version: an 8-cost-competitive deterministic algorithm, a 2e ~ 5.44-cost-competitive randomized algorithm, a (24+epsilon)-cost-competitive, poly-time deterministic algorithm, and a (6e+epsilon ~ .31)-cost-competitive, poly-time randomized algorithm. The algorithm is s-size-competitive if the cost of the kth set is at most the minimum cost of any set of size k, and has size at most s k. The optimal size-competitive ratios for this problem are 4 (deterministic) and e (randomized). We present the first poly-time O(log m)-size-approximation algorithm for the offline problem and first poly-time O(log m)-size-competitive algorithm for the incremental problem. Our proofs reduce incremental medians to the following online bidding problem: faced with an unknown threshold T, an algorithm submits "bids" until it submits a bid that is at least the threshold. It pays the sum of all its bids. We prove that folklore algorithms for online bidding are optimally competitive.Comment: conference version appeared in LATIN 2006 as "Oblivious Medians via Online Bidding

Multipliers for p-Bessel sequences in Banach spaces

Multipliers have been recently introduced as operators for Bessel sequences and frames in Hilbert spaces. These operators are defined by a fixed multiplication pattern (the symbol) which is inserted between the analysis and synthesis operators. In this paper, we will generalize the concept of Bessel multipliers for p-Bessel and p-Riesz sequences in Banach spaces. It will be shown that bounded symbols lead to bounded operators. Symbols converging to zero induce compact operators. Furthermore, we will give sufficient conditions for multipliers to be nuclear operators. Finally, we will show the continuous dependency of the multipliers on their parameters.Comment: 17 page