Effect of stoichiometry on oxygen incorporation in MgB2 thin films

The amount of oxygen incorporated into MgB2 thin films upon exposure to atmospheric gasses is found to depend strongly on the material's stoichiometry. Rutherford backscattering spectroscopy was used to monitor changes in oxygen incorporation resulting from exposure to: (a) ambient atmosphere, (b) humid atmospheres, (c) anneals in air and (d) anneals in oxygen. The study investigated thin-film samples with compositions that were systematically varied from Mg0.9B2 to Mg1.1B2. A significant surface oxygen contamination was observed in all of these films. The oxygen content in the bulk of the film, on the other hand, increased significantly only in Mg rich films and in films exposed to humid atmospheres.Comment: 10 pages, 6 figures, 1 tabl

Swelling of acetylated wood in organic liquids

To investigate the affinity of acetylated wood for organic liquids, Yezo spruce wood specimens were acetylated with acetic anhydride, and their swelling in various liquids were compared to those of untreated specimens. The acetylated wood was rapidly and remarkably swollen in aprotic organic liquids such as benzene and toluene in which the untreated wood was swollen only slightly and/or very slowly. On the other hand, the swelling of wood in water, ethylene glycol and alcohols remained unchanged or decreased by the acetylation. Consequently the maximum volume of wood swollen in organic liquids was always larger than that in water. The effect of acetylation on the maximum swollen volume of wood was greater in liquids having smaller solubility parameters. The easier penetration of aprotic organic liquids into the acetylated wood was considered to be due to the scission of hydrogen bonds among the amorphous wood constituents by the substitution of hydroxyl groups with hydrophobic acetyl groups.Comment: to be published in J Wood Science (Japanese wood research society

MgB2 tunnel junctions with native or thermal oxide barriers

MgB2 tunnel junctions (MgB2/barrier/MgB2) were fabricated using a native oxide grown on the bottom MgB2 film as the tunnel barrier. Such barriers therefore survive the deposition of the second electrode at 300oC, even over junction areas of ~1 mm2. Studies of such junctions, and those of the type MgB2/native or thermal oxide/metal (Pb, Au, or Ag) show that tunnel barriers grown on MgB2 exhibit a wide range of barrier heights and widths.Comment: 9 pages, 3 figure

Zurek-Kibble Mechanism for the Spontaneous Vortex Formation in Nb−Al/Alox/NbNb-Al/Al_{ox}/Nb Josephson Tunnel Junctions: New Theory and Experiment

New scaling behavior has been both predicted and observed in the spontaneous production of fluxons in quenched $Nb-Al/Al_{ox}/Nb$ annular Josephson tunnel junctions as a function of the quench time, $\tau_{Q}$. The probability $f_{1}$ to trap a single defect during the N-S phase transition clearly follows an allometric dependence on $\tau_{Q}$ with a scaling exponent $\sigma = 0.5$, as predicted from the Zurek-Kibble mechanism for {\it realistic} JTJs formed by strongly coupled superconductors. This definitive experiment replaces one reported by us earlier, in which an idealised model was used that predicted $\sigma = 0.25$, commensurate with the then much poorer data. Our experiment remains the only condensed matter experiment to date to have measured a scaling exponent with any reliability.Comment: Four pages, one figur

Role of Quantum Confinement in Luminescence Efficiency of Group IV Nanostructures

Experimental results obtained previously for the photoluminescence efficiency (PL$_{eff}$) of Ge quantum dots (QDs) are theoretically studied. A $\log$-$\log$ plot of PL$_{eff}$ versus QD diameter ($D$) resulted in an identical slope for each Ge QD sample only when $E_{G}\sim (D^2+D)^{-1}$. We identified that above $D\approx$ 6.2 nm: $E_{G}\sim D^{-1}$ due to a changing effective mass (EM), while below $D\approx$ 4.6 nm: $E_{G}\sim D^{-2}$ due to electron/ hole confinement. We propose that as the QD size is initially reduced, the EM is reduced, which increases the Bohr radius and interface scattering until eventually pure quantum confinement effects dominate at small $D$

Timing analysis techniques at large core distances for multi-TeV gamma ray astronomy

We present an analysis technique that uses the timing information of Cherenkov images from extensive air showers (EAS). Our emphasis is on distant, or large core distance gamma-ray induced showers at multi-TeV energies. Specifically, combining pixel timing information with an improved direction reconstruction algorithm, leads to improvements in angular and core resolution as large as ~40% and ~30%, respectively, when compared with the same algorithm without the use of timing. Above 10 TeV, this results in an angular resolution approaching 0.05 degrees, together with a core resolution better than ~15 m. The off-axis post-cut gamma-ray acceptance is energy dependent and its full width at half maximum ranges from 4 degrees to 8 degrees. For shower directions that are up to ~6 degrees off-axis, the angular resolution achieved by using timing information is comparable, around 100 TeV, to the on-axis angular resolution. The telescope specifications and layout we describe here are geared towards energies above 10 TeV. However, the methods can in principle be applied to other energies, given suitable telescope parameters. The 5-telescope cell investigated in this study could initially pave the way for a larger array of sparsely spaced telescopes in an effort to push the collection area to >10 km2. These results highlight the potential of a `sparse array' approach in effectively opening up the energy range above 10 TeV.Comment: Published in Astroparticle Physic

Skilful prediction of Sahel summer rainfall on inter-annual and multi-year timescales

This is the final version of the article. Available from Springer Nature via the DOI in this record.Summer rainfall in the Sahel region of Africa exhibits one of the largest signals of climatic variability and with a population reliant on agricultural productivity, the Sahel is particularly vulnerable to major droughts such as occurred in the 1970s and 1980s. Rainfall levels have subsequently recovered, but future projections remain uncertain. Here we show that Sahel rainfall is skilfully predicted on inter-annual and multi-year (that is, >5 years) timescales and use these predictions to better understand the driving mechanisms. Moisture budget analysis indicates that on multi-year timescales, a warmer north Atlantic and Mediterranean enhance Sahel rainfall through increased meridional convergence of low-level, externally sourced moisture. In contrast, year-to-year rainfall levels are largely determined by the recycling rate of local moisture, regulated by planetary circulation patterns associated with the El Niño-Southern Oscillation. Our findings aid improved understanding and forecasting of Sahel drought, paramount for successful adaptation strategies in a changing climate.This work was supported by the Joint DECC/Defra Met Office Hadley Centre Climate Programme (GA01101) and the EU FP7 SPECS project. The contribution of D.P.R. has received funding from the NERC/DFID Future Climate for Africa programme under the AMMA-2050 project, grant number NE/M019977/1

Simulation Study of TenTen: A new Multi-TeV IACT array

TenTen is a proposed array of Imaging Atmospheric Cherenkov Telescopes (IACT) optimized for the gamma ray energy regime of 10 TeV to 100 TeV, but with a threshold of ~1 to a few TeV. It will offer a collecting area of 10 km2 above energies of 10 TeV. In the initial phase, a cell of 3 to 5 modest-sized telescopes, each with 10-30 m2 mirror area, is suggested for an Australian site. A possible expansion of the array could comprise many such cells. Here we present work on configuration and technical issues from our simulation studies of the array. Working topics include array layout, telescope size and optics, camera field of view, telescope trigger system, electronics, and site surveys.Comment: 4 pages, 7 figures, submitted to Proceedings of the ICRC 2007, pdf forma