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Now showing 1 - 10 of 6331
  • (2009) Lim, Wee Han; Huebl, Hans; Willems van Beveren, Laurens; Rubanov, Sergey; Spizzirri, Paul; Angus, Susan; Clark, Robert; Dzurak, Andrew
    Journal Article
    A few-electron double quantum dot was fabricated using metal-oxide-semiconductor-compatible technology and low-temperature transport measurements were performed to study the energy spectrum of the device. The double dot structure is electrically tunable, enabling the inter-dot coupling to be adjusted over a wide range, as observed in the charge stability diagram. Resonant single-electron tunneling through ground and excited states of the double dot was clearly observed in bias spectroscopy measurements.

  • (2007) Warton, David
    Working Paper
    The Wald statistic is known to vary under reparameterization. This raises the question: which parameterization should be chosen, in order to optimize power of the Wald statistic? We specifically consider k-sample tests of generalized linear models and generalized estimating equations in which the alternative hypothesis contains only two parameters. Amongst a general class of parameterizations, we find the parameterization that maximizes power via analysis of the non-centrality parameter, and show how the effect on power of reparameterization depends on sampling design and the differences in variance across samples. There is no single parameterization with optimal power across all alternatives. The Wald statistic commonly used, that under the canonical parameterization, is optimal in some instances but it performs very poorly in others. We demonstrate results by example and by simulation, and describe their implications for likelihood ratio statistics and score statistics. We conclude that due to poor power properties, the routine use of score statistics and Wald statistics under the canonical parameterization for generalized estimating equations is a questionable practice.

  • (2009) Highfield, Linda D; Ward, Michael P; Laffan, Shawn; Norby, Bo; Wagner, Gale G
    Journal Article
    Modeling potential disease spread in wildlife populations is important for predicting, responding to and recovering from a foreign animal disease incursion such as foot and mouth disease (FMD). We conducted a series of simulation experiments to determine how seasonal estimates of the spatial distribution of white-tailed deer impact the predicted magnitude and distribution of potential FMD outbreaks. Outbreaks were simulated in a study area comprising 2 distinct ecoregions in south Texas, USA, using a susceptible-latent-infectious-resistant geographic automata model (Sirca). Seasonal deer distributions were estimated by spatial autoregressive lag models and the normalized difference vegetative index. Significant (P < 0.0001) differences in both the median predicted number of deer infected and number of herds infected were found both between seasons and between ecoregions. Larger outbreaks occurred in winter within the higher deer-density ecoregion, whereas larger outbreaks occurred in summer and fall within the lower deer-density ecoregion. Results of this simulation study suggest that the outcome of an FMD incursion in a population of wildlife would depend on the density of the population infected and when during the year the incursion occurs. It is likely that such effects would be seen for FMD incursions in other regions and countries, and for other diseases, in cases in which a potential wildlife reservoir exists. Study findings indicate that the design of a mitigation strategy needs to take into account population and seasonal characteristics.

  • (2009) Wang, Daming; Laffan, Shawn; Yu, Liu; Wu, Lun
    Journal Article
    We describe a method of morphometric characterisation of landform from DEMs. The method is implemented by first classifying every location into morphometric classes based on the mathematical shape of a locally fitted quadratic surface and its positional relationship with the analysis window. Single-scale fuzzy terrain indices of peakness, pitness, passness, ridgeness, and valleyness are then calculated based on the distance of the analysis location from the ideal cases. These can then be combined into multi-scale terrain indices to summarise terrain information across different operational scales. The algorithm has four characteristics: (1) the ideal cases of different geomorphometric features are simply and clearly defined; (2) the output is spatially continuous to reflect the inherent fuzziness of geomorphometric features; (3) the output is easily combined into a multi-scale index across a range of operational scales; and (4) the standard general morphometric parameters are quantified as the first and second order derivatives of the quadratic surface. An additional benefit of the quadratic surface is the derivation of the R2 goodness of fit statistic, which allows an assessment of both the reliability of the results and the complexity of the terrain. An application of the method using a test DEM indicates that the single- and multi-scale terrain indices perform well when characterising the different geomorphometric features.

  • (2009) Huang, Zhi; Laffan, Shawn
    Journal Article
    We analysed the sensitivity of a decision tree derived forest type mapping to simulated data errors in input DEM, geology and remotely sensed (Landsat Thematic Mapper) variables. We used a stochastic Monte Carlo simulation model coupled with a one-at-a-time approach. The DEM error was assumed to be spatially autocorrelated with its magnitude being a percentage of the elevation value. The error of categorical geology data was assumed to be positional and limited to boundary areas. The Landsat data error was assumed to be spatially random and follow a Gaussian distribution. Each layer was perturbed using its error model with increasing levels of error, and the effect on the forest type mapping was assessed. The results of the three sensitivity analyses were markedly different, with the classification being most sensitive to the DEM error, then to the Landsat data errors, but with only a limited sensitivity to the geology data error used. A linear increase in error resulted in non-linear increases in effect for the DEM and Landsat errors, while it was linear for geology. As an example, a DEM error of as small as ±2% reduced the overall test accuracy by more than 2%. More importantly, the same uncertainty level has caused nearly 10% of the study area to change its initial class assignment at each perturbation, on average. A spatial assessment of the sensitivities indicates that most of the pixel changes occurred within those forest classes expected to be more sensitive to data error. In addition to characterising the effect of errors on forest type mapping using decision trees, this study has demonstrated the generality of employing Monte Carlo analysis for the sensitivity and uncertainty analysis of categorical outputs which have distinctive characteristics from that of numerical outputs.

  • (2006) Gooding, John; Hibbert, D. Brynn; Zhao, Min
    Journal Article
    A simple method is described to determine sulfite in beer samples using a fill and flow channel biosensor. A droplet of sample is placed into the inlet of a rectangular flow cell and begins to flow through the channel by capillarity. The flow is maintained and controlled by a porous outlet plug of defined porosity. In a rectangular flow cell, the sample solution flows through three consecutive zones: over a predictor electrode, an enzyme layer and a detector electrode. Together these three zones enable the differentiation between current due to sulfite and current due to other electroactive species in the sample. The predictor electrode is located upstream, and on the opposite channel wall to the enzyme layer and detector electrode, and is poised at the same potential (+0.65 V versus Ag/AgCl) as the detector electrode. On this electrode, the current contribution from all species in the sample solution that are oxidized at that potential is determined. The enzyme layer contains sulfite oxidase, which, in the process of oxidizing sulfite, produces hydrogen peroxide, which itself is reduced by excess sulfite. The current at the downstream detector electrode is therefore different from that at the predictor electrode as a result of the enzyme reaction and the difference of the currents, corrected for the dimensions of the electrodes, is proportional to the concentration of sulfite. The method enables a straightforward correction of the interfering current at the detector electrode and a determination of the analyte concentration. The effect of interferences from ascorbic acid, ethanol, sorbic acid and tartaric acid in the detection of sulfite is efficiently removed. The concentration of sulfite in a sample of beer measured by the biosensor is equivalent to that measured using a reference method based on the AOAC-recommended Monier-Williams method. © 2005 Elsevier B.V. All rights reserved.

  • (2006) Heness, Gregory; Tam, W.; Bandyopadhyay, Srikanta
    Conference Paper
    A binary system of nano-reinforcement was used to increase flexural strength, flexural modulus and impact strength of a bisphenol-A type epoxy resin. Montmorillonite and alumina particles were used as modifiers. Each modifier, by itself, was found to increase these properties and further the use of both was found to have a synergistic effect.

  • (2006) Maercker, M; Burton, Michael
    Journal Article
    L-band data of 30 Doradus at 3.5 mu m taken with SPIREX (South Pole Infrared Explorer) is presented. The photometry was combined with 2MASS JHK data at 1.25- 2.2 mu m. Colour-colour and colour-magnitude diagrams are constructed and used to determine the sources with infrared excess. These are interpreted as circumstellar disks, and enable the fraction of sources with disks (the cluster disk fraction or CDF) to be determined. We find that similar to 42% of the sources detected at L-band in 30 Doradus have an IR-excess.

  • (2005) Burton, Michael; Minier, Vincent; Hill, Tracey; Purcell, Cormac; Walsh, Andrew; Longmore, Steven; Pestalozzi, M; Garay, Guido
    Journal Article
    We present a multiwavelength study of five methanol maser sites which are not directly associated with a strong (>100 mJy) radio continuum source: G31.28+0.06, G59.78+0.06, G173.49+2.42 (S231, S233IR), G188.95+0.89 (S252, AFGL5180) and G192.60-0.05 (S255IR). These radio-quiet methanol maser sites are often interpreted as precursors of ultracompact H regions or massive protostar sites. In this work, the environment of methanol masers is probed from mid-IR to millimetre wavelengths at angular resolutions of 8−34. Spectral energy distribution (SED) diagrams for each site are presented, together with mass and luminosity estimates. Each radio-quiet maser site is always associated with a massive (>50 M), deeply embedded (Av > 40 mag) and very luminous (>104 L) molecular clump, with Ltotal ∝ M0.75 gas . These physical properties characterise massive star-forming clumps in earlier evolutionary phases than H regions. In addition, colder gas clumps seen only at mm-wavelengths are also found near the methanol maser sites. These colder clumps may represent an even earlier phase of massive star formation. These results suggest an evolutionary sequence for massive star formation from a cold clump, seen only at mm wavelengths, evolving to a hot molecular core with a two-component SED with peaks at far-IR and mid-IR wavelengths, to an (ultra-compact) H region. Alternatively, the cold clumps might be clusters of low-mass YSOs, in formation near the massive star-forming clusters. Finally, the values of the dust grain emissivity index (β) range between 1.6 and 1.9.

  • (2007) Wright, Christopher; Burton, Michael; Van Dishoeck, Ewine; van Langevelde, Huib-Jan; Wilner, David; Hughes, Annie; Lommen, Dave; Maddison, Sarah; Jorgensen, Jes; Bourke, Tyler
    Journal Article
    Context. Low-mass stars form with disks in which the coagulation of grains may eventually lead to the formation of planets. It is not known when and where grain growth occurs, as models that explain the observations are often degenerate. A way to break this degeneracy is to resolve the sources under study. Aims. Our aim is to find evidence for the existence of grains of millimetre sizes in disks around T Tauri stars, implying grain growth. Methods. The Australia Telescope Compact Array (ATCA) was used to observe 15 southern T Tauri stars, five in the constellation Lupus and ten in Chamaeleon, at 3.3 mm. The five Lupus sources were also observed with the SubMillimeter Array (SMA) at 1.4 mm. Our new data are complemented with data from the literature to determine the slopes of the spectral energy distributions in the millimetre regime. Results. Ten sources were detected at better than 3 sigma with the ATCA, with sigma approximate to 1-2 mJy, and all sources that were observed with the SMA were detected at better than 15 sigma, with sigma approximate to 4 mJy. Six of the sources in our sample are resolved to physical radii of similar to 100 AU. Assuming that the emission from such large disks is predominantly optically thin, the millimetre slope can be related directly to the opacity index. For the other sources, the opacity indices are lower limits. Four out of six resolved sources have opacity indices <= 1, indicating grain growth to millimetre sizes and larger. The masses of the disks range from < 0.01 to 0.08 M-circle dot, which is comparable to the minimum mass solar nebula. A tentative correlation is found between the millimetre slope and the strength and shape of the 10-mu m silicate feature, indicating that grain growth occurs on similar (short) timescales in both the inner and outer disk.