1,019 research outputs found
Ballistic electron motion in a random magnetic field
Using a new scheme of the derivation of the non-linear -model we
consider the electron motion in a random magnetic field (RMF) in two
dimensions. The derivation is based on writing quasiclassical equations and
representing their solutions in terms of a functional integral over
supermatrices with the constraint . Contrary to the standard scheme,
neither singling out slow modes nor saddle-point approximation are used. The
-model obtained is applicable at the length scale down to the electron
wavelength. We show that this model differs from the model with a random
potential (RP).However, after averaging over fluctuations in the Lyapunov
region the standard -model is obtained leading to the conventional
localization behavior.Comment: 10 pages, no figures, to be submitted in PRB v2: Section IV is
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Acid pH activation of the PmrA/PmrB two-component regulatory system of Salmonella enterica
Acid pH often triggers changes in gene expression. However, little is known about the identity of the gene products that sense fluctuations in extracytoplasmic pH. The Gram-negative pathogen Salmonella enterica serovar Typhimurium experiences a number of acidic environments both inside and outside animal hosts. Growth in mild acid (pH 5.8) promotes transcription of genes activated by the response regulator PmrA, but the signalling pathway(s) that mediates this response has thus far remained unexplored. Here we report that this activation requires both PmrA's cognate sensor kinase PmrB, which had been previously shown to respond to Fe(3+) and Al(3+), and PmrA's post-translational activator PmrD. Substitution of a conserved histidine or of either one of four conserved glutamic acid residues in the periplasmic domain of PmrB severely decreased or abolished the mild acid-promoted transcription of PmrA-activated genes. The PmrA/PmrB system controls lipopolysaccharide modifications mediating resistance to the antibiotic polymyxin B. Wild-type Salmonella grown at pH 5.8 were > 100 000-fold more resistant to polymyxin B than organisms grown at pH 7.7. Our results suggest that protonation of the PmrB periplasmic histidine and/or of the glutamic acid residues activate the PmrA protein, and that mild acid promotes cellular changes resulting in polymyxin B resistance
High strain-rate material model validation for laser peening simulation
Finite element modeling can be a powerful tool for predicting residual stresses induced by laser peening; however the sign and magnitude of the stress predictions depend strongly on how the material model captures the high strain rate response. Although a Johnson-Cook formulation is often employed, its suitability for modeling phenomena at very high strain rates has not been rigorously evaluated. In this paper, we address the effectiveness of the Johnson-Cook model, with parameters developed from lower strain rate material data (∼10^3 s^–1), to capture the higher strain rate response (∼10^5–10^6 s^–1) encountered during the laser peening process. Published Johnson-Cook parameters extracted from split Hopkinson bar testing were used to predict the shock response of aluminum samples during high-impact flyer plate tests. Additional quasi-static and split Hopkinson bar tests were also conducted to study the model response in the lower strain rate regime. The overall objective of the research was to ascertain whether a material model based on conventional test data (quasi-static compression testing and split Hopkinson bar measurements) can credibly be used in FE simulations to predict laser peen-induced stresses
A Quantitative Model of Energy Release and Heating by Time-dependent, Localized Reconnection in a Flare with a Thermal Loop-top X-ray Source
We present a quantitative model of the magnetic energy stored and then
released through magnetic reconnection for a flare on 26 Feb 2004. This flare,
well observed by RHESSI and TRACE, shows evidence of non-thermal electrons only
for a brief, early phase. Throughout the main period of energy release there is
a super-hot (T>30 MK) plasma emitting thermal bremsstrahlung atop the flare
loops. Our model describes the heating and compression of such a source by
localized, transient magnetic reconnection. It is a three-dimensional
generalization of the Petschek model whereby Alfven-speed retraction following
reconnection drives supersonic inflows parallel to the field lines, which form
shocks heating, compressing, and confining a loop-top plasma plug. The
confining inflows provide longer life than a freely-expanding or
conductively-cooling plasma of similar size and temperature. Superposition of
successive transient episodes of localized reconnection across a current sheet
produces an apparently persistent, localized source of high-temperature
emission. The temperature of the source decreases smoothly on a time scale
consistent with observations, far longer than the cooling time of a single
plug. Built from a disordered collection of small plugs, the source need not
have the coherent jet-like structure predicted by steady-state reconnection
models. This new model predicts temperatures and emission measure consistent
with the observations of 26 Feb 2004. Furthermore, the total energy released by
the flare is found to be roughly consistent with that predicted by the model.
Only a small fraction of the energy released appears in the super-hot source at
any one time, but roughly a quarter of the flare energy is thermalized by the
reconnection shocks over the course of the flare. All energy is presumed to
ultimately appear in the lower-temperature T<20 MK, post-flare loops
Electromagnetic and Hadron Calorimeters in the MIPP Experiment
The purpose of the MIPP experiment is to study the inclusive production of
photons, pions, kaons and nucleons in pi, K and p interactions on various
targets using beams from the Main Injector at Fermilab. The function of the
calorimeters is to measure the production of forward-going neutrons and
photons. The electromagnetic calorimeter consist of 10 lead plates interspersed
with proportional chambers. It was followed by the hadron calorimeter with 64
steel plates interspersed with scintillator. The data presented were collected
with a variety of targets and beam momenta from 5 GeV/c to 120 GeV/c. The
energy calibration of both calorimeters with electrons, pions, kaons, and
protons is discussed. The resolution for electrons was found to be
0.27/sqrt(E), and for hadrons the resolution was 0.554/sqrt(E) with a constant
term of 2.6%. The performance of the calorimeters was tested on a neutron
sample
Teaching word recognition to children with severe learning difficulties: an exploratory comparison of teaching methods
Background: Some children with severe learning difficulties fail to begin word recognition. For these children there is a need for an effective and appropriate pedagogy. However, conflicting advice can be found regarding this derived from teaching approaches that are not based on a shared understanding of how reading develops or the skills that the non-reader needs to master.
Purpose: In this research, three techniques for teaching word recognition in this context are described and compared: (1) the handle technique, (2) morphing method and (3) word alone. It also discusses whether it is appropriate for such small-scale research to influence pedagogy.
Programme description: The handle technique uses an abstract mnemonic cue used to teach word recognition, and previous research indicates it is more successful than the presentation of words alone. The morphing method transforms a word into a photographic picture and a previous study suggested that it might also be more effective that presenting words alone.
Sample: Six children between 11 and 13 years of age were selected. The criterion for selection was being unable to recognise any words from the British Ability Scales Reading Test. All the children attended a school for children with severe learning difficulties.
Design and methods: A three-condition related design was used. The order in which the conditions were presented was counterbalanced and each child was taught 12 words, four words in each experimental condition. The children encountered each of the three methods and overall each word was taught via each method. Within conditions (teaching methods), the presentation of words was randomised. The number of words that the children could read (without cues) before each session was recorded, following the presentation of the uncued words in a random order. The difference in the number of words recognised between the three conditions was considered using a non-parametric statistical analysis.
Results: The results suggest that the handle approach might be a more effective method of teaching word recognition.
Conclusion: Research in this area is necessarily small in scale. However, it is ongoing and cumulative, and can give insights into potentially beneficial changes in classroom practice
A review of Monte Carlo simulations of polymers with PERM
In this review, we describe applications of the pruned-enriched Rosenbluth
method (PERM), a sequential Monte Carlo algorithm with resampling, to various
problems in polymer physics. PERM produces samples according to any given
prescribed weight distribution, by growing configurations step by step with
controlled bias, and correcting "bad" configurations by "population control".
The latter is implemented, in contrast to other population based algorithms
like e.g. genetic algorithms, by depth-first recursion which avoids storing all
members of the population at the same time in computer memory. The problems we
discuss all concern single polymers (with one exception), but under various
conditions: Homopolymers in good solvents and at the point, semi-stiff
polymers, polymers in confining geometries, stretched polymers undergoing a
forced globule-linear transition, star polymers, bottle brushes, lattice
animals as a model for randomly branched polymers, DNA melting, and finally --
as the only system at low temperatures, lattice heteropolymers as simple models
for protein folding. PERM is for some of these problems the method of choice,
but it can also fail. We discuss how to recognize when a result is reliable,
and we discuss also some types of bias that can be crucial in guiding the
growth into the right directions.Comment: 29 pages, 26 figures, to be published in J. Stat. Phys. (2011
Ethnicity and consumption: South Asian food shopping patterns in Britain 1947-75
Authors' draft version also available on University of Surrey e-print repository. Final version published by Sage and available at http://joc.sagepub.com/This article reviews the literature that explores the relationship between ethnic
identities and food consumption, with particular reference to business management
studies. It focuses on the food shopping practices of south Asians in Britain in the
period 1947 to 1975, to illustrate the need for more historically contextualized studies
that can provide a more nuanced exploration of any interconnections between ethnic
identity and shopping behaviour. The article draws on a reasonably long-standing
interest in ethnicity and consumption in marketing studies, and explores the
conceptual use of acculturation within this literature. The arguments put forward are
framed by recent interdisciplinary studies of the broader relationship between
consumption and identity, which stress the importance of contextualizing any
influence of ethnic identifications through a wider consideration of other factors
including societal status, gender and age, rather than giving it singular treatment. The
article uses a body of empirical research drawn from recent oral histories, to explore
how these factors informed everyday shopping practices among south Asians in Britain. It examines some of the shopping and wider food provisioning strategies
adopted by early immigrants on arrival in Britain. It considers the interaction between
the south Asian population and the changing retail structure, in the context of the
development of self-service and the supermarket. Finally, it demonstrates how age,
gender and socioeconomic status interacted with ethnic identities to produce
variations in shopping patterns
Changes in Herdsmen's Pastoral Behaviour Triggered by Rangeland Degradation in the Source Region of the Yellow River, Qinghai-Tibetan Plateau
An Integrated TCGA Pan-Cancer Clinical Data Resource to Drive High-Quality Survival Outcome Analytics
For a decade, The Cancer Genome Atlas (TCGA) program collected clinicopathologic annotation data along with multi-platform molecular profiles of more than 11,000 human tumors across 33 different cancer types. TCGA clinical data contain key features representing the democratized nature of the data collection process. To ensure proper use of this large clinical dataset associated with genomic features, we developed a standardized dataset named the TCGA Pan-Cancer Clinical Data Resource (TCGA-CDR), which includes four major clinical outcome endpoints. In addition to detailing major challenges and statistical limitations encountered during the effort of integrating the acquired clinical data, we present a summary that includes endpoint usage recommendations for each cancer type. These TCGA-CDR findings appear to be consistent with cancer genomics studies independent of the TCGA effort and provide opportunities for investigating cancer biology using clinical correlates at an unprecedented scale. Analysis of clinicopathologic annotations for over 11,000 cancer patients in the TCGA program leads to the generation of TCGA Clinical Data Resource, which provides recommendations of clinical outcome endpoint usage for 33 cancer types
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