599 research outputs found
Dielectric response of modified Hubbard models with neutral-ionic and Peierls transitions
The dipole P(F) of systems with periodic boundary conditions (PBC) in a
static electric field F is applied to one-dimensional Peierls-Hubbard models
for organic charge-transfer (CT) salts. Exact results for P(F) are obtained for
finite systems of N = 14 and 16 sites that are almost converged to infinite
chains in deformable lattices subject to a Peierls transition. The electronic
polarizability per site, \alpha_{el} = (\partial P/\partial F)_0, of rigid
stacks with alternating transfer integrals t(1 +/- \delta) diverges at the
neutral-ionic transition for \delta = 0 but remains finite for \delta > 0 in
dimerized chains. The Peierls or dimerization mode couples to charge
fluctuations along the stack and results in large vibrational contributions,
\alpha_{vib}, that are related to \partial P/\partial \delta and that peak
sharply at the Peierls transition. The extension of P(F) to correlated
electronic states yields the dielectric response \kappa of models with
neutral-ionic or Peierls transitions, where \kappa peaks >100 are found with
parameters used previously for variable ionicity \rho and vibrational spectra
of CT salts. The calculated \kappa accounts for the dielectric response of CT
salts based on substituted TTFs (tetrathiafulvalene) and substituted CAs
(chloranil). The role of lattice stiffness appears clearly in models: soft
systems have a Peierls instability at small \rho and continuous crossover to
large \rho, while stiff stacks such as TTF-CA have a first-order transition
with discontinuous \rho that is both a neutral-ionic and Peierls transition.
The transitions are associated with tuning the electronic ground state of
insulators via temperature or pressure in experiments, or via model parameters
in calculations.Comment: 10 pages, 9 figures; J.Chem.Phys., in pres
The Influence of Formulation, Buffering, pH and Divalent Cations on the Activity of Endothall on Hydrilla.
Endothall has been used as an aquatic herbicide for more
than 40 years and provides very effective weed control of
many weeds. Early research regarding the mechanism-of-action
of endothall contradicts the symptomology normally associated
with the product. Recent studies suggest endothall
is a respiratory toxin but the mechanism-of-action remains
unknown. To further elucidate the activity of endothall, several
endothall formulations were evaluated for their effects
on ion leakage, oxygen consumption and photosynthetic oxygen
evolution from hydrilla shoot tips. The influence of pH,
buffering and divalent cations was also evaluated. (PDF contains 6 pages.
Charge fluctuations and electron-phonon coupling in organic charge-transfer salts with neutral-ionic and Peierls transitions
The first-order transition of the charge-transfer complex TTF-CA
(tetrathiafulvalene-chloranil) is both a neutral-ionic and a Peierls
transition. In related organic charge transfer complexes, cooling at ambient
pressure increases the ionicity in strikingly different ways, and is
sometimes accompanied by a dielectric peak, that we relate to lattice
stiffness, to structural and energetic disorder, and to the softening of the
Peierls mode in the far-IR. The position operator for systems with periodic
boundary conditions makes possible a systematic treatment of electron-phonon
interactions in extended donor-acceptor stacks in terms of correlated
Peierls-Hubbard models. The IR intensity of the Peierls mode peaks at the
Peierls transition at small in soft lattices, where the dielectric
constant also has a large peak. In dimerized stacks, the IR intensity of
totally symmetric, Raman active, molecular vibrations is related to charge
fluctuations that modulate site energies. Combination bands of molecular and
Peierls modes are identified in regular TTF-CA stacks above Tc. Energetic
disorder can suppress the Peierls transition and rationalize a continuous
crossover from small to large . The TTF-CA scenario of a neutral-regular
to ionic-dimerized transition must be broadened considerably in view of charge
transfer salts that dimerize on the neutral side, that become ionic without a
structural change, or that show vibrational evidence for dimerization at
constant .Comment: 26 pages including figure
A Study of the Scintillation Induced by Alpha Particles and Gamma Rays in Liquid Xenon in an Electric Field
Scintillation produced in liquid xenon by alpha particles and gamma rays has
been studied as a function of applied electric field. For back scattered gamma
rays with energy of about 200 keV, the number of scintillation photons was
found to decrease by 64+/-2% with increasing field strength. Consequently, the
pulse shape discrimination power between alpha particles and gamma rays is
found to reduce with increasing field, but remaining non-zero at higher fields.Comment: 15 pages, 12 figures, accepted by Nuclear Instruments and Methods in
Physics Research
WIMP-nucleon cross-section results from the second science run of ZEPLIN-III
We report experimental upper limits on WIMP-nucleon elastic scattering cross
sections from the second science run of ZEPLIN-III at the Boulby Underground
Laboratory. A raw fiducial exposure of 1,344 kg.days was accrued over 319 days
of continuous operation between June 2010 and May 2011. A total of eight events
was observed in the signal acceptance region in the nuclear recoil energy range
7-29 keV, which is compatible with background expectations. This allows the
exclusion of the scalar cross-section above 4.8E-8 pb near 50 GeV/c^2 WIMP mass
with 90% confidence. Combined with data from the first run, this result
improves to 3.9E-8 pb. The corresponding WIMP-neutron spin-dependent
cross-section limit is 8.0E-3 pb. The ZEPLIN programme reaches thus its
conclusion at Boulby, having deployed and exploited successfully three liquid
xenon experiments of increasing reach
An exploratory cluster randomised trial of a university halls of residence based social norms marketing campaign to reduce alcohol consumption among 1st year students
<p>Aims: This exploratory trial examines the feasibility of implementing a social norms marketing campaign to reduce student drinking in universities in Wales, and evaluating it using cluster randomised trial methodology.</p>
<p>Methods: Fifty residence halls in 4 universities in Wales were randomly assigned to intervention or control arms. Web and paper surveys were distributed to students within these halls (nâ=â3800), assessing exposure/contamination, recall of and evaluative responses to intervention messages, perceived drinking norms and personal drinking behaviour. Measures included the Drinking Norms Rating Form, the Daily Drinking Questionnaire and AUDIT-C.</p>
<p>Results: A response rate of 15% (nâ=â554) was achieved, varying substantially between sites. Intervention posters were seen by 80% and 43% of students in intervention and control halls respectively, with most remaining materials seen by a minority in both groups. Intervention messages were rated as credible and relevant by little more than half of students, though fewer felt they would influence their behaviour, with lighter drinkers more likely to perceive messages as credible. No differences in perceived norms were observed between intervention and control groups. Students reporting having seen intervention materials reported lower descriptive and injunctive norms than those who did not.</p>
<p>Conclusions: Attention is needed to enhancing exposure, credibility and perceived relevance of intervention messages, particularly among heavier drinkers, before definitive evaluation can be recommended. A definitive evaluation would need to consider how it would achieve sufficient response rates, whilst hall-level cluster randomisation appears subject to a significant degree of contamination.</p>
Results from the First Science Run of the ZEPLIN-III Dark Matter Search Experiment
The ZEPLIN-III experiment in the Palmer Underground Laboratory at Boulby uses
a 12kg two-phase xenon time projection chamber to search for the weakly
interacting massive particles (WIMPs) that may account for the dark matter of
our Galaxy. The detector measures both scintillation and ionisation produced by
radiation interacting in the liquid to differentiate between the nuclear
recoils expected from WIMPs and the electron recoil background signals down to
~10keV nuclear recoil energy. An analysis of 847kg.days of data acquired
between February 27th 2008 and May 20th 2008 has excluded a WIMP-nucleon
elastic scattering spin-independent cross-section above 8.1x10(-8)pb at
55GeV/c2 with a 90% confidence limit. It has also demonstrated that the
two-phase xenon technique is capable of better discrimination between electron
and nuclear recoils at low-energy than previously achieved by other xenon-based
experiments.Comment: 12 pages, 17 figure
Hip fracture risk assessment: Artificial neural network outperforms conditional logistic regression in an age- and sex-matched case control study
Copyright @ 2013 Tseng et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly cited.Background - Osteoporotic hip fractures with a significant morbidity and excess mortality among the elderly have imposed huge health and economic burdens on societies worldwide. In this age- and sex-matched case control study, we examined the risk factors of hip fractures and assessed the fracture risk by conditional logistic regression (CLR) and ensemble artificial neural network (ANN). The performances of these two classifiers were compared.
Methods - The study population consisted of 217 pairs (149 women and 68 men) of fractures and controls with an age older than 60 years. All the participants were interviewed with the same standardized questionnaire including questions on 66 risk factors in 12 categories. Univariate CLR analysis was initially conducted to examine the unadjusted odds ratio of all potential risk factors. The significant risk factors were then tested by multivariate analyses. For fracture risk assessment, the participants were randomly divided into modeling and testing datasets for 10-fold cross validation analyses. The predicting models built by CLR and ANN in modeling datasets were applied to testing datasets for generalization study. The performances, including discrimination and calibration, were compared with non-parametric Wilcoxon tests.
Results - In univariate CLR analyses, 16 variables achieved significant level, and six of them remained significant in multivariate analyses, including low T score, low BMI, low MMSE score, milk intake, walking difficulty, and significant fall at home. For discrimination, ANN outperformed CLR in both 16- and 6-variable analyses in modeling and testing datasets (p?<?0.005). For calibration, ANN outperformed CLR only in 16-variable analyses in modeling and testing datasets (p?=?0.013 and 0.047, respectively).
Conclusions - The risk factors of hip fracture are more personal than environmental. With adequate model construction, ANN may outperform CLR in both discrimination and calibration. ANN seems to have not been developed to its full potential and efforts should be made to improve its performance.National Health Research Institutes in Taiwa
The ZEPLIN-III dark matter detector: instrument design, manufacture and commissioning
We present details of the technical design and manufacture of the ZEPLIN-III
dark matter experiment. ZEPLIN-III is a two-phase xenon detector which measures
both the scintillation light and the ionisation charge generated in the liquid
by interacting particles and radiation. The instrument design is driven by both
the physics requirements and by the technology requirements surrounding the use
of liquid xenon. These include considerations of key performance parameters,
such as the efficiency of scintillation light collection, restrictions placed
on the use of materials to control the inherent radioactivity levels,
attainment of high vacuum levels and chemical contamination control. The
successful solution has involved a number of novel design and manufacturing
features which will be of specific use to future generations of direct dark
matter search experiments as they struggle with similar and progressively more
demanding requirements.Comment: 25 pages, 19 figures. Submitted to Astropart. Phys. Some figures down
sampled to reduce siz
- âŠ