232 research outputs found
Dilepton and photon production in the presence of a nontrivial Polyakov loop
We calculate the production of dileptons and photons in the presence of a
nontrivial Polyakov loop in QCD. This is applicable to the semi-Quark Gluon
Plasma (QGP), at temperatures above but near the critical temperature for
deconfinement. The Polyakov loop is small in the semi-QGP, and near unity in
the perturbative QGP. Working to leading order in the coupling constant of QCD,
we find that there is a mild enhancement, ~ 20%, for dilepton production in the
semi-QGP over that in the perturbative QGP. In contrast, we find that photon
production is strongly suppressed in the semi-QGP, by about an order of
magnitude, relative to the perturbative QGP. In the perturbative QGP photon
production contains contributions from 2->2 scattering and collinear emission
with the Landau- Pomeranchuk-Migdal (LPM) effect. In the semi-QGP we show that
the two contributions are modified differently. The rate for 2->2 scattering is
suppressed by a factor which depends upon the Polyakov loop. In contrast, in an
SU(N) gauge theory the collinear rate is suppressed by 1/N, so that the LPM
effect vanishes at infinite N. To leading order in the semi-QGP at large N, we
compute the rate from 2->2 scattering to the leading logarithmic order and the
collinear rate to leading order
Electronic superlattice revealed by resonant scattering from random impurities in Sr3Ru2O7
Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive
tool for the study of subtle charge, orbital, and spin superlattice orders
driven by the valence electrons, which therefore escape detection in
conventional x-ray diffraction (XRD). Although the power of REXS has been
demonstrated by numerous studies of complex oxides performed in the soft x-ray
regime, the cross section and photon wavelength of the material-specific
elemental absorption edges ultimately set the limit to the smallest
superlattice amplitude and periodicity one can probe. Here we show -- with
simulations and REXS on Mn-substituted SrRuO -- that these
limitations can be overcome by performing resonant scattering experiments at
the absorption edge of a suitably-chosen, dilute impurity. This establishes
that -- in analogy with impurity-based methods used in electron-spin-resonance,
nuclear-magnetic resonance, and M\"ossbauer spectroscopy -- randomly
distributed impurities can serve as a non-invasive, but now momentum-dependent
probe, greatly extending the applicability of resonant x-ray scattering
techniques
Selective factors involved in oil flotation isolation of black yeasts from the environment
The oil flotation isolation technique has been successfully applied to
recover chaetothyrialean black yeasts and relatives from the environment. The
selective mechanisms playing a role in isolation are unknown. The fungi
concerned are supposed to occupy specialized microniches in nature, taking
advantage of (1) oligotrophism. Mineral oil as a main selective agent may be
based on (2) hydrophobicity or on (3) assimilation. All three hypotheses are
tested in this paper. Results show that cell wall hydrophobicity is unlikely
to be a selective factor. Incubation under poor nutrient conditions provides
competitive advantage for black yeasts, especially for Exophiala
strains, which are subsequently enriched by mineral oil which enhances growth
in this group of fungi. Incubation under mineral media and mineral oil can be
used as selective factor
Crystal-field level inversion in lightly Mn-doped Sr3Ru2O7
Sr3(Ru1-xMnx)2O7, in which 4d-Ru is substituted by the more localized 3d-Mn,
is studied by x-ray dichroism and spin-resolved density functional theory. We
find that Mn impurities do not exhibit the same 4+ valence of Ru, but act as 3+
acceptors; the extra eg electron occupies the in-plane 3dx2-y2 orbital instead
of the expected out-of-plane 3d3z2-r2. We propose that the 3d-4d interplay, via
the ligand oxygen orbitals, is responsible for this crystal-field level
inversion and the material's transition to an antiferromagnetic, possibly
orbitally-ordered, low-temperature state.Comment: A high-resolution version can be found at
http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/MnSr3Ru2O7_XAS.pd
WARNING: Physics Envy May Be Hazardous To Your Wealth!
The quantitative aspirations of economists and financial analysts have for
many years been based on the belief that it should be possible to build models
of economic systems - and financial markets in particular - that are as
predictive as those in physics. While this perspective has led to a number of
important breakthroughs in economics, "physics envy" has also created a false
sense of mathematical precision in some cases. We speculate on the origins of
physics envy, and then describe an alternate perspective of economic behavior
based on a new taxonomy of uncertainty. We illustrate the relevance of this
taxonomy with two concrete examples: the classical harmonic oscillator with
some new twists that make physics look more like economics, and a quantitative
equity market-neutral strategy. We conclude by offering a new interpretation of
tail events, proposing an "uncertainty checklist" with which our taxonomy can
be implemented, and considering the role that quants played in the current
financial crisis.Comment: v3 adds 2 reference
What controls the isotopic composition of Greenland surface snow?
International audienceWater stable isotopes in Greenland ice core data provide key paleoclimatic information, and have been compared with precipitation isotopic composition simulated by isotopically enabled atmospheric models. However, post-depositional processes linked with snow metamorphism remain poorly documented. For this purpose, monitoring of the isotopic composition (d18O, dD) of near-surface water vapor, precipitation and samples of the top (0.5 cm) snow surface has been conducted during two summers (2011-2012) at NEEM, NW Greenland. The samples also include a subset of 17O-excess measurements over 4 days, and the measurements span the 2012 Greenland heat wave. Our observations are consistent with calculations assuming isotopic equilibrium between surface snow and water vapor. We observe a strong correlation between near-surface vapor d18O and air temperature (0.85 ± 0.11‰ °C-1 (R = 0.76) for 2012). The correlation with air temperature is not observed in precipitation data or surface snow data. Deuterium excess (d-excess) is strongly anti-correlated with d18O with a stronger slope for vapor than for precipitation and snow surface data. During nine 1-5-day periods between precipitation events, our data demonstrate parallel changes of d18O and d-excess in surface snow and near-surface vapor. The changes in d18O of the vapor are similar or larger than those of the snow d18O. It is estimated using the CROCUS snow model that 6 to 20% of the surface snow mass is exchanged with the atmosphere. In our data, the sign of surface snow isotopic changes is not related to the sign or magnitude of sublimation or deposition. Comparisons with atmospheric models show that day-to-day variations in near-surface vapor isotopic composition are driven by synoptic variations and changes in air mass trajectories and distillation histories. We suggest that, in between precipitation events, changes in the surface snow isotopic composition are driven by these changes in near-surface vapor isotopic composition. This is consistent with an estimated 60% mass turnover of surface snow per day driven by snow recrystallization processes under NEEM summer surface snow temperature gradients. Our findings have implications for ice core data interpretation and model-data comparisons, and call for further process studies. © Author(s) 2014
Environmental isolation of black yeast-like fungi involved in human infection
The present study focuses on potential agents of chromoblastomycosis and
other endemic diseases in the state of Paraná, Southern Brazil. Using a
highly selective protocol for chaetothyrialean black yeasts and relatives,
environmental samples from the living area of symptomatic patients were
analysed. Additional strains were isolated from creosote-treated wood and
hydrocarbon-polluted environments, as such polluted sites have been supposed
to enhance black yeast prevalence. Isolates showed morphologies compatible
with the traditional etiological agents of chromoblastomycosis, e.g.
Fonsecaea pedrosoi and Phialophora verrucosa, and of agents of
subcutaneous or systemic infections like Cladophialophora bantiana
and Exophiala jeanselmei. Some agents of mild disease were indeed
encountered. However, molecular analysis proved that most environmental
strains differed from known etiologic agents of pronounced disease syndromes:
they belonged to the same order, but mostly were undescribed species. Agents
of chromoblastomycosis and systemic disease thus far are prevalent on the
human host. The hydrocarbon-polluted environments yielded yet another spectrum
of chaetothyrialean fungi. These observations are of great relevance because
they allow us to distinguish between categories of opportunists, indicating
possible differences in pathogenicity and virulence
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