21 research outputs found
On the Implications of Discrete Symmetries for the Beta Function of Quantum Hall Systems
We argue that the large discrete symmetry group of quantum Hall systems is
insufficient in itself to determine the complete beta function for the scaling
of the conductivities, and . We illustrate this
point by showing that a recent ansatz for this function is one of a
many-parameter family. A clean prediction for the delocalization exponents for
these systems therefore requires the specification of more information, such as
past proposals that the beta function is either holomorphic or
quasi-holomorphic in the variable .Comment: Minor typographical errors corrected. 6 pages, LaTeX, no figure
Distribution of adipose tissue: Quantification and relationship with hepatic steatosis and vascular profiles of type 2 diabetic patients with metabolic syndrome
AimAs the distribution of fat is increasingly related to cardiovascular events, we examined whether or not abdominal-fat quantification using magnetic resonance imaging (MRI) software is reliable, and whether or not it is related to clinical markers of fat distribution as well as to metabolic and vascular status. Methods We recorded the anthropometric measurements of 34 obese type 2 diabetic patients with metabolic syndrome. The patients were enrolled to evaluate their abdominal (visceral and subcutaneous) adipose tissue by single-slice L3–L4 MRI. Manual and automated analyses were compared. The relationships between anthropometric measurements, biological markers and intima-media thickness of the common carotid artery were also assessed. Results We validated the automated software to quantify abdominal-fat deposition with MRI compared with manual measurements (r2 = 0.95). The waist-to-hip-circumference ratio (WHR) was the only clinical parameter that correlated with the proportion and quantity of visceral and subcutaneous abdominal-adipose tissue evaluated by MRI (r = 0.60). In addition, fat repartition as evaluated by WHR was related to hepatic steatosis parameters (ferritin and ALAT) and to intima-media thickness, whereas simple waist circumference was not a determinant in these obese patients. We also showed that the adiponectin-to-leptin ratio was related to adipose tissue distribution. Conclusion Distribution of abdominal fat, as evaluated by MRI, can be reflected by clinical determination of the WHR. Differences in regional accumulations of abdominal fat may be specifically related to variations in the risks of steatosis and vascular rigidity among obese type 2 diabetic patients
A Note on Domain Walls and the Parameter Space of N=1 Gauge Theories
We study the spectrum of BPS domain walls within the parameter space of N=1
U(N) gauge theories with adjoint matter and a cubic superpotential. Using a low
energy description obtained by compactifying the theory on R^3 x S^1, we
examine the wall spectrum by combining direct calculations at special points in
the parameter space with insight drawn from the leading order potential between
minimal walls, i.e those interpolating between adjacent vacua. We show that the
multiplicity of composite BPS walls -- as characterised by the CFIV index --
exhibits discontinuities on marginal stability curves within the parameter
space of the maximally confining branch. The structure of these marginal
stability curves for large N appears tied to certain singularities within the
matrix model description of the confining vacua.Comment: 33 pages, LaTeX, 6 eps figures; v2: references adde
Duality in the Quantum Hall Effect - the Role of Electron Spin
At low temperatures the phase diagram for the quantum Hall effect has a
powerful symmetry arising from the Law of Corresponding States. This symmetry
gives rise to an infinite order discrete group which is a generalisation of
Kramers-Wannier duality for the two dimensional Ising model. The duality group,
which is a subgroup of the modular group, is analysed and it is argued that
there is a quantitative difference between a situation in which the spin
splitting of electron energy levels is comparable to the cyclotron energy and
one in which the spin splitting is much less than the cyclotron energy. In the
former case the group of symmetries is larger than in the latter case. These
duality symmetries are used to constrain the scaling functions of the theory
and, under an assumption of complex meromorphicity, a unique functional form is
obtained for the crossover of the conductivities between Hall states as a
function of the external magnetic field. This analytic form is shown to give
good agreement with experimental data.
The analysis requires a consideration of the way in which longitudinal
resistivities are extracted from the experimentally measured longitudinal
resistances and a novel method is proposed for determining the correct
normalisation for the former.Comment: 22 pages, 8 figures, typeset in LaTe
The Minimal Model of Nonbaryonic Dark Matter: A Singlet Scalar
We propose the simplest possible renormalizable extension of the Standard
Model - the addition of just one singlet scalar field - as a minimalist model
for non-baryonic dark matter. Such a model is characterized by only three
parameters in addition to those already appearing within the Standard Model: a
dimensionless self-coupling and a mass for the new scalar, and a dimensionless
coupling, \lambda, to the Higgs field. If the singlet is the dark matter, these
parameters are related to one another by the cosmological abundance constraint,
implying that the coupling of the singlet to the Higgs field is large, \lambda
\sim O(0.1 - 1). Since this parameter also controls couplings to ordinary
matter, we obtain predictions for the elastic cross section of the singlet with
nuclei. The resulting scattering rates are close to current limits from both
direct and indirect searches. The existence of the singlet also has
implications for current Higgs searches, as it gives a large contribution to
the invisible Higgs width for much of parameter space. These scalars can be
strongly self-coupled in the cosmologically interesting sense recently proposed
by Spergel and Steinhardt, but only for very low masses (< 1 GeV), which is
possible only at the expense of some fine-tuning of parameters.Comment: 26 pages, latex. Minor revisions, few references adde
Impact of Soil Type, Biology and Temperature on the Survival of Non-Toxigenic Escherichia Coli O157
peer-reviewedThe occurrence of microbial enteropathogens in the environment can represent a serious risk to human health. The fate of enteropathogens introduced into the soil environment is dependent on a wide range of complex interacting environmental factors. While the effect of abiotic factors on enteropathogen survival has been widely examined, the interaction of enteropathogens with the soil microbial community is poorly understood. This study investigated the effect of soil biology and soil type on the survival of a non-toxigenic strain of Escherichia coli O157 under different temperature regimes. Soil microcosms of two soil types, with and without an intact microbial community, were inoculated with the enteropathogen surrogate, and survival was determined over a 64-day period, encompassing a shift from cold to ambient temperatures. In both soil types bacterial numbers decreased in soil with an intact microflora, while in the absence of an intact community E. coli populations increased. This effect was temperature specific, with E. coli populations remaining stable at low temperature, regardless of treatment. Soil type was of importance in survival at both cold and ambient temperatures. This work highlights the signifi cance of the soil microbial community in suppressing enteropathogens in soil, and of investigating die-off in a multi-factorial manner.Teagasc Walsh Fellowship Programm