64 research outputs found
Higher Order Effects in the Dielectric Constant of Percolative Metal-Insulator Systems above the Critical Point
The dielectric constant of a conductor-insulator mixture shows a pronounced
maximum above the critical volume concentration. Further experimental evidence
is presented as well as a theoretical consideration based on a phenomenological
equation. Explicit expressions are given for the position of the maximum in
terms of scaling parameters and the (complex) conductances of the conductor and
insulator. In order to fit some of the data, a volume fraction dependent
expression for the conductivity of the more highly conductive component is
introduced.Comment: 4 pages, Latex, 4 postscript (*.epsi) files submitted to Phys Rev.
Twist-3 Distribute Amplitude of the Pion in QCD Sum Rules
We apply the background field method to calculate the moments of the pion
two-particles twist-3 distribution amplitude (DA) in QCD sum
rules. In this paper,we do not use the equation of motion for the quarks inside
the pion since they are not on shell and introduce a new parameter to
be determined. We get the parameter in this approach. If
assuming the expansion of in the series in Gegenbauer polynomials
, one can obtain its approximate expression which can be
determined by its first few moments.Comment: 12 pages, 3 figure
The color gauge invariance and a possible origin of a mass gap in QCD
The general scale parameter, having the dimensions of mass squared, is
dynamically generated in the QCD gluon sector. It is introduced through the
difference between the regularized full gluon self-energy and its value at some
finite point. It violates transversality of the full gluon self-energy. The
Slavnov-Taylor identity for the full gluon propagator, when it is given by the
corresponding equation of motion, is also violated by it. So in order to
maintain both transversality and the identity it should be disregarded from the
very beginning, i.e., put formally zero everywhere. However, we have shown how
to preserve the above-mentioned identity at non-zero mass squared parameter.
This allows one to establish the structure of the full gluon propagator when it
is explicitly present. Its contribution does not survive in the perturbation
theory regime when the gluon momentum goes to infinity. At the same time, its
contribution dominates the structure of the full gluon propagator when the
gluon momentum goes to zero. We have also proposed a method how to restore
transversality of the relevant gluon propagator in a gauge invariant way, while
keeping the mass squared parameter "alive".Comment: 20 pages, 1 figure, no tables, some final additional material is
added minor correction introduced and references rearrange
Crossover and scaling in a nearly antiferromagnetic Fermi liquid in two dimensions
We consider two-dimensional Fermi liquids in the vicinity of a quantum
transition to a phase with commensurate, antiferromagnetic long-range order.
Depending upon the Fermi surface topology, mean-field spin-density-wave theory
predicts two different types of such transitions, with mean-field dynamic
critical exponents (when the Fermi surface does not cross the magnetic
zone boundary, type ) and (when the Fermi surface crosses the magnetic
zone boundary, type ). The type system only displays behavior at
all energies and its scaling properties are similar (though not identical) to
those of an insulating Heisenberg antiferromagnet. Under suitable conditions
precisely stated in this paper, the type system displays a crossover from
relaxational behavior at low energies to type behavior at high energies. A
scaling hypothesis is proposed to describe this crossover: we postulate a
universal scaling function which determines the entire, temperature-,
wavevector-, and frequency-dependent, dynamic, staggered spin susceptibility in
terms of 4 measurable, , parameters (determining the distance, energy, and
order parameter scales, plus one crossover parameter). The scaling function
contains the full scaling behavior in all regimes for both type and
systems. The crossover behavior of the uniform susceptibility and the specific
heat is somewhat more complicated and is also discussed. Explicit computation
of the crossover functions is carried out in a large expansion on a
mean-field model. Some new results for the critical properties on the ordered
side of the transition are also obtained in a spin-density wave formalism. The
possible relevance of our results to the doped cuprate compounds is briefly
discussed.Comment: 20 pages, REVTeX, 6 figures (uuencoded compressed PostScript file for
figures is appended
NMR and NQR Fluctuation Effects in Layered Superconductors
We study the effect of thermal fluctuations of the s-wave order parameter of
a quasi two dimensional superconductor on the nuclear spin relaxation rate near
the transition temperature Tc. We consider both the effects of the amplitude
fluctuations and the Berezinskii-Kosterlitz-Thouless (BKT) phase fluctuations
in weakly coupled layered superconductors. In the treatment of the amplitude
fluctuations we employ the Gaussian approximation and evaluate the longitudinal
relaxation rate 1/T1 for a clean s-wave superconductor, with and without pair
breaking effects, using the static pair fluctuation propagator D. The increase
in 1/T1 due to pair breaking in D is overcompensated by the decrease arising
from the single particle Green's functions. The result is a strong effect on
1/T1 for even a small amount of pair breaking. The phase fluctuations are
described in terms of dynamical BKT excitations in the form of pancake
vortex-antivortex (VA) pairs. We calculate the effect of the magnetic field
fluctuations caused by the translational motion of VA excitations on 1/T1 and
on the transverse relaxation rate 1/T2 on both sides of the BKT transitation
temperature T(BKT)<Tc. The results for the NQR relaxation rates depend strongly
on the diffusion constant that governs the motion of free and bound vortices as
well as the annihilation of VA pairs. We discuss the relaxation rates for real
multilayer systems where the diffusion constant can be small and thus increase
the lifetime of a VA pair, leading to an enhancement of the rates. We also
discuss in some detail the experimental feasibility of observing the effects of
amplitude fluctuations in layered s-wave superconductors such as the
dichalcogenides and the effects of phase fluctuations in s- or d-wave
superconductors such as the layered cuprates.Comment: 38 pages, 12 figure
Holographic description of strongly correlated electrons in external magnetic fields
We study the Fermi level structure of (2+1)-dimensional strongly interacting
electron systems in external magnetic field using the AdS/CFT correspondence.
The gravity dual of a finite density fermion system is a Dirac field in the
background of the dyonic AdS-Reissner-Nordstrom black hole. In the probe limit
the magnetic system can be reduced to the non-magnetic one, with
Landau-quantized momenta and rescaled thermodynamical variables. We find that
at strong enough magnetic fields, the Fermi surface vanishes and the
quasiparticle is lost either through a crossover to conformal regime or through
a phase transition to an unstable Fermi surface. In the latter case, the
vanishing Fermi velocity at the critical magnetic field triggers the non-Fermi
liquid regime with unstable quasiparticles and a change in transport properties
of the system. We associate it with a metal-"strange metal" phase transition.
We compute the DC Hall and longitudinal conductivities using the
gravity-dressed fermion propagators. As expected, the Hall conductivity is
quantized according to integer Quantum Hall Effect (QHE) at weak magnetic
fields. At strong magnetic fields, new plateaus typical for the fractional QHE
appear. Our pattern closely resembles the experimental results on graphite
which are described using the fractional filling factor proposed by Halperin.Comment: 39 pages, 11 figures, Ch. 21, Lect. Notes Phys. "Strongly interacting
matter in magnetic fields" (Springer), edited by D. Kharzeev, K. Landsteiner,
A. Schmitt, H.-U. Yee. arXiv admin note: substantial text overlap with
arXiv:1011.405
RANTES/CCL5 and Risk for Coronary Events: Results from the MONICA/KORA Augsburg Case-Cohort, Athero-Express and CARDIoGRAM Studies
BACKGROUND: The chemokine RANTES (regulated on activation, normal T-cell expressed and secreted)/CCL5 is involved in the pathogenesis of cardiovascular disease in mice, whereas less is known in humans. We hypothesised that its relevance for atherosclerosis should be reflected by associations between CCL5 gene variants, RANTES serum concentrations and protein levels in atherosclerotic plaques and risk for coronary events.
METHODS AND FINDINGS: We conducted a case-cohort study within the population-based MONICA/KORA Augsburg studies. Baseline RANTES serum levels were measured in 363 individuals with incident coronary events and 1,908 non-cases (mean follow-up: 10.2±4.8 years). Cox proportional hazard models adjusting for age, sex, body mass index, metabolic factors and lifestyle factors revealed no significant association between RANTES and incident coronary events (HR [95% CI] for increasing RANTES tertiles 1.0, 1.03 [0.75-1.42] and 1.11 [0.81-1.54]). None of six CCL5 single nucleotide polymorphisms and no common haplotype showed significant associations with coronary events. Also in the CARDIoGRAM study (>22,000 cases, >60,000 controls), none of these CCL5 SNPs was significantly associated with coronary artery disease. In the prospective Athero-Express biobank study, RANTES plaque levels were measured in 606 atherosclerotic lesions from patients who underwent carotid endarterectomy. RANTES content in atherosclerotic plaques was positively associated with macrophage infiltration and inversely associated with plaque calcification. However, there was no significant association between RANTES content in plaques and risk for coronary events (mean follow-up 2.8±0.8 years).
CONCLUSIONS: High RANTES plaque levels were associated with an unstable plaque phenotype. However, the absence of associations between (i) RANTES serum levels, (ii) CCL5 genotypes and (iii) RANTES content in carotid plaques and either coronary artery disease or incident coronary events in our cohorts suggests that RANTES may not be a novel coronary risk biomarker. However, the potential relevance of RANTES levels in platelet-poor plasma needs to be investigated in further studies
RANTES/CCL5 and risk for coronary events: Results from the MONICA/KORA Augsburg case-cohort, Athero-express and CARDIoGRAM studies
Background: The chemokine RANTES (regulated on activation, normal T-cell expressed and secreted)/CCL5 is involved in the pathogenesis of cardiovascular disease in mice, whereas less is known in humans. We hypothesised that its relevance for atherosclerosis should be reflected by associations between CCL5 gene variants, RANTES serum concentrations and protein levels in atherosclerotic plaques and risk for coronary events. Methods and Findings: We conducted a case-cohort study within the population-based MONICA/KORA Augsburg studies. Baseline RANTES serum levels were measured in 363 individuals with incident coronary events and 1,908 non-cases (mean follow-up: 10.2±
Integrating sequence and array data to create an improved 1000 Genomes Project haplotype reference panel
A major use of the 1000 Genomes Project (1000GP) data is genotype imputation in genome-wide association studies (GWAS). Here we develop a method to estimate haplotypes from low-coverage sequencing data that can take advantage of single-nucleotide polymorphism (SNP) microarray genotypes on the same samples. First the SNP array data are phased to build a backbone (or 'scaffold') of haplotypes across each chromosome. We then phase the sequence data 'onto' this haplotype scaffold. This approach can take advantage of relatedness between sequenced and non-sequenced samples to improve accuracy. We use this method to create a new 1000GP haplotype reference set for use by the human genetic community. Using a set of validation genotypes at SNP and bi-allelic indels we show that these haplotypes have lower genotype discordance and improved imputation performance into downstream GWAS samples, especially at low-frequency variants. © 2014 Macmillan Publishers Limited. All rights reserved
National identity predicts public health support during a global pandemic
Changing collective behaviour and supporting non-pharmaceutical interventions is an important component in mitigating virus transmission during a pandemic. In a large international collaboration (Study 1, N = 49,968 across 67 countries), we investigated self-reported factors associated with public health behaviours (e.g., spatial distancing and stricter hygiene) and endorsed public policy interventions (e.g., closing bars and restaurants) during the early stage of the COVID-19 pandemic (April-May 2020). Respondents who reported identifying more strongly with their nation consistently reported greater engagement in public health behaviours and support for public health policies. Results were similar for representative and non-representative national samples. Study 2 (N = 42 countries) conceptually replicated the central finding using aggregate indices of national identity (obtained using the World Values Survey) and a measure of actual behaviour change during the pandemic (obtained from Google mobility reports). Higher levels of national identification prior to the pandemic predicted lower mobility during the early stage of the pandemic (r = −0.40). We discuss the potential implications of links between national identity, leadership, and public health for managing COVID-19 and future pandemics.publishedVersio
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