514 research outputs found
Parallel finite element density functional computations exploiting grid refinement and subspace recycling
In this communication computational methods that facilitate finite element analysis of density functional computations are developed. They are: (i) hÂżadaptive grid refinement techniques that reduce the total number of degrees of freedom in the real space grid while improving on the approximate resolution of the wanted solution; and (ii) subspace recycling of the approximate solution in self-consistent cycles with the aim of improving the performance of the generalized eigenproblem solver. These techniques are shown to give a convincing speed-up in the computation process by alleviating the overhead normally associated with computing systems with many degrees-of-freedom.The anonymous referees whose comments improved the presentation of this work are gratefully acknowledged. The work was supported by the Polish Ministry of Science and Higher Education N N519402837 and by the Spanish Ministry of Science and Innovation TIN2009-07519 and TIN2012-32846. The resources provided by the Barcelona Supercomputing Center are also acknowledged.Young, TD.; Romero Alcalde, E.; RomĂĄn MoltĂł, JE. (2013). Parallel finite element density functional computations exploiting grid refinement and subspace recycling. Computer Physics Communications. 184(1):66-72. doi:10.1016/j.cpc.2012.08.011S6672184
Limitations of the heavy-baryon expansion as revealed by a pion-mass dispersion relation
The chiral expansion of nucleon properties such as mass, magnetic moment, and
magnetic polarizability are investigated in the framework of chiral
perturbation theory, with and without the heavy-baryon expansion. The analysis
makes use of a pion-mass dispersion relation, which is shown to hold in both
frameworks. The dispersion relation allows an ultraviolet cutoff to be
implemented without compromising the symmetries. After renormalization, the
leading-order heavy-baryon loops demonstrate a stronger dependence on the
cutoff scale, which results in weakened convergence of the expansion. This
conclusion is tested against the recent results of lattice quantum
chromodynamics simulations for nucleon mass and isovector magnetic moment. In
the case of the polarizability, the situation is even more dramatic as the
heavy-baryon expansion is unable to reproduce large soft contributions to this
quantity. Clearly, the heavy-baryon expansion is not suitable for every
quantity.Comment: Accepted for publication in EPJ C. Made changes based on referee
comments: clarifying sentences to conclusion 1. of Section IV, beginning of
Section V, and new footnote in Section VI, page 8. Added more detailed
explanation in paragraph 4 of Section III. Added citations of Phys.Rev. D60,
034014, and Phys.Lett. B716, 33
Hypoxia modulates platelet purinergic signalling pathways.
BACKGROUND: âHypoxia resulting from ascent to high-altitude or pathological states at sea level is known to increase platelet reactivity. Previous work from our group has suggested that this may be adenosine diphosphate (ADP)-specific. Given the clinical importance of drugs targeting ADP pathways, research into the impact of hypoxia on platelet ADP pathways is highly important.
METHODS: âOptimul aggregometry was performed on plasma from 29 lowland residents ascending to 4,700 m, allowing systematic assessment of platelet reactivity in response to several platelet agonists. Aggregometry was also performed in response to ADP in the presence of inhibitors of the two main ADP receptors, P2Y1 and P2Y12 (MRS2500 and cangrelor, respectively). Phosphorylation of vasodilator-stimulated phosphoprotein (VASP), a key determinant of platelet aggregation, was analysed using the VASPFix assay.
RESULTS: âHypobaric hypoxia significantly reduced the ability of a fixed concentration of cangrelor to inhibit ADP-induced aggregation and increased basal VASP phosphorylation. However, in the absence of P2Y receptor inhibitors, we did not find evidence of increased platelet sensitivity to any of the agonists tested and found reduced sensitivity to thrombin receptor-activating peptide-6 amide.
CONCLUSION: âOur results provide evidence of increased P2Y1 receptor activity at high altitude and suggest down-regulation of the P2Y12 pathway through increased VASP phosphorylation. These changes in ADP pathway activity are of potential therapeutic significance to high-altitude sojourners and hypoxic sea level patients prescribed platelet inhibitors and warrant further investigation
The two-dimensional random-bond Ising model, free fermions and the network model
We develop a recently-proposed mapping of the two-dimensional Ising model
with random exchange (RBIM), via the transfer matrix, to a network model for a
disordered system of non-interacting fermions. The RBIM transforms in this way
to a localisation problem belonging to one of a set of non-standard symmetry
classes, known as class D; the transition between paramagnet and ferromagnet is
equivalent to a delocalisation transition between an insulator and a quantum
Hall conductor. We establish the mapping as an exact and efficient tool for
numerical analysis: using it, the computational effort required to study a
system of width is proportional to , and not exponential in as
with conventional algorithms. We show how the approach may be used to calculate
for the RBIM: the free energy; typical correlation lengths in quasi-one
dimension for both the spin and the disorder operators; even powers of
spin-spin correlation functions and their disorder-averages. We examine in
detail the square-lattice, nearest-neighbour RBIM, in which bonds are
independently antiferromagnetic with probability , and ferromagnetic with
probability . Studying temperatures , we obtain precise
coordinates in the plane for points on the phase boundary between
ferromagnet and paramagnet, and for the multicritical (Nishimori) point. We
demonstrate scaling flow towards the pure Ising fixed point at small , and
determine critical exponents at the multicritical point.Comment: 20 pages, 25 figures, figures correcte
Recommended from our members
Analysis automation with paving: A new quadrilateral meshing technique
This paper describes the impact of paving, a new automatic mesh generation algorithm, on the analysis portion of the design process. Paving generates an all-quadrilateral mesh in arbitrary 2D geometries. The paving technique significantly impacts the analysis process by drastically reducing the time and expertise requirements of traditional mesh generation. Paving produces a high quality mesh based on geometric boundary definitions and user specified element sizing constraints. In this paper we describe the paving algorithm, discuss varying aspects of the impact of the technique on design automation, and elaborate on current research into 3D all-hexahedral mesh generation. 11 refs., 10 figs
Extreme scenarios: the tightest possible constraints on the power spectrum due to primordial black holes
Observational constraints on the abundance of primordial black holes (PBHs) constrain the allowed amplitude of the primordial power spectrum on both the smallest and the largest ranges of scales, covering over 20 decades from 1 - 10^20=Mpc. Despite tight constraints on the allowed fraction of PBHs at their time of formation near horizon entry in the early Universe, the corresponding constraints on the primordial power spectrum are quite weak, typically PR . 10<~2 assuming Gaussian perturbations. Motivated by recent claims that the evaporation of just one PBH would destabilise the Higgs vacuum and collapse the Universe, we calculate the constraints which follow from assuming there are zero PBHs within the observable Universe. Even if evaporating PBHs do not collapse the Universe, this scenario represents the ultimate limit of observational constraints. Constraints can be extended on to smaller scales right down to the horizon scale at the end of in ation, but where power spectrum constraints already exist they do not tighten significantly, even though the constraint on PBH abundance can decrease by up to 46 orders of magnitude. This shows that no future improvement in observational constraints can ever lead to a significant tightening in constraints on in ation (via the power spectrum amplitude). The power spectrum constraints are weak because an order unity perturbation is required in order to overcome pressure forces. We therefore consider an early matter dominated era, during which exponentially more PBHs form for the same initial conditions. We show this leads to far tighter constraints, which approach PR . 10^-9, albeit over a smaller range of scales and are very sensitive to when the early matter dominated era ends. Finally, we show that an extended early matter era is incompatible with the argument that an evaporating PBH would destroy the Universe, unless the power spectrum amplitude decreases by up to ten orders of magnitude
Astroparticle Physics with a Customized Low-Background Broad Energy Germanium Detector
The MAJORANA Collaboration is building the MAJORANA DEMONSTRATOR, a 60 kg
array of high purity germanium detectors housed in an ultra-low background
shield at the Sanford Underground Laboratory in Lead, SD. The MAJORANA
DEMONSTRATOR will search for neutrinoless double-beta decay of 76Ge while
demonstrating the feasibility of a tonne-scale experiment. It may also carry
out a dark matter search in the 1-10 GeV/c^2 mass range. We have found that
customized Broad Energy Germanium (BEGe) detectors produced by Canberra have
several desirable features for a neutrinoless double-beta decay experiment,
including low electronic noise, excellent pulse shape analysis capabilities,
and simple fabrication. We have deployed a customized BEGe, the MAJORANA
Low-Background BEGe at Kimballton (MALBEK), in a low-background cryostat and
shield at the Kimballton Underground Research Facility in Virginia. This paper
will focus on the detector characteristics and measurements that can be
performed with such a radiation detector in a low-background environment.Comment: Submitted to NIMA Proceedings, SORMA XII. 9 pages, 4 figure
Automated Detection of EUV Polar Coronal Holes During Solar Cycle 23
A new method for automated detection of polar coronal holes is presented.
This method, called perimeter tracing, uses a series of 171, 195, and 304 \AA\
full disk images from the Extreme ultraviolet Imaging Telescope (EIT) on SOHO
over solar cycle 23 to measure the perimeter of polar coronal holes as they
appear on the limbs. Perimeter tracing minimizes line-of-sight obscurations
caused by the emitting plasma of the various wavelengths by taking measurements
at the solar limb. Perimeter tracing also allows for the polar rotation period
to emerge organically from the data as 33 days. We have called this the Harvey
rotation rate and count Harvey rotations starting 4 January 1900. From the
measured perimeter, we are then able to fit a curve to the data and derive an
area within the line of best fit. We observe the area of the northern polar
hole area in 1996, at the beginning of solar cycle 23, to be about 4.2% of the
total solar surface area and about 3.6% in 2007. The area of the southern polar
hole is observed to be about 4.0% in 1996 and about 3.4% in 2007. Thus, both
the north and south polar hole areas are no more than 15% smaller now than they
were at the beginning of cycle 23. This compares to the polar magnetic field
measured to be about 40% less now than it was a cycle ago.Comment: 18 pagers, 7 figures, accepted to Solar Physic
Reducing bumblefoot lesions in a group of captive Magellanic penguins (Spheniscus magellanicus) with the use of environmental enrichment
Model-independent search for CP violation in D0âKâK+ÏâÏ+ and D0âÏâÏ+Ï+Ïâ decays
A search for CP violation in the phase-space structures of D0 and View the MathML source decays to the final states KâK+ÏâÏ+ and ÏâÏ+Ï+Ïâ is presented. The search is carried out with a data set corresponding to an integrated luminosity of 1.0 fbâ1 collected in 2011 by the LHCb experiment in pp collisions at a centre-of-mass energy of 7 TeV. For the KâK+ÏâÏ+ final state, the four-body phase space is divided into 32 bins, each bin with approximately 1800 decays. The p-value under the hypothesis of no CP violation is 9.1%, and in no bin is a CP asymmetry greater than 6.5% observed. The phase space of the ÏâÏ+Ï+Ïâ final state is partitioned into 128 bins, each bin with approximately 2500 decays. The p-value under the hypothesis of no CP violation is 41%, and in no bin is a CP asymmetry greater than 5.5% observed. All results are consistent with the hypothesis of no CP violation at the current sensitivity
- âŠ