13,311 research outputs found
A Deformable Model for Magnetic Vortex Pinning
A two-parameter analytical model of the magnetic vortex in a thin disk of
soft magnetic material is constructed. The model is capable of describing the
change in evolution of net vortex state magnetization and of core position when
the vortex core interacts with a magnetic pinning site. The model employs a
piecewise, physically continuous, magnetization distribution obtained by the
merger of two extensively used one-parameter analytical models of the vortex
state in a disk. Through comparison to numerical simulations of ideal disks
with and without pinning sites, the model is found to accurately predict the
magnetization, vortex position, hysteretic transitions, and 2-D displacement of
the vortex in the presence of pinning sites. The model will be applicable to
the quantitative determination of vortex pinning energies from measurements of
magnetization.Comment: 27 pages, 7 figures, including supplementary information, ancillary
files:3 supplementary movie
Specialization of the rostral prefrontal cortex for distinct analogy processes
Analogical reasoning is central to learning and abstract thinking. It involves using a more familiar situation (source) to make inferences about a less familiar situation (target). According to the predominant cognitive models, analogical reasoning includes 1) generation of structured mental representations and 2) mapping based on structural similarities between them. This study used functional magnetic resonance imaging to specify the role of rostral prefrontal cortex (PFC) in these distinct processes. An experimental paradigm was designed that enabled differentiation between these processes, by temporal separation of the presentation of the source and the target. Within rostral PFC, a lateral subregion was activated by analogy task both during study of the source (before the source could be compared with a target) and when the target appeared. This may suggest that this subregion supports fundamental analogy processes such as generating structured representations of stimuli but is not specific to one particular processing stage. By contrast, a dorsomedial subregion of rostral PFC showed an interaction between task (analogy vs. control) and period (more activated when the target appeared). We propose that this region is involved in comparison or mapping processes. These results add to the growing evidence for functional differentiation between rostral PFC subregions
Duality, the Semi-Circle Law and Quantum Hall Bilayers
There is considerable experimental evidence for the existence in Quantum Hall
systems of an approximate emergent discrete symmetry, . The evidence consists of the robustness of the tests of a suite a
predictions concerning the transitions between the phases of the system as
magnetic fields and temperatures are varied, which follow from the existence of
the symmetry alone. These include the universality of and quantum numbers of
the fixed points which occur in these transitions; selection rules governing
which phases may be related by transitions; and the semi-circular trajectories
in the Ohmic-Hall conductivity plane which are followed during the transitions.
We explore the implications of this symmetry for Quantum Hall systems involving
{\it two} charge-carrying fluids, and so obtain predictions both for bilayer
systems and for single-layer systems for which the Landau levels have a spin
degeneracy. We obtain similarly striking predictions which include the novel
new phases which are seen in these systems, as well as a prediction for
semicircle trajectories which are traversed by specific combinations of the
bilayer conductivities as magnetic fields are varied at low temperatures.Comment: 12 pages, 8 figures; discussion of magnetic field dependence modified
and figures and references updated in v
Swift and Fermi observations of X-ray flares: the case of Late Internal Shock
Simultaneous Swift and Fermi observations of gamma-ray bursts (GRBs) offer a
unique broadband view of their afterglow emission, spanning more than ten
decades in energy. We present the sample of X-ray flares observed by both Swift
and Fermi during the first three years of Fermi operations. While bright in the
X-ray band, X-ray flares are often undetected at lower (optical), and higher
(MeV to GeV) energies. We show that this disfavors synchrotron self-Compton
processes as origin of the observed X-ray emission. We compare the broadband
properties of X-ray flares with the standard late internal shock model, and
find that, in this scenario, X-ray flares can be produced by a late-time
relativistic (Gamma>50) outflow at radii R~10^13-10^14 cm. This conclusion
holds only if the variability timescale is significantly shorter than the
observed flare duration, and implies that X-ray flares can directly probe the
activity of the GRB central engine.Comment: 13 pages, 4 figures, accepted for publication in Ap
An Abdominal Presentation of Churg-Strauss Syndrome
Churg-Strauss syndrome is a small and medium vessel vasculitis that is also known as allergic granulomatous angiitis. It most commonly presents with an asthma like symptoms. It was first described in Mount Siani Hospital, New York in 1951 by Jacob Churg and Lotte Stauss and was recognised after the study of a series of 13 patients who had asthma, eosinophilia, granulomatous
inflammation necrotising systemic vasculitis and necrotising glomerulonephritis. We describe a case of Churg-Strauss syndrome presenting with abdominal pain and later during the hospital admission a mono-neuritis multiplex syndrome affecting the lower limbs. The patient presented in such an atypical fashion with abdominal signs and symptoms that they required laparotomy and the diagnosis was made after histological examination of tissue taken at the time of surgery. Treatment with immunosuppression and aggressive rehabilitation achieved a progressive recovery which continued on discharge from hospital
Biological removal of nitrogen species from metal-processing wastewater
Although several nitrification/denitrification processes are established for the removal of ammonia and nitrate from municipal and industrial wastewaters, there are few reported results on the removal of these ions from metal-processing and finishing wastewaters. Unlike municipal wastewater, there is very little organic content in metal-processing wastewaters. Sources of ammonia and nitrate in the wastewater include the use of ammonium-nitrate-fuel oil as a blasting agent, and the use of other nitrogen-containing reagents during processing. The objective of this work was to investigate a biological process for the removal of nitrogenous compounds from real metal-processing wastewater. The system comprised an aerobic continuously stirred tank reactor (CSTR) followed by an anaerobic packed column and was run using real wastewater from a metal-processing operation. The system was inoculated using humus sludge from a municipal trickling filter and a period of approximately four weeks was required for a denitrifying biofilm to develop. Results showed that ammonia removal occurred readily in the CSTR while nitrite oxidation was slower to develop. The CSTR was found to be suitable for ammonia oxidation; up to 89% ammonia removal was achieved. By employing an integrated process comprising nitrification and denitrification, high ammonia removal efficiencies can be obtained. An effluent that is low in ammonia can be obtained with this system with additional carbon introduced after the CSTR. The gravel-packed column reactor was found to be unsuitable for the removal of nitrate in the configuration used (maximum 15% removal efficiency). The critical parameters for denitrification are nitrate concentration, temperature, influent flow rate and mean cell retention time. Nitrate removal did not meet the expectations projected by previous authors' work using synthetic wastewater
Electromagnetic Response and Approximate SO(5) Symmetry in High-Tc Superconductors
It has been proposed that the effective Hamiltonian describing high T_c
superconductivity in cuprate materials has an approximate SO(5) symmetry
relating the superconducting (SC) and antiferromagnetic (AF) phases of these
systems. We show that robust consequences of this proposal are potentially
large optical conductivities and Raman scattering rates in the AF phase, due to
the electromagnetic response of the doubly-charged pseudo Goldstone bosons
which must exist there. This provides strong constraints on the properties of
the bosons, such as their mass gap and velocity.Comment: 4 pages, 3 figure
Multi-Throat Brane Inflation
We present a scenario where brane inflation arises more generically. We start
with D3 and anti-D3-branes at the infrared ends of two different throats. This
setup is a natural consequence of the assumption that in the beginning we have
a multi-throat string compactification with many wandering anti-D3-branes. A
long period of inflation is triggered when D3-branes slowly exit the highly
warped infrared region, under a potential generically arising from the moduli
stabilization. In this scenario, the usual slow-roll conditions are not
required, and a large warping is allowed to incorporate the Randall-Sundrum
model.Comment: 11 pages; v3: minor revision, PRD versio
Quantum fields in disequilibrium: neutral scalar bosons with long-range, inhomogeneous perturbations
Using Schwinger's quantum action principle, dispersion relations are obtained
for neutral scalar mesons interacting with bi-local sources. These relations
are used as the basis of a method for representing the effect of interactions
in the Gaussian approximation to field theory, and it is argued that a marked
inhomogeneity, in space-time dependence of the sources, forces a discrete
spectrum on the field. The development of such a system is characterized by
features commonly associated with chaos and self-organization (localization by
domain or cell formation). The Green functions play the role of an iterative
map in phase space. Stable systems reside at the fixed points of the map. The
present work can be applied to self-interacting theories by choosing suitable
properties for the sources. Rapid transport leads to a second order phase
transition and anomalous dispersion. Finally, it is shown that there is a
compact representation of the non-equilibrium dynamics in terms of generalized
chemical potentials, or equivalently as a pseudo-gauge theory, with an
imaginary charge. This analogy shows, more clearly, how dissipation and entropy
production are related to the source picture and transform a flip-flop like
behaviour between two reservoirs into the Landau problem in a constant
`magnetic field'. A summary of conventions and formalism is provided as a basis
for future work.Comment: 23 pages revte
- …