1,920 research outputs found
Density of states in graphene with vacancies: midgap power law and frozen multifractality
The density of states (DoS), , of graphene is investigated
numerically and within the self-consistent T-matrix approximation (SCTMA) in
the presence of vacancies within the tight binding model. The focus is on
compensated disorder, where the concentration of vacancies, and
, in both sub-lattices is the same. Formally, this model belongs to
the chiral symmetry class BDI. The prediction of the non-linear sigma-model for
this class is a Gade-type singularity . Our numerical data is compatible with this
result in a preasymptotic regime that gives way, however, at even lower
energies to , . We take this finding as an evidence that similar to the case
of dirty d-wave superconductors, also generic bipartite random hopping models
may exhibit unconventional (strong-coupling) fixed points for certain kinds of
randomly placed scatterers if these are strong enough. Our research suggests
that graphene with (effective) vacancy disorder is a physical representative of
such systems.Comment: References updated onl
A Spreading Layer Origin for Dwarf Nova Oscillations
Dwarf nova outbursts often show coherent () sinusoidal
oscillations with the largest pulsed fraction in the extreme ultraviolet.
Called dwarf nova oscillations (DNOs), they have periods of and scale with luminosity as with
. We propose that DNOs may be produced by nonradial
oscillations in a thin hydrostatic layer of freshly accreted material, the
``spreading layer'' (SL), at the white dwarf (WD) equator. This would naturally
explain a number of key properties of DNOs, including their frequency range,
sinusoidal nature, sensitivity to accretion rate, and why they are only seen
during outburst. In support of this hypothesis we construct a simple model that
treats the SL as a cavity containing shallow surface waves, each with the same
radial structure, but split into three different modes denoted by their
azimuthal wavenumber, . The latitudinally propagating mode best
matches the periods and scalings associated with most DNOs, and DNOs with
periods shorter than the WD Keplerian period are explained by the
prograde mode. We also predict a third set of oscillations, produced by the
retrograde mode, and show its expected dependence on accretion rate.Comment: Accepted for publication in Astrophysical Journal Letters, 4 pages, 2
figure
Pathologic motion patterns in patients with progressive pseudorheumatoid arthropathy of childhood
Effects of Juvenile Idiopathic Arthritis on Kinematics and Kinetics of the Lower Extremities Call for Consequences in Physical Activities Recommendations
Juvenile idiopathic arthritis (JIA) patients (n = 36) with symmetrical polyarticular joint involvement of the lower extremities and healthy controls (n = 20) were compared concerning differences in kinematic, kinetic, and spatio-temporal parameters with 3D gait analysis. The aims of this study were to quantify the differences in gait between JIA patients and healthy controls and to provide data for more detailed sport activities recommendations. JIA-patients showed reduced walking speed and step length, strongly anterior tilted pelvis, reduced maximum hip extension, reduced knee extension during single support phase and reduced plantar flexion in push off. Additionally the roll-off procedure of the foot was slightly decelerated. The reduced push off motion in the ankle was confirmed by lower peaks in ankle moment and power. The gait of JIA-patients can be explained as a crouch-like gait with hyperflexion in hip and knee joints and less plantar flexion in the ankle. A preventive mobility workout would be recommendable to reduce these restrictions in the future. Advisable are sports with emphasis on extension in hip, knee, and ankle plantar flexion
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Survey of Dynamic Simulation Programs for Nuclear Fuel Reprocessing
The absence of any industrial scale nuclear fuel reprocessing in the U.S. has precluded the necessary driver for developing the advanced simulation capability now prevalent in so many other industries. Modeling programs to simulate the dynamic behavior of nuclear fuel separations and processing were originally developed to support the US government’s mission of weapons production and defense fuel recovery. Consequently there has been little effort is the US devoted towards improving this specific process simulation capability during the last two or three decades. More recent work has been focused on elucidating chemical thermodynamics and developing better models of predicting equilibrium in actinide solvent extraction systems. These equilibrium models have been used to augment flowsheet development and testing primarily at laboratory scales. The development of more robust and complete process models has not kept pace with the vast improvements in computational power and user interface and is significantly behind simulation capability in other chemical processing and separation fields
A comparative analysis of parallel processing and super-individual methods for improving the computational performance of a large individual-based model
Individual-based modelling approaches are being used to simulate larger complex spatial systems in ecology and in other fields of research. Several novel model development issues now face researchers: in particular how to simulate large numbers of individuals with high levels of complexity, given finite computing resources. A case study of a spatially-explicit simulation of aphid population dynamics was used to assess two strategies for coping with a large number of individuals: the use of ‘super-individuals’ and parallel computing. Parallelisation of the model maintained the model structure and thus the simulation results were comparable to the original model. However, the super-individual implementation of the model caused significant changes to the model dynamics, both spatially and temporally. When super-individuals represented more than around 10 individuals it became evident that aggregate statistics generated from a super-individual model can hide more detailed deviations from an individual-level model. Improvements in memory use and model speed were perceived with both approaches. For the parallel approach, significant speed-up was only achieved when more than five processors were used and memory availability was only increased once five or more processors were used. The super-individual approach has potential to improve model speed and memory use dramatically, however this paper cautions the use of this approach for a density-dependent spatially-explicit model, unless individual variability is better taken into account
3D Gamma-ray and Neutron Mapping in Real-Time with the Localization and Mapping Platform from Unmanned Aerial Systems and Man-Portable Configurations
Nuclear Scene Data Fusion (SDF), implemented in the Localization and Mapping
Platform (LAMP) fuses three-dimensional (3D), real-time volumetric
reconstructions of radiation sources with contextual information (e.g. LIDAR,
camera, etc.) derived from the environment around the detector system. This
information, particularly when obtained in real time, may be transformative for
applications, including directed search for lost or stolen sources, consequence
management after the release of radioactive materials, or contamination
avoidance in security-related or emergency response scenarios. 3D
reconstructions enabled by SDF localize contamination or hotspots to specific
areas or objects, providing higher resolution over larger areas than
conventional 2D approaches, and enabling more efficient planning and response,
particularly in complex 3D environments.
In this work, we present the expansion of these gamma-ray mapping concepts to
neutron source localization. Here we integrate LAMP with a custom
(CLLBC) scintillator detector sensitive to both
gamma-rays and neutrons, which we dub Neutron Gamma LAMP (NG-LAMP). NG-LAMP
enables simultaneous neutron and gamma-ray mapping with high resolution
gamma-ray spectroscopy. We demonstrate the ability to detect and localize
surrogate Special Nuclear Materials (SNM) in real-time and in 3D based on
neutron signatures alone, which is critical for the detection of heavily
shielded SNM, when gamma-ray signatures are attenuated. In this work, we show
for the first time the ability to localize, in 3D and realtime, a neutron
source in the presence of a strong gamma-ray source, simultaneous and
spectroscopic localization of three gamma-ray sources and a neutron source, and
finally the localization of a surrogate SNM source based on neutron signatures
alone, where gamma-ray data are consistent with background
DE Canum Venaticorum : a bright, eclipsing red dwarf–white dwarf binary
Context. Close white dwarf–red dwarf binaries must have gone through a common-envelope phase during their evolution. DE CVn is a detached white dwarf–red dwarf binary with a relatively short (∼8.7 h) orbital period. Its brightness and the presence of eclipses makes this system ideal for a more detailed study.
Aims. From a study of photometric and spectroscopic observations of DE CVn we derive the system parameters that we discuss in the framework of common-envelope evolution.
Methods. Photometric observations of the eclipses are used to determine an accurate ephemeris. From a model fit to an average lowresolution spectrum of DE CVn, we constrain the temperature of the white dwarf and the spectral type of the red dwarf. The eclipse light curve is analysed and combined with the radial velocity curve of the red dwarf determined from time-resolved spectroscopy to derive constraints on the inclination and the masses of the components in the system.
Results. The derived ephemeris is HJDmin = 2 452 784.5533(1) + 0.3641394(2) × E. The red dwarf in DE CVn has a spectral type of M3V and the white dwarf has an effective temperature of 8 000 K. The inclination of the system is 86+3◦ −2 and the mass and radius of the red dwarf are 0.41 ± 0.06 M and 0.37+0.06 −0.007 R, respectively, and the mass and radius of the white dwarf are 0.51+0.06
−0.02 M and 0.0136+0.0008 −0.0002 R, respectively.
Conclusions. We found that the white dwarf has a hydrogen-rich atmosphere (DA-type). Given that DE CVn has experienced a common-envelope phase, we can reconstruct its evolution and we find that the progenitor of the white dwarf was a relatively lowmass star (M ≤ 1.6 M). The current age of this system is 3.3−7.3 × 109 years, while it will take longer than the Hubble time for DE CVn to evolve into a semi-detached system
A Detailed Investigation of the Proposed NN Serpentis Planetary System
The post-main sequence eclipsing binary NN Serpentis was recently announced
as the potential host of at least two massive planetary companions. In that
work, the authors put forward two potential architectures that fit the
observations of the eclipsing binary with almost identical precision. In this
work, we present the results of a dynamical investigation of the orbital
stability of both proposed system architectures, finding that they are only
stable for scenarios in which the planets are locked in mutual mean motion
resonance. In the discovery work, the authors artificially fixed the orbital
eccentricity of the more massive planet, NN Ser(AB) c, at 0. Here, we reanalyse
the observational data on NN Serpentis without this artificial constraint, and
derive a new orbital solution for the two proposed planets. We detail the
results of further dynamical simulations investigating the stability of our new
orbital solution, and find that allowing a small non-zero eccentricity for the
outer planet renders the system unstable. We conclude that, although the
original orbits proposed for the NN Serpentis planetary system prove
dynamically feasible, further observations of the system are vital in order to
better constrain the system's true architecture.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society; 5 figures, 2 table
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