2,507 research outputs found
First-order transition between a small-gap semiconductor and a ferromagnetic metal in the isoelectronic alloys FeSiGe
The contrasting groundstates of isoelectronic and isostructural FeSi and FeGe
can be explained within an extended local density approximation scheme (LDA+U)
by an appropriate choice of the onsite Coulomb repulsion, on the Fe-sites.
A minimal two-band model with interband interactions allows us to obtain a
phase diagram for the alloys FeSiGe. Treating the model in a mean
field approximation, gives a first order transition between a small-gap
semiconductor and a ferromagnetic metal as a function of magnetic field,
temperature, and concentration, . Unusually the transition from metal to
insulator is driven by broadening, not narrowing, the bands and it is the
metallic state that shows magnetic order.Comment: 4 pages, 5 figure
Dense Cloud Formation and Star Formation in a Barred Galaxy
We investigate the properties of massive, dense clouds formed in a barred
galaxy and their possible relation to star formation, performing a
two-dimensional hydrodynamical simulation with the gravitational potential
obtained from the 2Mass data from the barred spiral galaxy, M83. Since the
environment for cloud formation and evolution in the bar region is expected to
be different from that in the spiral arm region, barred galaxies are a good
target to study the environmental effects on cloud formation and the subsequent
star formation. Our simulation uses for an initial 80 Myr an isothermal flow of
non-self gravitating gas in the barred potential, then including radiative
cooling, heating and self-gravitation of the gas for the next 40 Myr, during
which dense clumps are formed. We identify many cold, dense gas clumps for
which the mass is more than (a value corresponding to the
molecular clouds) and study the physical properties of these clumps. The
relation of the velocity dispersion of the identified clump's internal motion
with the clump size is similar to that observed in the molecular clouds of our
Galaxy. We find that the virial parameters for clumps in the bar region are
larger than that in the spiral arm region. From our numerical results, we
estimate star formation in the bar and spiral arm regions by applying the
simple model of Krumholtz and McKee (2005). The mean relation between star
formation rate and gas surface density agrees well with the observed
Kennicutt-Schmidt relation. The SFE in the bar region is of the
spiral arm region. This trend is consistent with observations of barred
galaxies.Comment: 9 pages, 16 figures. Accepted for publication in the MNRA
Structure of a Bathtub Vortex : Importance of the Bottom Boundary Layer
A bathtub vortex in a cylindrical tank rotating at a constant angular velocity [omega] is studied by meansof a laboratory experiment, a numerical experiment and a boundary layer theory. The laboratory and numerical experiments show that two regimes of vortices in the steady-state can occur depending on [omega] and the volume flux Q through the drain hole: when Q is large and [omega] is small, a potential vortex is formed in which angular momentum outside the vortex core is constant in the non-rotating frame. However, when Q is small or [omega] is large, a vortex is generated in which the angular momentum decreases with decreasing radius. Boundary layertheory shows that the vortex regimes strongly depend on the theoretical radial volume flux through the bottomboundary layer under a potential vortex : when the ratio of Q to the theoretical boundary-layer radial volume flux Qb (scaled by 2Ï R2([omega] Îœ)12 ) at the outer rim of the vortex core is larger than a critical value (of order 1), the radial flow in the interior exists at all radiiand Regime I is realized, where R is the inner radius of the tank and Îœ the kinematicviscosity.When the ratio is less than the critical value, the radial flow in the interior nearlyvanishes inside a critical radius and almost all of the radial volume flux occurs only in the boundary layer,resulting in Regime II in which the angular momentum is not constant with radius. This criterion is found to explain the results of the laboratory and numerical experiments very well
Giant Pulses with Nanosecond Time Resolution detected from the Crab Pulsar at 8.5 and 15.1 GHz
We present a study of shape, spectra and polarization properties of giant
pulses (GPs) from the Crab pulsar at the very high frequencies of 8.5 and 15.1
GHz. Studies at 15.1 GHz were performed for the first time. Observations were
conducted with the 100-m radio telescope in Effelsberg in Oct-Nov 2007 at the
frequencies of 8.5 and 15.1 GHz as part of an extensive campaign of
multi-station multi-frequency observations of the Crab pulsar. A selection of
the strongest pulses was recorded with a new data acquisition system, based on
a fast digital oscilloscope, providing nanosecond time resolution in two
polarizations in a bandwidth of about 500 MHz. We analyzed the pulse shapes,
polarisation and dynamic spectra of GPs as well as the cross-correlations
between their LHC and RHC signals. No events were detected outside main pulse
and interpulse windows. GP properties were found to be very different for GPs
emitted at longitudes of the main pulse and the interpulse. Cross-correlations
of the LHC and RHC signals show regular patterns in the frequency domain for
the main pulse, but these are missing for the interpulse GPs. We consider
consequences of application of the rotating vector model to explain the
apparent smooth variation in the position angle of linear polarization for main
pulse GPs.
We also introduce a new scenario of GP generation as a direct consequence of
the polar cap discharge. We find further evidence for strong nano-shot
discharges in the magnetosphere of the Crab pulsar. The repetitive frequency
spectrum seen in GPs at the main pulse phase is interpreted as a diffraction
pattern of regular structures in the emission region. The interpulse GPs
however have a spectrum that resembles that of amplitude modulated noise.
Propagation effects may be the cause of the differences.Comment: Astronomy & Astrophysics (accepted
Glassy Transition and Aging in a Model without Disorder
We study the off-equilibrium relaxational dynamics of the Amit-Roginsky
field theory, for which the mode coupling approximation is exact. We
show that complex phenomena such as aging and ergodicity breaking are present
at low temperature, similarly to what is found in long range spin glasses. This
is a generalization of mode coupling theory of the structural glass transition
to off-equilibrium situations.Comment: 9 pages, 1 uuencoded figure, LaTex, preprint NORDITA 94/3
Allocation in Practice
How do we allocate scarcere sources? How do we fairly allocate costs? These
are two pressing challenges facing society today. I discuss two recent projects
at NICTA concerning resource and cost allocation. In the first, we have been
working with FoodBank Local, a social startup working in collaboration with
food bank charities around the world to optimise the logistics of collecting
and distributing donated food. Before we can distribute this food, we must
decide how to allocate it to different charities and food kitchens. This gives
rise to a fair division problem with several new dimensions, rarely considered
in the literature. In the second, we have been looking at cost allocation
within the distribution network of a large multinational company. This also has
several new dimensions rarely considered in the literature.Comment: To appear in Proc. of 37th edition of the German Conference on
Artificial Intelligence (KI 2014), Springer LNC
Causes of death among undocumented migrants in Sweden, 1997â2010
Background: Undocumented migrants are one of the most vulnerable groups in Swedish society, where they generally suffer from poor health and limited health care access. Due to their irregular status, such migrants are an under-researched group and are not included in the country's Cause of Death Register (CDR). Objective: To determine the causes of death among undocumented migrants in Sweden and to ascertain whether there are patterns in causes of death that differ between residents and undocumented migrants. Design: This is a cross-sectional study of death certificates issued from 1997 to 2010 but never included in the CDR from which we established our study sample of undocumented migrants. As age adjustments could not be performed due to lack of data, comparisons between residents and undocumented migrants were made at specific age intervals, based on the study sample's mean age at death±a half standard deviation. Results: Out of 7,925 individuals surveyed, 860 were classified as likely to have been undocumented migrants. External causes (49.8%) were the most frequent cause of death, followed by circulatory system diseases, and then neoplasms. Undocumented migrants had a statistically significant increased risk of dying from external causes (odds ratio [OR] 3.57, 95% confidence interval [CI]: 2.83â4.52) and circulatory system diseases (OR 2.20, 95% CI: 1.73â2.82) compared to residents, and a lower risk of dying from neoplasms (OR 0.07, 95% CI: 0.04â0.14). Conclusions: We believe our study is the first to determine national figures on causes of death of undocumented migrants. We found inequity in health as substantial differences in causes of death between undocumented migrants and residents were seen. Legal ambiguities regarding health care provision must be addressed if equity in health is to be achieved in a country otherwise known for its universal health coverage
Separation between coherent and turbulent fluctuations. What can we learn from the Empirical Mode Decomposition?
The performances of a new data processing technique, namely the Empirical
Mode Decomposition, are evaluated on a fully developed turbulent velocity
signal perturbed by a numerical forcing which mimics a long-period flapping.
First, we introduce a "resemblance" criterion to discriminate between the
polluted and the unpolluted modes extracted from the perturbed velocity signal
by means of the Empirical Mode Decomposition algorithm. A rejection procedure,
playing, somehow, the role of a high-pass filter, is then designed in order to
infer the original velocity signal from the perturbed one. The quality of this
recovering procedure is extensively evaluated in the case of a "mono-component"
perturbation (sine wave) by varying both the amplitude and the frequency of the
perturbation. An excellent agreement between the recovered and the reference
velocity signals is found, even though some discrepancies are observed when the
perturbation frequency overlaps the frequency range corresponding to the
energy-containing eddies as emphasized by both the energy spectrum and the
structure functions. Finally, our recovering procedure is successfully
performed on a time-dependent perturbation (linear chirp) covering a broad
range of frequencies.Comment: 23 pages, 13 figures, submitted to Experiments in Fluid
Ageing memory and glassiness of a driven vortex system
Many systems in nature, glasses, interfaces and fractures being some
examples, cannot equilibrate with their environment, which gives rise to novel
and surprising behaviour such as memory effects, ageing and nonlinear dynamics.
Unlike their equilibrated counterparts, the dynamics of out-of- equilibrium
systems is generally too complex to be captured by simple macroscopic laws.
Here we investigate a system that straddles the boundary between glass and
crystal: a Bragg glass formed by vortices in a superconductor. We find that the
response to an applied force evolves according to a stretched exponential, with
the exponent reflecting the deviation from equilibrium. After the force is
removed, the system ages with time and its subsequent response time scales
linearly with its age (simple ageing), meaning that older systems are slower
than younger ones. We show that simple ageing can occur naturally in the
presence of sufficient quenched disorder. Moreover, the hierarchical
distribution of timescales, arising when chunks of loose vortices cannot move
before trapped ones become dislodged, leads to a stretched-exponential
response.Comment: 16 pages, 5 figure
Realizing a 140\ua0GHz Gap WaveguideâBased Array Antenna by Low-Cost Injection Molding and Micromachining
This paper presents a novel micromachining process to fabricate a 140\ua0GHz planar antenna based on gap waveguide technology to be used in the next-generation backhauling links. The 140\ua0GHz planar array antenna consists of three layers, all of which have been fabricated using polymer-based microfabrication and injection molding. The 140\ua0GHz antenna has the potential to be used as an element in a bigger 3D array in a line-of-sight (LOS) multiple input multiple output (MIMO) configuration to boost the network capacity. In this work, we focus on the fabrication of a single antenna array element based on gap waveguide technology. Depending on the complexity of each antenna layerâs design, three different micromachining techniques, SU8 fabrication, polydimethylsiloxane (PDMS) molding, and injection molding of the polymer (OSTEMER), together with gold (Au) coating, have been utilized to fabricate a single 140\ua0GHz planar array antenna. The input reflection coefficient was measured to be below â 11\ua0dB over a 14% bandwidth from 132 to 152\ua0GHz, and the antenna gain was measured to be 31 dBi at 140\ua0GHz, both of which are in good agreement with the simulations
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