4,106 research outputs found
First-principles calculations of exchange interactions, spin waves, and temperature dependence of magnetization in inverse-Heusler-based spin gapless semiconductors
Employing first principles electronic structure calculations in conjunction
with the frozen-magnon method we calculate exchange interactions, spin-wave
dispersion, and spin-wave stiffness constants in inverse-Heusler-based spin
gapless semiconductor (SGS) compounds MnCoAl, TiMnAl, CrZnSi,
TiCoSi and TiVAs. We find that their magnetic behavior is similar to
the half-metallic ferromagnetic full-Heusler alloys, i.e., the intersublattice
exchange interactions play an essential role in the formation of the magnetic
ground state and in determining the Curie temperature, . All
compounds, except TiCoSi possess a ferrimagnetic ground state. Due to the
finite energy gap in one spin channel, the exchange interactions decay sharply
with the distance, and hence magnetism of these SGSs can be described
considering only nearest and next-nearest neighbor exchange interactions. The
calculated spin-wave dispersion curves are typical for ferrimagnets and
ferromagnets. The spin-wave stiffness constants turn out to be larger than
those of the elementary 3-ferromagnets. Calculated exchange parameters are
used as input to determine the temperature dependence of the magnetization and
of the SGSs. We find that the of all compounds is
much above the room temperature. The calculated magnetization curve for
MnCoAl as well as the Curie temperature are in very good agreement with
available experimental data. The present study is expected to pave the way for
a deeper understanding of the magnetic properties of the inverse-Heusler-based
SGSs and enhance the interest in these materials for application in spintronic
and magnetoelectronic devices.Comment: Accepted for publ;ication in Physical Review
Coercion-resistant Proxy Voting
In general, most elections follow the principle of equality, or as it came to be known, the principle of “one man – one vote”. However, this principle might pose difficulties for voters, who are not well informed regarding the particular matter that is voted on. In order to address this issue, a new form of voting has been proposed, namely proxy voting. In proxy voting, each voter has the possibility to delegate her voting right to another voter, so called proxy, that she considers a trusted expert on the matter. In this paper we propose an end-to-end verifiable Internet voting scheme, which to the best of our knowledge is the first scheme to address voter coercion in the proxy voting setting
The host of the SN-less GRB 060505 in high resolution
The spiral host galaxy of GRB 060505 at z=0.089 was the site of a puzzling
long duration burst without an accompanying supernova. Studies of the burst
environment by Th\"one et al. (2008) suggested that this GRB came from the
collapse of a massive star and that the GRB site was a region with properties
different from the rest of the galaxy. We reobserved the galaxy in high spatial
resolution using the VIMOS integral-field unit (IFU) at the VLT with a spaxel
size of 0.67 arcsec. Furthermore, we use long slit high resolution data from
HIRES/Keck at two different slit positions covering the GRB site, the center of
the galaxy and an HII region next to the GRB region. We compare the properties
of different HII regions in the galaxy with the GRB site and study the global
and local kinematic properties of this galaxy. The resolved data show that the
GRB site has the lowest metallicity in the galaxy with around 1/3 Z_solar, but
its specific SFR (SSFR) of 7.4 M_solar/yr/L/L* and age (determined by the
Halpha EW) are similar to other HII regions in the host. The galaxy shows a
gradient in metallicity and SSFR from the bulge to the outskirts as it is
common for spiral galaxies. This gives further support to the theory that GRBs
prefer regions of higher star-formation and lower metallicity, which, in S-type
galaxies, are more easily found in the spiral arms than in the centre.
Kinematic measurements of the galaxy do not show evidence for large
perturbations but a minor merger in the past cannot be excluded. This study
confirms the collapsar origin of GRB060505 but reveals that the properties of
the HII region surrounding the GRB were not unique to that galaxy. Spatially
resolved observations are key to know the implications and interpretations of
unresolved GRB hosts observations at higher redshifts.Comment: 15 pages, 16 figures, 6 tables; resubmitted to MNRAS after minor
revision
Tuning the Curie temperature of FeCo compounds by tetragonal distortion
Combining density-functional theory calculations with a classical Monte Carlo
method, we show that for B2-type FeCo compounds tetragonal distortion gives
rise to a strong reduction of the Curie temperature . The
monotonically decreases from 1575 K (for ) to 940 K
(for c/a=\sqrtwo). We find that the nearest neighbor Fe-Co exchange
interaction is sufficient to explain the behavior of the
. Combination of high magnetocrystalline anisotropy energy with
a moderate value suggests tetragonal FeCo grown on the Rh
substrate with to be a promising material for heat-assisted magnetic
recording applications.Comment: 4 pages, 2 figure
Lens magnification by CL0024+1654 in the U and R band
[ABRIDGED] We estimate the total mass distribution of the galaxy cluster
CL0024+1654 from the measured source depletion due to lens magnification in the
R band. Within a radius of 0.54Mpc/h, a total projected mass of
(8.1+/-3.2)*10^14 M_sol/h (EdS) is measured, which corresponds to a mass-
to-light ratio of M/L(B)=470+/-180. We compute the luminosity function of
CL0024+1654 in order to estimate contamination of the background source counts
from cluster galaxies. Three different magnification-based reconstruction
methods are employed using both local and non-local techniques. We have
modified the standard single power-law slope number count theory to incorporate
a break and applied this to our observations. Fitting analytical magnification
profiles of different cluster models to the observed number counts, we find
that the cluster is best described either by a NFW model with scale radius
r_s=334+/-191 kpc/h and normalisation kappa_s=0.23+/-0.08 or a power-law
profile with slope xi=0.61+/-0.11, central surface mass density
kappa_0=1.52+/-0.20 and assuming a core radius of r_core=35 kpc/h. The NFW
model predicts that the cumulative projected mass contained within a radius R
scales as M(<R)=2.9*10^14*(R/1')^[1.3-0.5lg (R/1')] M_sol/h. Finally, we have
exploited the fact that flux magnification effectively enables us to probe
deeper than the physical limiting magnitude of our observations in searching
for a change of slope in the U band number counts. We rule out both a total
flattening of the counts with a break up to U_AB<=26.6 and a change of slope,
reported by some studies, from dlog N/dm=0.4->0.15 up to U_AB<=26.4 with 95%
confidence.Comment: 19 pages, 12 figures, submitted to A&A. New version includes more
robust U band break analysis and contamination estimates, plus new plot
Cast-as-Intended Mechanism with Return Codes Based on PETs
We propose a method providing cast-as-intended verifiability for remote
electronic voting. The method is based on plaintext equivalence tests (PETs),
used to match the cast ballots against the pre-generated encrypted code tables.
Our solution provides an attractive balance of security and functional
properties. It is based on well-known cryptographic building blocks and relies
on standard cryptographic assumptions, which allows for relatively simple
security analysis. Our scheme is designed with a built-in fine-grained
distributed trust mechanism based on threshold decryption. It, finally, imposes
only very little additional computational burden on the voting platform, which
is especially important when voters use devices of restricted computational
power such as mobile phones. At the same time, the computational cost on the
server side is very reasonable and scales well with the increasing ballot size
The Radio Afterglow and Host Galaxy of the Dark GRB 020819
Of the fourteen gamma-ray bursts (GRBs) localized to better than 2' radius
with the SXC on HETE-2, only two lack optical afterglow detections, and the
high recovery rate among this sample has been used to argue that the fraction
of truly dark bursts is ~10%. While a large fraction of earlier dark bursts can
be explained by the failure of ground-based searches to reach appropriate
limiting magnitudes, suppression of the optical light of these SXC dark bursts
seems likely. Here we report the discovery and observation of the radio
afterglow of GRB 020819, an SXC dark burst, which enables us to identify the
likely host galaxy (probability of 99.2%) and hence the redshift (z=0.41) of
the GRB. The radio light curve is qualitatively similar to that of several
other radio afterglows, and may include an early-time contribution from the
emission of the reverse shock. The proposed host is a bright R = 19.5 mag
barred spiral galaxy, with a faint R ~ 24.0 mag "blob'' of emission, 3" from
the galaxy core (16 kpc in projection), that is coincident with the radio
afterglow. Optical photometry of the galaxy and blob, beginning 3 hours after
the burst and extending over more than 100 days, establishes strong upper
limits to the optical brightness of any afterglow or associated supernova.
Combining the afterglow radio fluxes and our earliest R-band limit, we find
that the most likely afterglow model invokes a spherical expansion into a
constant-density (rather than stellar wind-like) external environment; within
the context of this model, a modest local extinction of A_V ~ 1 mag is
sufficient to suppress the optical flux below our limits.Comment: 7 pages, 2 figures. ApJ, in press. For more info on dark bursts, see
http://www.astro.ku.dk/~pallja/dark.htm
Diversity of multiwavelength emission bumps in the GRB 100219A afterglow
Context. Multi-wavelength observations of gamma-ray burst (GRB) afterglows
provide important information about the activity of their central engines and
their environments. In particular, the short timescale variability, such as
bumps and/or rebrightening features visible in the multi-wavelength light
curves, is still poorly understood.
Aims. We analyze the multi-wavelength observations of the GRB100219A
afterglow at redshift 4.7. In particular, we attempt to identify the physical
origin of the late achromatic flares/bumps detected in the X-ray and optical
bands.
Methods. We present ground-based optical photometric data and Swift X-ray
observations on GRB100219A. We analyzed the temporal behavior of the X-ray and
optical light curves, as well as the X-ray spectra.
Results. The early flares in the X-ray and optical light curves peak
simultaneously at about 1000 s after the burst trigger, while late achromatic
bumps in the X-ray and optical bands appear at about 20000 s after the burst
trigger. These are uncommon features in the afterglow phenomenology.
Considering the temporal and spectral properties, we argue that both optical
and X-ray emissions come from the same mechanism. The late flares/bumps may be
produced by late internal shocks from long-lasting activity of the central
engine. An off-axis origin for a structured jet model is also discussed to
interpret the bump shapes. The early optical bump can be interpreted as the
afterglow onset, while the early X-ray flare could be caused by the internal
activity. GRB 100219A exploded in a dense environment as revealed by the strong
attenuation of X-ray emission and the optical-to-X-ray spectral energy
distribution.Comment: A&A accepte
Subspace-based Fundamental Frequency Estimation
Publication in the conference proceedings of EUSIPCO, Viena, Austria, 200
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