906 research outputs found
Magnetic properties of single-crystalline CeCuGa3
The magnetic behavior of single-crystalline CeCuGa3 has been investigated.
The compound forms in a tetragonal BaAl4-type structure consisting of
rare-earth planes separated by Cu-Ga layers. If the Cu-Ga site disorder is
reduced, CeCuGa3 adopts the related, likewise tetragonal BaNiSn3-type
structure, in which the Ce ion are surrounded by different Cu and Ga layers and
the inversion symmetry is lost. In the literature conflicting reports about the
magnetic order of CeCuGa3 have been published. Single crystals with the
centrosymmetric structure variant exhibit ferromagnetic order below approx. 4 K
with a strong planar anisotropy. The magnetic behavior above the transition
temperature can be well understood by the crystal-field splitting of the 4f
Hund's rule ground-state multiplet of the Ce ions
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A Photometric Redshift of z ~ 9.4 for GRB 090429B
Gamma-ray bursts (GRBs) serve as powerful probes of the early universe, with their luminous afterglows revealing the locations and physical properties of star-forming galaxies at the highest redshifts, and potentially locating first-generation (Population III) stars. Since GRB afterglows have intrinsically very simple spectra, they allow robust redshifts from low signal-to-noise spectroscopy, or photometry. Here we present a photometric redshift of z ~ 9.4 for the Swift detected GRB 090429B based on deep observations with Gemini-North, the Very Large Telescope, and the GRB Optical and Near-infrared Detector. Assuming a Small Magellanic Cloud dust law (which has been found in a majority of GRB sight lines), the 90% likelihood range for the redshift is 9.06 7. The non-detection of the host galaxy to deep limits (Y(AB) ~ 28, which would correspond roughly to 0.001L* at z = 1) in our late-time optical and infrared observations with the Hubble Space Telescope strongly supports the extreme-redshift origin of GRB 090429B, since we would expect to have detected any low-z galaxy, even if it were highly dusty. Finally, the energetics of GRB 090429B are comparable to those of other GRBs and suggest that its progenitor is not greatly different from those of lower redshift bursts
Radiation-induced oscillatory magnetoresistance as a sensitive probe of the zero-field spin splitting in high mobility GaAs/AlGaAs devices
We suggest an approach for characterizing the zero-field spin splitting of
high mobility two-dimensional electron systems, when beats are not readily
observable in the Shubnikov-de Haas effect. The zero-field spin splitting and
the effective magnetic field seen in the reference frame of the electron is
evaluated from a quantitative study of beats observed in radiation-induced
magnetoresistance oscillations.Comment: 4 pages, 4 color figure
The Spectroscopic Classification and Explosion Properties of SN2009nz Associated with GRB091127 at z=0.490
We present spectroscopic observations of GRB091127 (z=0.490) at the peak of
the putative associated supernova, SN2009nz. Subtracting a late-time spectrum
of the host galaxy, we isolate the contribution of SN2009nz and uncover broad
features typical of nearby GRB-SNe. This establishes unambiguously that
GRB091127 was accompanied by a broad-lined Type Ic SN, and links a cosmological
long burst with a standard energy release (E_gamma,iso ~ 1.1e52 erg) to a
massive star progenitor. The spectrum of SN2009nz closely resembles that of
SN2006aj, with SN2003dh also providing an acceptable match, but has
significantly narrower features than SNe 1998bw and 2010bh, indicative of a
lower expansion velocity. The photospheric velocity inferred from the SiII 6355
absorption feature, v_ph ~ 17,000 km/s, is indeed closer to that of SNe 2006aj
and 2003dh than to the other GRB-SNe. Combining the measured velocity with the
light curve peak brightness and width, we estimate the following explosion
parameters: M_Ni ~ 0.35 M_sun, E_K ~ 2.3e51 erg, and M_ej ~ 1.4 M_sun, similar
to those of SN2006aj. These properties indicate that SN2009nz follows a trend
of lower M_Ni for GRB-SNe with lower E_K and M_ej. Equally important, since
GRB091127 is a typical cosmological burst, the similarity of SN2009nz to
SN2006aj either casts doubt on the claim that XRF060218/SN2006aj was powered by
a neutron star, or indicates that the nature of the central engine is encoded
in the SN properties but not in the prompt emission. Future spectra of GRB-SNe
at z > 0.3, including proper subtraction of the host galaxy contribution, will
shed light on the full dispersion of SN properties for standard long GRBs, on
the relation between SNe associated with sub-energetic and standard GRBs, and
on a potential dispersion in the associated SN types.Comment: Submitted to ApJ; 11 pages; 2 tables; 4 figures; emulateapj styl
Observations of Milky Way Dwarf Spheroidal galaxies with the Fermi-LAT detector and constraints on Dark Matter models
We report on the observations of 14 dwarf spheroidal galaxies with the Fermi
Gamma-Ray Space Telescope taken during the first 11 months of survey mode
operations. The Fermi telescope provides a new opportunity to test particle
dark matter models through the expected gamma-ray emission produced by pair
annihilation of weakly interacting massive particles (WIMPs). Local Group dwarf
spheroidal galaxies, the largest galactic substructures predicted by the cold
dark matter scenario, are attractive targets for such indirect searches for
dark matter because they are nearby and among the most extreme dark matter
dominated environments. No significant gamma-ray emission was detected above
100 MeV from the candidate dwarf galaxies. We determine upper limits to the
gamma-ray flux assuming both power-law spectra and representative spectra from
WIMP annihilation. The resulting integral flux above 100 MeV is constrained to
be at a level below around 10^-9 photons cm^-2 s^-1. Using recent stellar
kinematic data, the gamma-ray flux limits are combined with improved
determinations of the dark matter density profile in 8 of the 14 candidate
dwarfs to place limits on the pair annihilation cross-section of WIMPs in
several widely studied extensions of the standard model. With the present data,
we are able to rule out large parts of the parameter space where the thermal
relic density is below the observed cosmological dark matter density and WIMPs
(neutralinos here) are dominantly produced non-thermally, e.g. in models where
supersymmetry breaking occurs via anomaly mediation. The gamma-ray limits
presented here also constrain some WIMP models proposed to explain the Fermi
and PAMELA e^+e^- data, including low-mass wino-like neutralinos and models
with TeV masses pair-annihilating into muon-antimuon pairs. (Abridged)Comment: 25 pages, 4 figures, accepted to ApJ, Corresponding authors: J.
Cohen-Tanugi, C. Farnier, T.E. Jeltema, E. Nuss, and S. Profum
Lateral Distribution of Muons in IceCube Cosmic Ray Events
In cosmic ray air showers, the muon lateral separation from the center of the
shower is a measure of the transverse momentum that the muon parent acquired in
the cosmic ray interaction. IceCube has observed cosmic ray interactions that
produce muons laterally separated by up to 400 m from the shower core, a factor
of 6 larger distance than previous measurements. These muons originate in high
pT (> 2 GeV/c) interactions from the incident cosmic ray, or high-energy
secondary interactions. The separation distribution shows a transition to a
power law at large values, indicating the presence of a hard pT component that
can be described by perturbative quantum chromodynamics. However, the rates and
the zenith angle distributions of these events are not well reproduced with the
cosmic ray models tested here, even those that include charm interactions. This
discrepancy may be explained by a larger fraction of kaons and charmed
particles than is currently incorporated in the simulations
TESLA Technical Design Report Part III: Physics at an e+e- Linear Collider
The TESLA Technical Design Report Part III: Physics at an e+e- Linear
ColliderComment: 192 pages, 131 figures. Some figures have reduced quality. Full
quality figures can be obtained from http://tesla.desy.de/tdr. Editors -
R.-D. Heuer, D.J. Miller, F. Richard, P.M. Zerwa
Spin dynamics in semiconductor nanocrystals
Time-resolved Faraday rotation is used to study both transverse and
longitudinal spin relaxation in chemically-synthesized CdSe nanocrystals (NCs)
22-80 Angstroms in diameter. The precession of optically-injected spins in a
transverse magnetic field occurs at distinct frequencies whose assignment to
electron and exciton spins is developed through systematic studies of the
size-dependence and theoretical calculations. It is shown that the transverse
spin lifetime is limited by inhomogeneous dephasing to a degree that cannot be
accounted for by the NC size distribution alone. Longitudinal spin relaxation
in these NCs occurs on several distinct timescales ranging from 100 ps-10
microseconds and exhibits markedly different dependencies on temperature and
field in comparison to transverse spin relaxation.Comment: 25 pages, 11 figures, tabl
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