229 research outputs found
Rejuvenating Power Spectra II: the Gaussianized galaxy density field
We find that, even in the presence of discreteness noise, a Gaussianizing
transform (producing a more-Gaussian one-point distribution) reduces
nonlinearities in the power spectra of cosmological matter and galaxy density
fields, in many cases drastically. Although Gaussianization does increase the
effective shot noise, it also increases the power spectrum's fidelity to the
linear power spectrum on scales where the shot noise is negligible.
Gaussianizing also increases the Fisher information in the power spectrum in
all cases and resolutions, although the gains are smaller in redshift space
than in real space. We also find that the gain in cumulative Fisher information
from Gaussianizing peaks at a particular grid resolution that depends on the
sampling level.Comment: Slight changes to match version accepted to ApJ. 7 pages, 8 figure
Observation of Changes in the Atomic and Electronic Structure of Single-Crystal YBa₂Cu₃O₆.₆ Accompanying Bromination
To ascertain the role of bromination in the recovery of superconductivity in underdoped YBa2Cu3O6+y (YBCO), we have performed polarized multiple-edge x-ray-absorption fine structure (XAFS) measurements on normal (y~0.6) and brominated (Br/Cu~1/30, y~0.6) single crystals with superconducting transitions at 63 and 89 K, respectively. The brominated sample becomes strongly heterogeneous on an atomic length scale. Approximately one-third of YBCO is locally decomposed yet incorporated as a well-ordered host lattice as nanoscale regions. The decomposed phase consists of heavily distorted domains with an order not following that of the host lattice. Structurally, these domains are fragments of the YBCO lattice that are discontinued along the Cu(1)-O(1) containing planes. The local structure is consistent with the cluster expansions: Y-O(2,3)8-Cu(2)8-..., Ba-O8-Cu(2)4Cu(1)2-..., and Cu-O4... about the Y, Ba, and Cu sites. Interatomic distances and Debye-Waller factors for the expansions were determined from fits to Y K-, Ba L3-, and Cu K-edge XAFS data at room temperature. Br K-edge data reveal that Br does not enter substitutionally or interstitially into the perfect YBCO lattice. However, Br does occupy the Cu(1) sites in a nanofragment of the YBCO lattice, forming Br-O(4)-Ba-Cu2(1)Cu(2)-... nanoclusters. From polarized measurements these nanoclusters were found to be almost randomly oriented with respect to the host crystal, and probably are the nucleus of the decomposed phase. This heterogeneity brings about the unusual structural and electronic properties of the normal state previously reported in the literature. Implications on for diffraction, transport, and magnetization measurements are discussed
Hydrodynamic Simulation of the Cosmological X-ray Background
(Abridged) We use a hydrodynamic simulation of a LambdaCDM model to predict
the extragalactic X-ray background (XRB), focussing on emission from the
intergalactic medium (IGM). We also include X-rays from point sources
associated with galaxies in the simulation, and make maps of the angular
distribution of the emission. We find that filaments in the maps are not
evident, being diluted by projection. In the soft (0.5-2 keV) band, the mean
intensity of radiation from intergalactic and cluster gas is 2.3*10^-12
ergdeg^-2cm^-2s^-1, 35% of the total soft band emission. This is compatible at
the ~1 sigma level with estimates of the unresolved soft background from ROSAT
and {\it Chandra}. Only 4% of the hard (2-10 keV) emission is associated with
the IGM. Relative to AGN flux, the IGM component peaks at a lower redshift
(median z~0.45) so its clustering makes an important contribution to that of
the total XRB. The angular correlations on 0.1-10 arcmin scales are
significant, with an amplitude roughly consistent with an extrapolation of
recent ROSAT results to small scales. A cross-correlation of the XRB against
nearby galaxies taken from a simulated redshift survey also yields a strong
signal from the IGM. Although some recent papers have argued that the expected
soft band intensity from gas in galaxy, group, and cluster halos would exceed
XRB limits unless much of the gas is expelled by supernova feedback, we obtain
reasonable compatibility with current observations in a simulation that
incorporates cooling, star formation, and only modest feedback. A prediction of
our model is that the unresolved portion of the soft XRB will remain mostly
unresolved.Comment: Improved referencing of related papers. Submitted to ApJ, 19 pages,
17 postscript figures, most reduced in resolution, emulateapj.sty, for full
resolution version, see http://cfa-www.harvard.edu/~rcroft/xray.ps.g
Designing Polar and Magnetic Oxides: Zn2FeTaO6 - in Search of Multiferroics
Polar oxides are technically of great interest but difficult to prepare. Our recent discoveries predicted that polar oxides can be synthesized in the corundum-derivative A2BB′O6 family with unusually small cations at the A-site and a d0 electron configuration ion at B′-site. When magnetic transition-metal ions are incorporated more interesting polar magnetic oxides can form. In this work we experimentally verified this prediction and prepared LiNbO3 (LN)-type polar magnetic Zn2FeTaO6 via high pressure and temperature synthesis. The crystal structure analysis indicates highly distorted ZnO6 and (Fe/Ta)O6 octahedra, and an estimated spontaneous polarization (PS) of ∼50 μC/cm2 along the c-axis was obtained from point charge model calculations. Zn2Fe3+Ta5+O6 has a lower magnetic transition temperature (TN ∼ 22 K) than the Mn2FeTaO6 analogue but is less conductive. The dielectric and polarization measurements indicate a potentially switchable component
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Designing Polar and Magnetic Oxides: Zn2FeTaO6 - in Search of Multiferroics
Polar oxides are technically of great interest but difficult to prepare. Our recent discoveries predicted that polar oxides can be synthesized in the corundum-derivative A2BB′O6 family with unusually small cations at the A-site and a d0 electron configuration ion at B′-site. When magnetic transition-metal ions are incorporated more interesting polar magnetic oxides can form. In this work we experimentally verified this prediction and prepared LiNbO3 (LN)-type polar magnetic Zn2FeTaO6 via high pressure and temperature synthesis. The crystal structure analysis indicates highly distorted ZnO6 and (Fe/Ta)O6 octahedra, and an estimated spontaneous polarization (PS) of ∼50 μC/cm2 along the c-axis was obtained from point charge model calculations. Zn2Fe3+Ta5+O6 has a lower magnetic transition temperature (TN ∼ 22 K) than the Mn2FeTaO6 analogue but is less conductive. The dielectric and polarization measurements indicate a potentially switchable component
Acute Exposure to Terrestrial Trunked Radio (TETRA) has effects on the electroencephalogram and electrocardiogram, consistent with vagal nerve stimulation
BACKGROUND: Terrestrial Trunked Radio (TETRA) is a telecommunications system widely used by police and emergency services around the world. The Stewart Report on mobile telephony and health raised questions about possible health effects associated with TETRA signals. This study investigates possible effects of TETRA signals on the electroencephalogram and electrocardiogram in human volunteers. METHODS: Blinded randomized provocation study with a standardized TETRA signal or sham exposure. In the first of two experiments, police officers had a TETRA set placed first against the left temple and then the upper-left quadrant of the chest and the electroencephalogram was recorded during rest and active cognitive processing. In the second experiment, volunteers were subject to chest exposure of TETRA whilst their electroencephalogram and heart rate variability derived from the electrocardiogram were recorded. RESULTS: In the first experiment, we found that exposure to TETRA had consistent neurophysiological effects on the electroencephalogram, but only during chest exposure, in a pattern suggestive of vagal nerve stimulation. In the second experiment, we observed changes in heart rate variability during exposure to TETRA but the electroencephalogram effects were not replicated. CONCLUSIONS: Observed effects of exposure to TETRA signals on the electroencephalogram (first experiment) and electrocardiogram are consistent with vagal nerve stimulation in the chest by TETRA. However given the small effect on heart rate variability and the lack of consistency on the electroencephalogram, it seems unlikely that this will have a significant impact on health. Long-term monitoring of the health of the police force in relation to TETRA use is on-going
WMAP constraints on inflationary models with global defects
We use the cosmic microwave background angular power spectra to place upper
limits on the degree to which global defects may have aided cosmic structure
formation. We explore this under the inflationary paradigm, but with the
addition of textures resulting from the breaking of a global O(4) symmetry
during the early stages of the Universe. As a measure of their contribution, we
use the fraction of the temperature power spectrum that is attributed to the
defects at a multipole of 10. However, we find a parameter degeneracy enabling
a fit to the first-year WMAP data to be made even with a significant defect
fraction. This degeneracy involves the baryon fraction and the Hubble constant,
plus the normalization and tilt of the primordial power spectrum. Hence,
constraints on these cosmological parameters are weakened. Combining the WMAP
data with a constraint on the physical baryon fraction from big bang
nucleosynthesis calculations and high-redshift deuterium abundance, limits the
extent of the degeneracy and gives an upper bound on the defect fraction of
0.13 (95% confidence).Comment: 10pp LaTeX/RevTeX, 6 eps figs; matches accepted versio
Cacao para la paz
Saber cómo funciona la cadena de valor de cacao en Colombia; Reconocer el rol del cacao en las economÃas rurales sostenibles; Conocer los puntos claves para mejorar la competitividad; Determinar las fortalezas del sector agrÃcola de Colombia; Identificar los limitantes para el acceso y sostenibilidad en mercados de alto valor; Orientar futuras inversiones de la cooperación internacional en cacao
Cacao para la Paz: Un Análisis de la Cadena Productiva de Cacao en Colombia
U.S. Agency for International Developmen
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