3,611 research outputs found
Systematic Errors in Future Weak Lensing Surveys: Requirements and Prospects for Self-Calibration
We study the impact of systematic errors on planned weak lensing surveys and
compute the requirements on their contributions so that they are not a dominant
source of the cosmological parameter error budget. The generic types of error
we consider are multiplicative and additive errors in measurements of shear, as
well as photometric redshift errors. In general, more powerful surveys have
stronger systematic requirements. For example, for a SNAP-type survey the
multiplicative error in shear needs to be smaller than 1%(fsky/0.025)^{-1/2} of
the mean shear in any given redshift bin, while the centroids of photometric
redshift bins need to be known to better than 0.003(fsky/0.025)^{-1/2}. With
about a factor of two degradation in cosmological parameter errors, future
surveys can enter a self-calibration regime, where the mean systematic biases
are self-consistently determined from the survey and only higher-order moments
of the systematics contribute. Interestingly, once the power spectrum
measurements are combined with the bispectrum, the self-calibration regime in
the variation of the equation of state of dark energy w_a is attained with only
a 20-30% error degradation.Comment: 20 pages, 9 figures, to be submitted to MNRAS. Comments are welcom
59Co-NMR Knight Shift of Superconducting Three-Layer NaxCoO2.yH2O
The superconducting state of NaxCoO2.yH2O with three CoO2 layers in a unit
cell has been studied by 59Co-NMR. The Knight shift measured for a peak of the
NMR spectra corresponding to the external magnetic field H along one of the
principal directions within the CoO2 plane, exhibits a rapid decrease with
decreasing temperature T below the superconducting transition temperature Tc,
indicating that the spin susceptibility is suppressed in the superconducting
phase, at least, for this field direction. Because differences of the
superconducting properties are rather small between this three-layer
NaxCoO2.yH2O and previously reported NaxCoO2.yH2O with two CoO2 layers within a
unit cell, the present result of the Knight shift studies indicates that the
Cooper pairs of the former system are in the singlet state as in the latter,
for which the spin susceptibility is suppressed for both directions of H
parallel and perpendicular to the CoO2 plane.Comment: 5 page
The Skewness of the Aperture Mass Statistic
We present simple formulae for calculating the skewness and kurtosis of the
aperture mass statistic for weak lensing surveys which is insensitive to
masking effects of survey geometry or variable survey depth. The calculation is
the higher order analog of the formula given by Schneider et al (2002) which
has been used to compute the variance of the aperture mass from several lensing
surveys. As our formula requires the three-point shear correlation function, we
also present an efficient tree-based algorithm for measuring it. We show how
our algorithm would scale in computing time and memory usage for future lensing
surveys. Finally, we apply the procedure to our CTIO survey data, originally
described in Jarvis et al (2003). We find that the skewness is positive
(inconsistent with zero) at the 2 sigma level. However, the signal is too noisy
from this data to usefully constrain cosmology.Comment: 16 pages, accepted by MNRAS. Minor revisions; this replacement
matches the accepted versio
The Surprisingly Steep Mass Profile of Abell 1689, from a Lensing Analysis of Subaru Images
Subaru observations of A1689 (z=0.183) are used to derive an accurate,
model-independent mass profile for the entire cluster, r<2 Mpc/h, by combining
magnification bias and distortion measurements. The projected mass profile
steepens quickly with increasing radius, falling away to zero at r~1.0 Mpc/h,
well short of the anticipated virial radius. Our profile accurately matches
onto the inner profile, r<200 kpc/h, derived from deep HST/ACS images. The
combined ACS and Subaru information is well fitted by an NFW profile with
virial mass, (1.93 \pm 0.20)10^15 M_sun, and surprisingly high concentration,
c_vir=13.7^{+1.4}_{-1.1}, significantly larger than theoretically expected
(c_vir~4), corresponding to a relatively steep overall profile. A slightly
better fit is achieved with a steep power-law model that has its 2D logarithmic
slope -3 and core radius theta_c~1.7' (r_c~210 kpc/h), whereas an isothermal
profile is strongly rejected. These results are based on a reliable sample of
background galaxies selected to be redder than the cluster E/S0 sequence. By
including the faint blue galaxy population a much smaller distortion signal is
found, demonstrating that blue cluster members significantly dilute the true
signal for r~400 kpc/h. This contamination is likely to affect most weak
lensing results to date.Comment: 5 pages, 3 figures, to appear in ApJ
Possibility of f-wave spin-triplet superconductivity in the CoO superconductor: a case study on a 2D triangular lattice in the repulsive Hubbard model
Stimulated by the recent finding of NaCoO.1.3HO
superconductor, we investigate superconducting instabilities on a 2D triangular
lattice in the repulsive Hubbard model. Using the third-order perturbation
expansion with respect to the on-site repulsion , we evaluate the linearized
Dyson-Gor'kov equation. We find that an -wave spin-triplet pairing is the
most stable in a wide range of the next nearest neighbor hopping integral
and an electron number density . The introduction of is crucial to
adjust the van Hove singularities to the neighborhood of the Fermi surface
crossing around K point. In this case, the bare spin susceptibility shows the
broad peak around point. These conditions stabilize the -wave
pairing. Although the -wave pairing is also given by the
fluctuation-exchange approximation, the transition temperature is too low to be
observed. This is because the depairing effect by the spin fluctuation is
over-estimated. Thus, the third-order vertex corrections are important for the
spin-triplet superconductivity, like the case in SrRuO.Comment: 4 pages, 7 figure
Effects of two dimensional plasmons on the tunneling density of states
We show that gapless plasmons lead to a universal
correction to the tunneling
density of states of a clean two dimensional Coulomb interacting electron gas.
We also discuss a counterpart of this effect in the "composite fermion metal"
which forms in the presence of a quantizing perpendicular magnetic field
corresponding to the half-filled Landau level. We argue that the latter
phenomenon might be relevant for deviations from a simple scaling observed by
A.Chang et al in the tunneling characteristics of Quantum Hall liquids.Comment: 12 pages, Latex, NORDITA repor
Behavior of a frustrated quantum spin chain with bond dimerization
We clarified behavior of the excitation gap in a frustrated S=1/2 quantum
spin chain with bond dimerization by using the numerical diagonalization of
finite systems and a variational approach. The model interpolates between the
independent dimer model and the S=1 spin chain by changing a strength of the
dimerization. The energy gap is minimum at the fully-frustrated point, where a
localized kink and a freely mobile anti-kink govern the low-lying excitations.
Away from the point, a kink and an antikink form a bound state by an effective
triangular potential between them. The consequential gap enhancement and the
localization length of the bound state is obtained exactly in the continuous
limit. The gap enhancement obeys a power law with exponent 2/3. The method and
the obtained results are common to other frustrated double spin-chain systems,
such as the one-dimensional J_1 - J_2 model, or the frustrated ladder model.Comment: 11 pages, REVTeX, 8 figures in eps-fil
Galactic chemical evolution : Carbon through zinc
Copyright © 2006. The American Astronomical Society. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the final published version of the work, which was originally published at https://doi.org/10.1086/508914We calculate the evolution of heavy-element abundances from C to Zn in the solar neighborhood, adopting our new nucleosynthesis yields. Our yields are calculated for wide ranges of metallicity (Z = 0-Z circle dot) and the explosion energy (normal supernovae and hypernovae), based on the light-curve and spectra fitting of individual supernovae. The elemental abundance ratios are in good agreement with observations. Among the alpha-elements, O, Mg, Si, S, and Ca show a plateau at [Fe/H] <= -1, while Ti is underabundant overall. The observed abundance of Zn ([Zn/Fe] similar to 0) can be explained only by the high-energy explosion models, as it requires a large contribution of hypernovae. The observed decrease in the odd-Z elements (Na, Al, and Cu) toward low [Fe/H] is reproduced by the metallicity effect on nucleosynthesis. The iron-peak elements (Cr, Mn, Co, and Ni) are consistent with the observed mean values at -2.5 less than or similar to [Fe/H] less than or similar to -1, and the observed trend at the lower metallicity can be explained by the energy effect. We also show the abundance ratios and the metallicity distribution functions of the Galactic bulge, halo, and thick disk. Our results suggest that the formation timescale of the thick disk is similar to 1-3 Gyr.Peer reviewe
NaxCoO2: Enhanced low-energy excitations of electrons on a 2D triangular lattice
To elucidate the low-energy excitation spectrum of correlated electrons on a
2D triangular lattice, we have studied the electrical resistance and specific
heat down to 0.5 K and in magnetic fields up to 14 T, in NaxCoO2 samples with a
Na content ranging from x \approx 0.5 to 0.82. Two distinct regimes are
observed: for x from about 0.6 to x \approx 0.75 the specific heat is strongly
enhanced, with a pronounced upturn of C/T below about 10 K, reaching 47 mJ/(mol
K^2). This enhancement is suppressed in a magnetic field indicative of strong
low-energy spin fluctuations. At higher Na content the fluctuations are reduced
and mu-SR data confirm the SDW ground state below 22 K and the much reduced
heat capacity is field independent.Comment: Accepted in Physica
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