27,907 research outputs found
On the Approximation in the Hermitian Treatment of Dyson Boson Expansion Theory
We discuss about the Hermitian treatment of Dyson-type boson expansion
theory. We show that the basic assumption of the conventional treatment does
not hold in general and the method is only approximately valid. We also show
that the approximation is the same order as that of truncation of the expansion
usually done in the Hermitian type boson expansion theory.Comment: 18 page, no figur
Superconductivity in three-layer Na0.3CoO2*1.3H2O
The observation of superconductivity at 4.3 K in a new crystalline form of
Na0.3CoO2*1.3H2O is reported. The new superconductor has three layers of CoO6
octahedra per crystallographic unit cell, in contrast to the previously
reported two-layer superconductor. The three-layer cell occurs because the
relative orientations of neighboring CoO2 layers are distinctly different from
what is seen in the two-layer superconducting phase. This type of structural
difference in materials that are otherwise chemically and structurally
identical is not possible to attain on the layered copper oxide
superconductors. The synthesis and stability of the new phase are described.Comment: 10 pages, 4 figures. Submitted to Physical Review Letter
The Impact of Non-Gaussian Errors on Weak Lensing Surveys
The weak lensing power spectrum carries cosmological information via its
dependence on the growth of structure and on geometric factors. Since much of
the cosmological information comes from scales affected by nonlinear
clustering, measurements of the lensing power spectrum can be degraded by
non-Gaussian covariances. Recently there have been conflicting studies about
the level of this degradation. We use the halo model to estimate it and include
new contributions related to the finite size of lensing surveys, following
Rimes and Hamilton's study of 3D simulations. We find that non-Gaussian
correlations between different multipoles can degrade the cumulative
signal-to-noise for the power spectrum amplitude by up to a factor of 2 (or 5
for a worst-case model that exceeds current N-body simulation predictions).
However, using an eight-parameter Fisher analysis we find that the marginalized
errors on individual parameters are degraded by less than 10% (or 20% for the
worst-case model). The smaller degradation in parameter accuracy is primarily
because: individual parameters in a high-dimensional parameter space are
degraded much less than the volume of the full Fisher ellipsoid; lensing
involves projections along the line of sight, which reduce the non-Gaussian
effect; some of the cosmological information comes from geometric factors which
are not degraded at all. We contrast our findings with those of Lee & Pen
(2008) who suggested a much larger degradation in information content. Finally,
our results give a useful guide for exploring survey design by giving the
cosmological information returns for varying survey area, depth and the level
of some systematic errors.Comment: To appear in MNRAS, 22 pages, 12 figures. Minor modifications made
according to the referee comment
On a Growing Transverse Mode as a Post-Newtonian Effect in the Large-Scale Structure Formation
We point out the existence of a new type of growing transverse mode in the
gravitational instability. This appears as a post-Newtonian effect to Newtonian
dynamics. We demonstrate this existence by formulating the Lagrangian
perturbation theory in the framework of the cosmological post-Newtonian
approximation in general relativity. Such post-Newtonian order effects might
produce characteristic appearances of large-scale structure formation, for
example, through the observation of anisotropy of the cosmic microwave
background radiation (CMB).Comment: 12 pages, no figure. To be published in Prog. Theor. Phys.(August
issue
Particle flows around an intruder
Particle flows injected as beams and scattered by an intruder are numerically
studied. We find a crossover of the drag force from Epstein's law to Newton's
law, depending on the ratio of the speed to the thermal speed. These laws can
be reproduced by a simple analysis of a collision model between the intruder
and particle flows. The crossover from Epstein's law to Stokes' law is also
found for the low-speed regime as the time evolution of the drag force caused
by beam particles. We also show the existence of turbulent-like behavior of the
particle flows behind the intruder with the aid of the second invariant of the
velocity gradient tensor and the relative mean square displacement for the
high-speed regime and a large intruder.Comment: 10 pages, 13 figure
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