12,857 research outputs found
Measuring Dark Energy with Gamma-Ray Bursts and Other Cosmological Probes
It has been widely shown that the cosmological parameters and dark energy can
be constrained by using data from type-Ia supernovae (SNe Ia), the cosmic
microwave background (CMB) anisotropy, the baryon acoustic oscillation (BAO)
peak from Sloan Digital Sky Survey (SDSS), the X-ray gas mass fraction in
clusters, and the linear growth rate of perturbations at z=0.15 as obtained
from the 2dF Galaxy Redshift Survey. Recently, gamma-ray bursts (GRBs) have
also been argued to be promising standard candles for cosmography. In this
paper, we present constraints on the cosmological parameters and dark energy by
combining a recent GRB sample including 69 events with the other cosmological
probes. First, we find that for the LambdaCDM cosmology this combination makes
the constraints stringent and the best fit is close to the flat universe.
Second, we fit the flat Cardassian expansion model and find that this model is
consistent with the LambdaCDM cosmology. Third, we present constraints on
several two-parameter dark energy models and find that these models are also
consistent with the LambdaCDM cosmology. Finally, we reconstruct the dark
energy equation-of-state parameter w(z) and the deceleration parameter q(z). We
see that the acceleration could have started at a redshift from
z_T=0.40_{-0.08}^{+0.14} to z_T=0.65_{-0.05}^{+0.10}. This difference in the
transition redshift is due to different dark energy models that we adopt. The
most stringent constraint on w(z) lies in the redshift range z\sim 0.3-0.6.Comment: 28 pages, 13 figures, accepted for publication in ApJ. One reference
added, one minor change in the final paragraph of section
Fourth Order Perturbation Theory for Normal Selfenergy in Repulsive Hubbard Model
We investigate the normal selfenergy and the mass enhancement factor in the
Hubbard model on the two-dimensional square lattice. Our purpose in this paper
is to evaluate the mass enhancement factor more quantitatively than the
conventional third order perturbation theory. We calculate it by expanding
perturbatively up to the fourth order with respect to the on-site repulsion
. We consider the cases that the system is near the half-filling, which are
similar situations to high- cuprates. As results of the calculations, we
obtain the large mass enhancement on the Fermi surface by introducing the
fourth order terms. This is mainly originated from the fourth order
particle-hole and particle-particle diagrams. Although the other fourth order
terms have effect of reducing the effective mass, this effect does not cancel
out the former mass enhancement completely and there remains still a large mass
enhancement effect. In addition, we find that the mass enhancement factor
becomes large with increasing the on-site repulsion and the density of
state (DOS) at the Fermi energy . According to many current reseaches,
such large and enhance the effective interaction between
quasiparticles, therefore the superconducting transition temperature
increases. On the other hand, the large mass enhancement leads the reduction of
the energy scale of quasiparticles, as a result, is reduced. When we
discuss , we have to estimate these two competitive effects.Comment: 6pages,8figure
Re-parameterization Invariance in Fractional Flux Periodicity
We analyze a common feature of a nontrivial fractional flux periodicity in
two-dimensional systems. We demonstrate that an addition of fractional flux can
be absorbed into re-parameterization of quantum numbers. For an exact
fractional periodicity, all the electronic states undergo the
re-parameterization, whereas for an approximate periodicity valid in a large
system, only the states near the Fermi level are involved in the
re-parameterization.Comment: 4 pages, 1 figure, minor changes, final version to appear in J. Phys.
Soc. Jp
M 33 X-7: ChASeM33 reveals the first eclipsing black hole X-ray binary
The first observations conducted as part of the Chandra ACIS survey of M 33
(ChASeM33) sampled the eclipsing X-ray binary M 33 X-7 over a large part of the
3.45 d orbital period and have resolved eclipse ingress and egress for the
first time. The occurrence of the X-ray eclipse allows us to determine an
improved ephemeris of mid-eclipse and binary period as HJD (2453639.119+-0.005)
+- N x (3.453014+-0.000020) and constrain the eclipse half angle to (26.5+-1.1)
degree. There are indications for a shortening of the orbital period. The X-ray
spectrum is best described by a disk blackbody spectrum typical for black hole
X-ray binaries in the Galaxy. We find a flat power density spectrum and no
significant regular pulsations were found in the frequency range of 10^{-4} to
0.15 Hz. HST WFPC2 images resolve the optical counterpart, which can be
identified as an O6III star with the help of extinction and colour corrections
derived from the X-ray absorption. Based on the optical light curve, the mass
of the compact object in the system most likely exceeds 9 M_sun. This mass, the
shape of the X-ray spectrum and the short term X-ray time variability identify
M 33 X-7 as the first eclipsing black hole high mass X-ray binary.Comment: 14 pages, 5 figures, ApJ accepte
16x125 Gb/s Quasi-Nyquist DAC-Generated PM-16QAM Transmission Over 3590 km of PSCF
We report on a transmission experiment over high-performance pure silica core fiber (PSCF) of 16 Nyquist wavelength-division-multiplexed (Nyquist-WDM) channels at a symbol rate of 15.625 GBaud, using polarization-multiplexed (PM) 16 symbols quadrature amplitude modulation (16QAM), resulting in a per-channel raw bit rate of 125 Gb/s. The channel spacing is 16 GHz, corresponding to 1.024 times the symbol rate. The interchannel crosstalk penalty is drastically reduced through the confinement of the signal spectrum within a near-Nyquist bandwidth, achieved with digital filtering and digital-to-analog converters (DACs) operating at 1.5 samples/symbol. The optical line is a recirculating loop composed of two spans of high-performance PSCF with erbium-doped fiber amplifiers only. The transmission distance of 3590 km at a target line bit-error rate (BER) of 1.5 10^-2 is achieved at a raw spectral efficiency (SE) of 7.81 b/s/Hz. Assuming a commercial hard forward error correction with 20.5% redundancy, capable of handling the target BER, the net SE is 6.48 b/s/Hz, the highest so far reported for multithousand kilometer transmission of PM-16QAM at ≥ 100 Gb/s per channel. These results demonstrate the feasibility of very high SE DAC-enabled ultra-long-haul quasi-Nyquist-WDM transmission using PM-16QAM with current technologies and manageable digital signal processing complexit
Topological surface transport in epitaxial SnTe thin films grown on Bi₂Te₃
The topological crystalline insulator SnTe has been grown epitaxially on a Bi₂Te₃ buffer layer by molecular beam epitaxy. In a 30-nm-thick SnTe film, p- and n-type carriers are found to coexist, and Shubnikov–de Haas oscillation data suggest that the n-type carriers are Dirac fermions residing on the SnTe (111) surface. This transport observation of the topological surface state in a p-type topological crystalline insulator became possible due to a downward band bending on the free SnTe surface, which appears to be of intrinsic origin
Phase transitions of a tethered surface model with a deficit angle term
Nambu-Goto model is investigated by using the canonical Monte Carlo
simulations on fixed connectivity surfaces of spherical topology. Three
distinct phases are found: crumpled, tubular, and smooth. The crumpled and the
tubular phases are smoothly connected, and the tubular and the smooth phases
are connected by a discontinuous transition. The surface in the tubular phase
forms an oblong and one-dimensional object similar to a one-dimensional linear
subspace in the Euclidean three-dimensional space R^3. This indicates that the
rotational symmetry inherent in the model is spontaneously broken in the
tubular phase, and it is restored in the smooth and the crumpled phases.Comment: 6 pages with 6 figure
Gauge Problem in the Gravitational Self-Force II. First Post Newtonian Force under Regge-Wheeler Gauge
We discuss the gravitational self-force on a particle in a black hole
space-time. For a point particle, the full (bare) self-force diverges. It is
known that the metric perturbation induced by a particle can be divided into
two parts, the direct part (or the S part) and the tail part (or the R part),
in the harmonic gauge, and the regularized self-force is derived from the R
part which is regular and satisfies the source-free perturbed Einstein
equations. In this paper, we consider a gauge transformation from the harmonic
gauge to the Regge-Wheeler gauge in which the full metric perturbation can be
calculated, and present a method to derive the regularized self-force for a
particle in circular orbit around a Schwarzschild black hole in the
Regge-Wheeler gauge. As a first application of this method, we then calculate
the self-force to first post-Newtonian order. We find the correction to the
total mass of the system due to the presence of the particle is correctly
reproduced in the force at the Newtonian order.Comment: Revtex4, 43 pages, no figure. Version to be published in PR
Coulomb suppression of NMR coherence peak in fullerene superconductors
The suppressed NMR coherence peak in the fullerene superconductors is
explained in terms of the dampings in the superconducting state induced by the
Coulomb interaction between conduction electrons. The Coulomb interaction,
modelled in terms of the onsite Hubbard repulsion, is incorporated into the
Eliashberg theory of superconductivity with its frequency dependence considered
self-consistently at all temperatures. The vertex correction is also included
via the method of Nambu. The frequency dependent Coulomb interaction induces
the substantial dampings in the superconducting state and, consequently,
suppresses the anticipated NMR coherence peak of fullerene superconductors as
found experimentally.Comment: 4 pages, Revtex, and 2 figures. Revised and final version to appear
in Phys. Rev. Lett. (1998
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