3,657 research outputs found
Enhancement of the Critical Current Density of YBa2Cu3Ox Superconductors under Hydrostatic Pressure
The dependence of the critical current density Jc on hydrostatic pressure to
0.6 GPa is determined for a single 25-degree [001]-tilt grain boundary in a
bicrystalline ring of nearly optimally doped melt-textured YBa2Cu3Ox. Jc is
found to increase rapidly under pressure at +20 %/GPa. A new diagnostic method
is introduced (pressure-induced Jc relaxation) which reveals a sizeable
concentration of vacant oxygen sites in the grain boundary region. Completely
filling such sites with oxygen anions should lead to significant enhancements
in Jc.Comment: revised manuscript, graphic errors in figures correcte
Probability-Changing Cluster Algorithm: Study of Three-Dimensional Ising Model and Percolation Problem
We present a detailed description of the idea and procedure for the newly
proposed Monte Carlo algorithm of tuning the critical point automatically,
which is called the probability-changing cluster (PCC) algorithm [Y. Tomita and
Y. Okabe, Phys. Rev. Lett. {\bf 86} (2001) 572]. Using the PCC algorithm, we
investigate the three-dimensional Ising model and the bond percolation problem.
We employ a refined finite-size scaling analysis to make estimates of critical
point and exponents. With much less efforts, we obtain the results which are
consistent with the previous calculations. We argue several directions for the
application of the PCC algorithm.Comment: 6 pages including 8 eps figures, to appear in J. Phys. Soc. Jp
Finite-size Scaling of Correlation Ratio and Generalized Scheme for the Probability-Changing Cluster Algorithm
We study the finite-size scaling (FSS) property of the correlation ratio, the
ratio of the correlation functions with different distances. It is shown that
the correlation ratio is a good estimator to determine the critical point of
the second-order transition using the FSS analysis. The correlation ratio is
especially useful for the analysis of the Kosterlitz-Thouless (KT) transition.
We also present a generalized scheme of the probability-changing cluster
algorithm, which has been recently developed by the present authors, based on
the FSS property of the correlation ratio. We investigate the two-dimensional
quantum XY model of spin 1/2 with this generalized scheme, obtaining the
precise estimate of the KT transition temperature with less numerical effort.Comment: 4 pages, RevTeX4, to appear in Phys. Rev. B, Rapid Communication
Renormalization Group Approach to Einstein Equation in Cosmology
The renormalization group method has been adapted to the analysis of the
long-time behavior of non-linear partial differential equation and has
demonstrated its power in the study of critical phenomena of gravitational
collapse. In the present work we apply the renormalization group to the
Einstein equation in cosmology and carry out detailed analysis of
renormalization group flow in the vicinity of the scale invariant fixed point
in the spherically symmetric and inhomogeneous dust filled universe model.Comment: 16 pages including 2 eps figures, RevTe
3+1 Approach to the Long Wavelength Iteration Scheme
Large-scale inhomogeneities and anisotropies are modeled using the Long
Wavelength Iteration Scheme. In this scheme solutions are obtained as
expansions in spatial gradients, which are taken to be small. It is shown that
the choice of foliation for spacetime can make the iteration scheme more
effective in two respects: (i) the shift vector can be chosen so as to dilute
the effect of anisotropy on the late-time value of the extrinsic curvature of
the spacelike hypersurfaces of the foliation; and (ii) pure gauge solutions
present in a similar calculation using the synchronous gauge vanish when the
spacelike hypersurfaces have extrinsic curvature with constant trace. We
furthermore verify the main conclusion of the synchronous gauge calculation
which is large-scale inhomogeneity decays if the matter--considered to be that
of a perfect-fluid with a barotropic equation of state--violates the
strong-energy condition. Finally, we obtain the solution for the lapse function
and discuss its late-time behaviour. It is found that the lapse function is
well-behaved when the matter violates the strong energy condition.Comment: 21 pages, TeX file, already publishe
Disk Properties and Density Structure of the Star-Forming Dense Core B335
We present subarcsecond resolution dust continuum observations of the
protostellar collapse candidate B335 made with the IRAM Plateau de Bure
Interferometer at wavelengths of 1.2 and 3.0 mm. These observations probe to <
100 AU size scales and reveal a compact source component that we identify with
a circumstellar disk. We analyze these data in concert with previous lower
resolution interferometer observations and find a best fit density structure
for B335 that consists of a power law envelope with index p=1.55 +/- 0.04 (r <
5000 AU) together with a disk (r < 100 AU) of flux F_{1.2 mm}=21 +/-2 mJy. We
estimate a systematic uncertainty in the power law index delta(p) < 0.15, where
the largest error comes from the assumed form of the dust temperature falloff
with radius. This determination of the inner density structure of B335 has a
precision unique amongst protostellar cores, and it is consistent with the
r^{-1.5} profile of gravitational free-fall, in accord with basic expectations
for the formation of a star. The flux (and implied mass) of the compact
component in B335 is typical of the disks around T Tauri stars.Comment: 16 pages, 2 figures. Accepted to the Astrophysical Journal, sched
v596 (2003 Oct 10
Observation of Antinormally Ordered Hanbury-Brown--Twiss Correlations
We have measured antinormally ordered Hanbury-Brown--Twiss correlations for
coherent states of electromagnetic field by using stimulated parametric
down-conversion process. Photons were detected by stimulated emission, rather
than by absorption, so that the detection responded not only to actual photons
but also to zero-point fluctuations via spontaneous emission. The observed
correlations were distinct from normally ordered ones as they showed excess
positive correlations, i.e., photon bunching effects, which arose from the
thermal nature of zero-point fluctuations.Comment: 5 pages, 3 figures, to appear in Physical Review Letter
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