3,274 research outputs found
Suppression of vortex channeling in meandered YBa2Cu3O7-d grain boundaries
We report on the in-plane magnetic field (H) dependence of the critical
current density (Jc) in meandered and planar single grain boundaries (GBs)
isolated in YBa2Cu3O7-d (YBCO) coated conductors. The Jc(H)properties of the
planar GB are consistent with those previously seen in single GBs of YBCO films
grown on SrTiO3 bi-crystals. In the straight boundary a characteristic flux
channeling regime when H is oriented near the GB plane, associated with a
reduced Jc, is seen. The meandered GB does not show vortex channeling since it
is not possible for a sufficient length of vortex line to lie within it.Comment: Submitted to AP
Bloch oscillations of Bose-Einstein condensates: Breakdown and revival
We investigate the dynamics of Bose-Einstein condensates (BEC) in a tilted
one-dimensional periodic lattice within the mean-field (Gross-Pitaevskii)
description. Unlike in the linear case the Bloch oscillations decay because of
nonlinear dephasing. Pronounced revival phenomena are observed. These are
analyzed in detail in terms of a simple integrable model constructed by an
expansion in Wannier-Stark resonance states. We also briefly discuss the pulsed
output of such systems for stronger static fields.Comment: RevTeX4, 9 pages, 14 figure
Hyperatlas: A New Framework for Image Federation
Hyperatlas is an open standard intended to facilitate the large-scale
federation of image-based data. The subject of hyperatlas is the space of
sphere-to-plane projection mappings (the FITS-WCS information), and the
standard consists of coherent collections of these on which data can be
resampled and thereby federated with other image data. We hope for a
distributed effort that will produce a multi-faceted image atlas of the sky,
made by federating many different surveys at different wavelengths and
different times. We expect that hyperatlas-compliant imagery will be published
and discovered through an International Virtual Observatory Alliance (IVOA)
registry, and that grid-based services will emerge for the required resampling
and mosaicking.Comment: Published in ADASS XIII proceeding
Atlasmaker: A Grid-based Implementation of the Hyperatlas
The Atlasmaker project is using Grid technology, in combination with NVO
interoperability, to create new knowledge resources in astronomy. The product
is a multi-faceted, multi-dimensional, scientifically trusted image atlas of
the sky, made by federating many different surveys at different wavelengths,
times, resolutions, polarizations, etc. The Atlasmaker software does resampling
and mosaicking of image collections, and is well-suited to operate with the
Hyperatlas standard. Requests can be satisfied via on-demand computations or by
accessing a data cache. Computed data is stored in a distributed virtual file
system, such as the Storage Resource Broker (SRB). We expect these atlases to
be a new and powerful paradigm for knowledge extraction in astronomy, as well
as a magnificent way to build educational resources. The system is being
incorporated into the data analysis pipeline of the Palomar-Quest synoptic
survey, and is being used to generate all-sky atlases from the 2MASS, SDSS, and
DPOSS surveys for joint object detection.Comment: Published in the Proceedings of ADASS XI
Renormalization group analysis of the 2D Hubbard model
Salmhofer [Commun. Math. Phys. 194, 249 (1998)] has recently developed a new
renormalization group method for interacting Fermi systems, where the complete
flow from the bare action of a microscopic model to the effective low-energy
action, as a function of a continuously decreasing infrared cutoff, is given by
a differential flow equation which is local in the flow parameter. We apply
this approach to the repulsive two-dimensional Hubbard model with nearest and
next-nearest neighbor hopping amplitudes. The flow equation for the effective
interaction is evaluated numerically on 1-loop level. The effective
interactions diverge at a finite energy scale which is exponentially small for
small bare interactions. To analyze the nature of the instabilities signalled
by the diverging interactions we extend Salmhofers renormalization group for
the calculation of susceptibilities. We compute the singlet superconducting
susceptibilities for various pairing symmetries and also charge and spin
density susceptibilities. Depending on the choice of the model parameters
(hopping amplitudes, interaction strength and band-filling) we find
commensurate and incommensurate antiferromagnetic instabilities or d-wave
superconductivity as leading instability. We present the resulting phase
diagram in the vicinity of half-filling and also results for the density
dependence of the critical energy scale.Comment: 16 pages, RevTeX, 16 eps figure
The decay pi0 to gamma gamma to next to leading order in Chiral Perturbation Theory
The two photon decay width of the neutral pion is analyzed within the
combined framework of Chiral Perturbation Theory and the 1/Nc expansion up to
order p^6 and p^4 times 1/Nc in the decay amplitude. The eta' is explicitly
included in the analysis. It is found that the decay width is enhanced by about
4.5% due to the isospin-breaking induced mixing of the pure U(3) states. This
effect, which is of leading order in the low energy expansion, is shown to
persist nearly unchanged at next to leading order. The chief prediction for the
width with its estimated uncertainty is 8.10+-0.08 eV. This prediction at the
1% level makes the upcomming precision measurement of the decay width even more
urgent.
Observations on the eta and eta' can also be made, especially about their
mixing, which is shown to be significantly affected by next to leading order
corrections.Comment: 21 pages, two figure
eta and eta' mesons and dimension 2 gluon condensate <A^2>
The study of light pseudoscalar quark-antiquark bound states in the
Dyson-Schwinger approach with the effective QCD coupling enhanced by the
interplay of the dimension 2 gluon condensate and dimension 4 gluon
condensate , is extended to the eta-eta' complex. We include the effects
of the gluon axial anomaly into the Dyson-Schwinger approach to mesons. The
calculated masses, mixing and two-photon decay widths of eta and eta' mesons
are in agreement with experiment. Also, in a model-independent way, we give the
modification of the Gell-Mann--Okubo and Schwinger nonet relations due to the
interplay of the gluon anomaly and SU(3) flavor symmetry breaking.Comment: 15 pages, 2 tables, 1 eps figure, revtex
Local Measurement of Current Density by Magneto-Optical Current Reconstruction in Normally and Overpressure Processed Bi-2223 Tapes
Magneto-optical current reconstruction has been used for detailed analysis of
the local critical current density (Jc) variation in monocore Bi-2223 tapes. We
find, even in high quality tapes with bulk transport Jc ~ 40 kA/cm^2 (77K, 0T),
that there exist local regions which possess current densities of more than 200
kA/cm^2. Overpressure processing at 148 bar significantly improved Jc to 48
kA/cm^2 by improving the connectivity. For the overpressure-processed sample we
find that the current distribution is more uniform and that the maximum local
current density at 77 K is increased almost to 300 kA/cm^2.Comment: Presented at Applied Superconductivity Conference, Houston, August
4th -9th, 200
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