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Oxygen diffusion in Sr<sub>0.75</sub>Y<sub>0.25</sub>CoO<sub>2.625</sub>: a molecular dynamics study
Oxygen diffusion in Sr0.75Y0.25CoO2.625 is investigated using molecular dynamics simulations in conjunction with an established set of Born model potentials. We predict an activation energy of diffusion for 1.56 eV in the temperature range of 1000-1400 K. We observe extensive disordering of the oxygen ions over a subset of lattice sites. Furthermore, oxygen ion diffusion both in the a-b plane and along the c axis requires the same set of rate-limiting ion hops. It is predicted that oxygen transport in Sr0.75Y0.25CoO2.625 is therefore isotropic
High pressure effects in fluorinated HgBa2Ca2Cu3O(8+d)
We have measured the pressure sensitivity of Tc in fluorinated
HgBa2Ca2Cu3O(8+d) (Hg-1223) ceramic samples with different F contents, applying
pressures up to 30 GPa. We obtained that Tc increases with increasing pressure,
reaching different maximum values, depending on the F doping level, and
decreases for a further increase of pressure. A new high Tc record (166 K +/- 1
K) was achieved by applying pressure (23 GPa) in a fluorinated Hg-1223 sample
near the optimum doping level. Our results show that all our samples are at the
optimal doping, and that fluorine incorporation decreases the crystallographic
-parameter concomitantly increasing the maximum attainable Tc. This effect
reveals that the compression of the axes is one of the keys that controls
the Tc of high temperature superconductors.Comment: 4 pages, 4 figures, submitted to Phys. Rev.
On the Existence of Heavy Pentaquarks: The large Nc and Heavy Quark Limits and Beyond
We present a very general argument that the analogue of a heavy pentaquark (a
state with the quantum numbers of a baryon combined with an additional light
quark and a heavy antiquark) must exist as a particle stable under strong
interactions in the combined heavy quark and large Nc limits of QCD. Moreover,
in the combined limit these heavy pentaquark states fill multiplets of
SU(4)xO(8)xSU(2). We explore the question of whether corrections in the
combined 1/Nc and 1/mQ expansions are sufficiently small to maintain this
qualitative result. Since no model-independent way is known to answer this
question, we use a class of ``realistic'' hadronic models in which a pentaquark
can be formed via nucleon-heavy meson binding through a pion-exchange
potential. These models have the virtue that they necessarily yield the correct
behavior in the combined limit, and the long-distance parts of the interactions
are model independent. If the long-distance attraction in these models were to
predict bound states in a robust way (i.e., largely insensitive to the details
of the short-range interaction), then one could safely conclude that heavy
pentaquarks do exist. However, in practice the binding does depend very
strongly on the details of the short-distance physics, suggesting that the real
world is not sufficiently near the combined large Nc, mQ limit to use it as a
reliable guide. Whether stable heavy pentaquarks exist remains an open
question.Comment: 11 pages; references adde
Micro-canonical pentaquark production in \ee annihilations
The existence of pentaquarks, namely baryonic states made up of four quarks
and one antiquark, became questionable, because the candidates, i.e. the
peak, are seen in certain reactions, i.e. p+p collisions, but not in
others, i.e. \ee annihilations. In this paper, we estimate the production of
and in \ee annihilations at different
energies using Fermi statistical model as originally proposed in its
microcanonical form. The results is compared with that from pp collisions at
SPS and RHIC energies. We find that, if pentaquark states exist, the production
is highly possible in \ee annihilations. For example, at LEP energy
=91.2 GeV, both and yield more
than in pp collisions at SPS and RHIC energy.Comment: 7 pages 2 figure
Radiative Corrections for Pion Polarizability Experiments
We use the semi-analytical program RCFORGV to evaluate radiative corrections
to one-photon radiative emission in the high-energy scattering of pions in the
Coulomb field of a nucleus with atomic number Z. It is shown that radiative
corrections can simulate a pion polarizability effect. The average effect was
estimated for pion energies 40-600 GeV. We also study the range of
applicability of the equivalent photon approximation in describing one-photon
radiative emission.Comment: 11 pages (LaTex), 6 figures, 1 table. No changes in the paper. New
submission because old files are corrupted in arXi
A Brief Introduction to Chiral Perturbation Theory
A brief introduction to the subject of chiral perturbation theory (pt)
is presented, including a discussion of effective field theory and applications
of pt in the arena of purely mesonic interactions as well as in the sector.Comment: 15 pages, talk given at TAPS detector workshop, Rez, Czech Republic,
Sept. 1999, to be published in Czech. J. Phy
Pion and Kaon Polarizabilities and Radiative Transitions
CERN COMPASS plans measurements of gamma-pi and gamma-K interactions using
50-280 GeV pion (kaon) beams and a virtual photon target. Pion (kaon)
polarizabilities and radiative transitions will be measured via Primakoff
effect reactions such as pi+gamma->pi'+gamma and pi+gamma->meson. The former
can test a precise prediction of chiral symmetry; the latter for
pi+gamma->a1(1260) is important for understanding the polarizability. The
radiative transition of a pion to a low mass two-pion system, pi+gamma->pi+pi0,
can also be studied to measure the chiral anomaly amplitude F(3pi)
(characterizing gamma->3pi), arising from the effective Chiral Lagrangian. We
review here the motivation for the above physics program. We describe the beam,
target, detector, and trigger requirements for these experiments. We also
describe FNAL SELEX attempts to study related physics via the interaction of
600 GeV pions with target electrons. Data analysis in progress aims to identify
the reactions pi+e->pi'+e'+pi0 related to the chiral anomaly, and
pi+e->pi'+e'+gamma related to pion polarizabilities.Comment: 16 pages, 6 figures, Latex Springer-Verlag style Tel Aviv U. Preprint
TAUP-2469-97, Contribution to the Workshop on Chiral Dynamics Theory and
Experiment, U. of Mainz, Sept. 1-5, 1997, to be published in Springer-Verlag,
Eds. A. Bernstein, Th. Walcher, 199
Search for the pentaquark resonance signature in lattice QCD
Claims concerning the possible discovery of the \Theta^+ pentaquark, with
minimal quark content uudd\bar{s}, have motivated our comprehensive study into
possible pentaquark states using lattice QCD. We review various pentaquark
interpolating fields in the literature and create a new candidate ideal for
lattice QCD simulations. Using these interpolating fields we attempt to isolate
a signal for a five-quark resonance. Calculations are performed using improved
actions on a large 20^3 x 40 lattice in the quenched approximation. The
standard lattice resonance signal of increasing attraction between baryon
constituents for increasing quark mass is not observed for spin-1/2 pentaquark
states. We conclude that evidence supporting the existence of a spin-1/2
pentaquark resonance does not exist in quenched QCD.Comment: 42 pages, 28 figures, 11 table
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