693 research outputs found
Neutron scattering study of the effects of dopant disorder on the superconductivity and magnetic order in stage-4 La_2CuO_{4+y}
We report neutron scattering measurements of the structure and magnetism of
stage-4 La_2CuO_{4+y} with T_c ~42 K. Our diffraction results on a single
crystal sample demonstrate that the excess oxygen dopants form a
three-dimensional ordered superlattice within the interstitial regions of the
crystal. The oxygen superlattice becomes disordered above T ~ 330 K, and a fast
rate of cooling can freeze-in the disordered-oxygen state. Hence, by
controlling the cooling rate, the degree of dopant disorder in our
La_2CuO_{4+y} crystal can be varied. We find that a higher degree of quenched
disorder reduces T_c by ~ 5 K relative to the ordered-oxygen state. At the same
time, the quenched disorder enhances the spin density wave order in a manner
analogous to the effects of an applied magnetic field.Comment: 4 figures included in text; submitted to PR
Interplay of structural and electronic phase separation in single crystalline La(2)CuO(4.05) studied by neutron and Raman scattering
We report a neutron and Raman scattering study of a single-crystal of
La(2)CuO(4.05) prepared by high temperature electrochemical oxidation. Elastic
neutron scattering measurements show the presence of two phases, corresponding
to the two edges of the first miscibility gap, all the way up to 300 K. An
additional oxygen redistribution, driven by electronic energies, is identified
at 250 K in Raman scattering (RS) experiments by the simultaneous onset of
two-phonon and two-magnon scattering, which are fingerprints of the insulating
phase. Elastic neutron scattering measurements show directly an
antiferromagnetic ordering below a N\'eel temperature of T_N =210K. The opening
of the superconducting gap manifests itself as a redistribution of electronic
Raman scattering below the superconducting transition temperature, T_c = 24K. A
pronounced temperature-dependent suppression of the intensity of the (100)
magnetic Bragg peak has been detected below T_c. We ascribe this phenomenon to
a change of relative volume fraction of superconducting and antiferromagnetic
phases with decreasing temperature caused by a form of a superconducting
proximity effect.Comment: 9 pages, including 9 eps figures, submitted to PR
Dynamics of escaping Earth ejecta and their collision probability with different Solar System bodies
It has been suggested that the ejection to interplanetary space of
terrestrial crustal material, accelerated in a large impact, may result in the
interchange of biological material between Earth and other Solar System bodies.
In this paper, we analyze the fate of debris ejected from Earth by means of
numerical simulations of the dynamics of a large collection of test particles.
This allows us to determine the probability and conditions for the collision of
ejecta with other planets of the Solar System. We also estimate the amount of
particles falling-back to Earth as a function of time after being ejected. We
find that, in general, the collision rates of Earth ejecta with Venus and the
Moon, as well as the fall-back rates, are consistent with results reported in
the literature. By considering a larger number of particles than in all
previous calculations we have also determined directly the collision
probability with Mars and, for the first time, computed collision probabilities
with Jupiter. We find that the collision probability with Mars is greater than
values determined from collision cross section estimations previously reported.Comment: 9 pages, 4 figures & 1 table. Submitted to Icaru
The Self Model and the Conception of Biological Identity in Immunology
The self/non-self model, first proposed by F.M. Burnet, has dominated immunology for sixty years now. According to this model, any foreign element will trigger an immune reaction in an organism, whereas endogenous elements will not, in normal circumstances, induce an immune reaction. In this paper we show that the self/non-self model is no longer an appropriate explanation of experimental data in immunology, and that this inadequacy may be rooted in an excessively strong metaphysical conception of biological identity. We suggest that another hypothesis, one based on the notion of continuity, gives a better account of immune phenomena. Finally, we underscore the mapping between this metaphysical deflation from self to continuity in immunology and the philosophical debate between substantialism and empiricism about identity
Surveying Standard Model Flux Vacua on
We consider the SU(2)LxSU(2)R Standard Model brane embedding in an
orientifold of T6/Z2xZ2. Within defined limits, we construct all such Standard
Model brane embeddings and determine the relative number of flux vacua for each
construction. Supersymmetry preserving brane recombination in the hidden sector
enables us to identify many solutions with high flux. We discuss in detail the
phenomenology of one model which is likely to dominate the counting of vacua.
While Kahler moduli stabilization remains to be fully understood, we define the
criteria necessary for generic constructions to have fixed moduli.Comment: 30 pages, LaTeX, v2: added reference
Review of two-photon exchange in electron scattering
We review the role of two-photon exchange (TPE) in electron-hadron
scattering, focusing in particular on hadronic frameworks suitable for
describing the low and moderate Q^2 region relevant to most experimental
studies. We discuss the effects of TPE on the extraction of nucleon form
factors and their role in the resolution of the proton electric to magnetic
form factor ratio puzzle. The implications of TPE on various other observables,
including neutron form factors, electroproduction of resonances and pions, and
nuclear form factors, are summarized. Measurements seeking to directly identify
TPE effects, such as through the angular dependence of polarization
measurements, nonlinear epsilon contributions to the cross sections, and via e+
p to e- p cross section ratios, are also outlined. In the weak sector, we
describe the role of TPE and gamma-Z interference in parity-violating electron
scattering, and assess their impact on the extraction of the strange form
factors of the nucleon and the weak charge of the proton.Comment: 73 pages, 40 figures, review article for Prog. Part. Nucl. Phys.
(dedicated to the memory of John A. Tjon
Pinned Balseiro-Falicov Model of Tunneling and Photoemission in the Cuprates
The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping
from a pseudogap state in the underdoped cuprates to a superconducting state at
optimal and overdoping, has been interpreted as evidence that the pseudogap
must be due to precursor pairing. We suggest an alternative explanation, that
the smoothness reflects a hidden SO(N) symmetry near the (pi,0) points of the
Brillouin zone (with N = 3, 4, 5, or 6). Because of this symmetry, the
pseudogap could actually be due to any of a number of nesting instabilities,
including charge or spin density waves or more exotic phases. We present a
detailed analysis of this competition for one particular model: the pinned
Balseiro-Falicov model of competing charge density wave and (s-wave)
superconductivity. We show that most of the anomalous features of both
tunneling and photoemission follow naturally from the model, including the
smooth crossover, the general shape of the pseudogap phase diagram, the
shrinking Fermi surface of the pseudogap phase, and the asymmetry of the
tunneling gap away from optimal doping. Below T_c, the sharp peak at Delta_1
and the dip seen in the tunneling and photoemission near 2Delta_1 cannot be
described in detail by this model, but we suggest a simple generalization to
account for inhomogeneity, which does provide an adequate description. We show
that it should be possible, with a combination of photoemission and tunneling,
to demonstrate the extent of pinning of the Fermi level to the Van Hove
singularity. A preliminary analysis of the data suggests pinning in the
underdoped, but not in the overdoped regime.Comment: 18 pages LaTeX, 26 ps. figure
A measurement of the tau mass and the first CPT test with tau leptons
We measure the mass of the tau lepton to be 1775.1+-1.6(stat)+-1.0(syst.) MeV
using tau pairs from Z0 decays. To test CPT invariance we compare the masses of
the positively and negatively charged tau leptons. The relative mass difference
is found to be smaller than 3.0 10^-3 at the 90% confidence level.Comment: 10 pages, 4 figures, Submitted to Phys. Letts.
Measurement of the B0 Lifetime and Oscillation Frequency using B0->D*+l-v decays
The lifetime and oscillation frequency of the B0 meson has been measured
using B0->D*+l-v decays recorded on the Z0 peak with the OPAL detector at LEP.
The D*+ -> D0pi+ decays were reconstructed using an inclusive technique and the
production flavour of the B0 mesons was determined using a combination of tags
from the rest of the event. The results t_B0 = 1.541 +- 0.028 +- 0.023 ps, Dm_d
= 0.497 +- 0.024 +- 0.025 ps-1 were obtained, where in each case the first
error is statistical and the second systematic.Comment: 17 pages, 4 figures, submitted to Phys. Lett.
First Measurement of Z/gamma* Production in Compton Scattering of Quasi-real Photons
We report the first observation of Z/gamma* production in Compton scattering
of quasi-real photons. This is a subprocess of the reaction e+e- to
e+e-Z/gamma*, where one of the final state electrons is undetected.
Approximately 55 pb-1 of data collected in the year 1997 at an e+e-
centre-of-mass energy of 183 GeV with the OPAL detector at LEP have been
analysed. The Z/gamma* from Compton scattering has been detected in the
hadronic decay channel. Within well defined kinematic bounds, we measure the
product of cross-section and Z/gamma* branching ratio to hadrons to be
(0.9+-0.3+-0.1) pb for events with a hadronic mass larger than 60 GeV,
dominated by (e)eZ production. In the hadronic mass region between 5 GeV and 60
GeV, dominated by (e)egamma* production, this product is found to be
(4.1+-1.6+-0.6) pb. Our results agree with the predictions of two Monte Carlo
event generators, grc4f and PYTHIA.Comment: 18 pages, LaTeX, 5 eps figures included, submitted to Physics Letters
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