1,208 research outputs found
Theory of Neutron Scattering in the Normal and Superconducting State of YBCO
We analyze neutron experiments on \ybco at various stoichiometries in the
superconducting state, within the context of a bi-layer theory which yields
good agreement with the normal state Cu-NMR and neutron data as a function of
\omega, q and T. A d-wave superconducting state exhibits peaks at q = ( \pi ,
\pi , \pi ) and sharp maxima as a function of \omega, at twice the gap
frequency. This behavior may have been observed experimentally. The counterpart
behavior for other choices of order parameter symmetry is discussed.Comment: uuencoded postscript file for the entire paper enclose
Neutron scattering and superconducting order parameter in YBa2Cu3O7
We discuss the origin of the neutron scattering peak at 41 meV observed in
YBaCuO below . The peak may occur due to spin-flip electron
excitations across the superconducting gap which are enhanced by the
antiferromagnetic interaction between Cu spins. In this picture, the experiment
is most naturally explained if the superconducting order parameter has -wave
symmetry and opposite signs in the bonding and antibonding electron bands
formed within a CuO bilayer.Comment: In this version, only few minor corrections and the update of
references were done in order to make perfect correspondence with the
published version. RevTeX, psfig, 5 pages, and 3 figure
The cosmic ray proton plus helium energy spectrum measured by the ARGO-YBJ experiment in the energy range 3-300 TeV
The ARGO-YBJ experiment is a full-coverage air shower detector located at the
Yangbajing Cosmic Ray Observatory (Tibet, People's Republic of China, 4300 m
a.s.l.). The high altitude, combined with the full-coverage technique, allows
the detection of extensive air showers in a wide energy range and offer the
possibility of measuring the cosmic ray proton plus helium spectrum down to the
TeV region, where direct balloon/space-borne measurements are available. The
detector has been in stable data taking in its full configuration from November
2007 to February 2013. In this paper the measurement of the cosmic ray proton
plus helium energy spectrum is presented in the region 3-300 TeV by analyzing
the full collected data sample. The resulting spectral index is . These results demonstrate the possibility of performing an accurate
measurement of the spectrum of light elements with a ground based air shower
detector.Comment: 18 pages, 8 figures, preprint submitted to Phys. Rev.
Study of the diffuse gamma-ray emission from the Galactic plane with ARGO-YBJ
The events recorded by ARGO-YBJ in more than five years of data collection
have been analyzed to determine the diffuse gamma-ray emission in the Galactic
plane at Galactic longitudes 25{\deg} < l < 100{\deg} and Galactic latitudes .
The energy range covered by this analysis, from ~350 GeV to ~2 TeV, allows the
connection of the region explored by Fermi with the multi-TeV measurements
carried out by Milagro. Our analysis has been focused on two selected regions
of the Galactic plane, i.e., 40{\deg} < l < 100{\deg} and 65{\deg} < l <
85{\deg} (the Cygnus region), where Milagro observed an excess with respect to
the predictions of current models. Great care has been taken in order to mask
the most intense gamma-ray sources, including the TeV counterpart of the Cygnus
cocoon recently identified by ARGO-YBJ, and to remove residual contributions.
The ARGO-YBJ results do not show any excess at sub-TeV energies corresponding
to the excess found by Milagro, and are consistent with the predictions of the
Fermi model for the diffuse Galactic emission. From the measured energy
distribution we derive spectral indices and the differential flux at 1 TeV of
the diffuse gamma-ray emission in the sky regions investigated.Comment: 11 pages, 6 figures, published in AP
4.5 years multi-wavelength observations of Mrk 421 during the ARGO-YBJ and Fermi common operation time
We report on the extensive multi-wavelength observations of the blazar
Markarian 421 (Mrk 421) covering radio to gamma-rays, during the 4.5 year
period of ARGO-YBJ and Fermi common operation time, from August 2008 to
February 2013. In particular, thanks to the ARGO-YBJ and Fermi data, the whole
energy range from 100 MeV to 10 TeV is covered without any gap. In the
observation period, Mrk 421 showed both low and high activity states at all
wavebands. The correlations among flux variations in different wavebands were
analyzed. Seven large flares, including five X-ray flares and two GeV gamma-ray
flares with variable durations (3-58 days), and one X-ray outburst phase were
identified and used to investigate the variation of the spectral energy
distribution with respect to a relative quiescent phase. During the outburst
phase and the seven flaring episodes, the peak energy in X-rays is observed to
increase from sub-keV to few keV. The TeV gamma-ray flux increases up to
0.9-7.2 times the flux of the Crab Nebula. The behavior of GeV gamma-rays is
found to vary depending on the flare, a feature that leads us to classify
flares into three groups according to the GeV flux variation. Finally, the
one-zone synchrotron self-Compton model was adopted to describe the emission
spectra. Two out of three groups can be satisfactorily described using injected
electrons with a power-law spectral index around 2.2, as expected from
relativistic diffuse shock acceleration, whereas the remaining group requires a
harder injected spectrum. The underlying physical mechanisms responsible for
different groups may be related to the acceleration process or to the
environment properties.Comment: 17 pages, 9 figures, 5 tables, Accepted for publication in ApJ
The Dynamical Cluster Approximation: Non-Local Dynamics of Correlated Electron Systems
We recently introduced the dynamical cluster approximation(DCA), a new
technique that includes short-ranged dynamical correlations in addition to the
local dynamics of the dynamical mean field approximation while preserving
causality. The technique is based on an iterative self-consistency scheme on a
finite size periodic cluster. The dynamical mean field approximation (exact
result) is obtained by taking the cluster to a single site (the thermodynamic
limit). Here, we provide details of our method, explicitly show that it is
causal, systematic, -derivable, and that it becomes conserving as the
cluster size increases. We demonstrate the DCA by applying it to a Quantum
Monte Carlo and Exact Enumeration study of the two-dimensional Falicov-Kimball
model. The resulting spectral functions preserve causality, and the spectra and
the CDW transition temperature converge quickly and systematically to the
thermodynamic limit as the cluster size increases.Comment: 19 pages, 13 postscript figures, revte
Observation of TeV gamma rays from the Cygnus region with the ARGO-YBJ experiment
We report the observation of TeV gamma-rays from the Cygnus region using the
ARGO-YBJ data collected from 2007 November to 2011 August. Several TeV sources
are located in this region including the two bright extended MGRO J2019+37 and
MGRO J2031+41. According to the Milagro data set, at 20 TeV MGRO J2019+37 is
the most significant source apart from the Crab Nebula. No signal from MGRO
J2019+37 is detected by the ARGO-YBJ experiment, and the derived flux upper
limits at 90% confidence level for all the events above 600 GeV with medium
energy of 3 TeV are lower than the Milagro flux, implying that the source might
be variable and hard to be identified as a pulsar wind nebula. The only
statistically significant (6.4 standard deviations) gamma-ray signal is found
from MGRO J2031+41, with a flux consistent with the measurement by Milagro.Comment: 14 pages, 4 figure
Magnetic and pair correlations of the Hubbard model with next-nearest-neighbor hopping
A combination of analytical approaches and quantum Monte Carlo simulations is
used to study both magnetic and pairing correlations for a version of the
Hubbard model that includes second-neighbor hopping as a
model for high-temperature superconductors. Magnetic properties are analyzed
using the Two-Particle Self-Consistent approach. The maximum in magnetic
susceptibility as a function of doping appears both at finite
and at but for two totally different physical reasons. When
, it is induced by antiferromagnetic correlations while at
it is a band structure effect amplified by interactions.
Finally, pairing fluctuations are compared with -matrix results to
disentangle the effects of van Hove singularity and of nesting on
superconducting correlations. The addition of antiferromagnetic fluctuations
increases slightly the -wave superconducting correlations despite the
presence of a van Hove singularity which tends to decrease them in the
repulsive model. Some aspects of the phase diagram and some subtleties of
finite-size scaling in Monte Carlo simulations, such as inverted finite-size
dependence, are also discussed.Comment: Revtex, 8 pages + 15 uuencoded postcript figure
Observation of TeV gamma-rays from the unidentified source HESS J1841-055 with the ARGO-YBJ experiment
We report the observation of a very high energy \gamma-ray source, whose
position is coincident with HESS J1841-055. This source has been observed for
4.5 years by the ARGO-YBJ experiment from November 2007 to July 2012. Its
emission is detected with a statistical significance of 5.3 standard
deviations. Parameterizing the source shape with a two-dimensional Gaussian
function we estimate an extension \sigma=(0.40(+0.32,-0.22}) degree, consistent
with the HESS measurement. The observed energy spectrum is dN/dE =(9.0-+1.6) x
10^{-13}(E/5 TeV)^{-2.32-+0.23} photons cm^{-2} s^{-1} TeV^{-1}, in the energy
range 0.9-50 TeV. The integral \gamma-ray flux above 1 TeV is 1.3-+0.4 Crab
units, which is 3.2-+1.0 times the flux derived by HESS. The differences in the
flux determination between HESS and ARGO-YBJ, and possible counterparts at
other wavelengths are discussed.Comment: 17 pages, 4 figures, have been accepted for publication in Ap
EAS age determination from the study of the lateral distribution of charged particles near the shower axis with the ARGO-YBJ experiment
The ARGO-YBJ experiment, a full coverage extensive air shower (EAS) detector
located at high altitude (4300 m a.s.l.) in Tibet, China, has smoothly taken
data, with very high stability, since November 2007 to the beginning of 2013.
The array consisted of a carpet of about 7000 m Resistive Plate Chambers
(RPCs) operated in streamer mode and equipped with both digital and analog
readout, providing the measurement of particle densities up to few particles
per cm. The unique detector features (full coverage, readout granularity,
wide dynamic range, etc) and location (very high altitude) allowed a detailed
study of the lateral density profile of charged particles at ground very close
to the shower axis and its description by a proper lateral distribution
function (LDF). In particular, the information collected in the first 10 m from
the shower axis have been shown to provide a very effective tool for the
determination of the shower development stage ("age") in the energy range 50
TeV - 10 PeV. The sensitivity of the age parameter to the mass composition of
primary Cosmic Rays is also discussed
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