371 research outputs found
Dispersion of the odd magnetic resonant mode in near-optimally doped Bi2Sr2CaCu2O8+d
We report a neutron scattering study of the spin excitation spectrum in the
superconducting state of slightly overdoped Bi2Sr2CaCu2O8+d system (Tc=87 K).
We focus on the dispersion of the resonance peak in the superconducting state
that is due to a S=1 collective mode. The measured spin excitation spectrum
bears a strong similarity to the spectrum of the YBa2Cu3O6+x system for a
similar doping level i.e. x= 0.95-1), which consists of intersecting upward-
and downward-dispersing branches. A close comparison of the threshold of the
electron-hole spin flip continuum, deduced from angle resolved photo-emission
measurements in the same system, indicates that the magnetic response in the
superconducting state is confined, in both energy and momentum, below the
gapped Stoner continuum. In contrast to YBa2Cu3O6+x, the spin excitation
spectrum is broader than the experimental resolution. In the framework of an
itinerant-electron model, we quantitatively relate this intrinsic energy width
to the superconducting gap distribution observed in scanning tunnelling
microscopy experiments. Our study further suggests a significant in-plane
anisotropy of the magnetic response.Comment: 10 figure
Dynamics of the magnetic flux trapped in fractal clusters of normal phase in a superconductor
The influence of geometry and morphology of superconducting structure on
critical currents and magnetic flux trapping in percolative type-II
superconductor is considered. The superconductor contains the clusters of a
normal phase, which act as pinning centers. It is found that such clusters have
significant fractal properties. The main features of these clusters are studied
in detail: the cluster statistics is analyzed; the fractal dimension of their
boundary is estimated; the distribution of critical currents is obtained, and
its peculiarities are explored. It is examined thoroughly how the finite
resolution capacity of the cluster geometrical size measurement affects the
estimated value of fractal dimension. The effect of fractal properties of the
normal phase clusters on the electric field arising from magnetic flux motion
is investigated in the case of an exponential distribution of cluster areas.
The voltage-current characteristics of superconductors in the resistive state
for an arbitrary fractal dimension are obtained. It is revealed that the
fractality of the boundaries of the normal phase clusters intensifies the
magnetic flux trapping and thereby raises the critical current of a
superconductor.Comment: revtex, 16 pages with 1 table and 5 figures; text and figures are
improved; more detailed version with geometric probability analisys of the
distribution of entry points into weak links over the perimeter of a normal
phase clusters and one additional figure is published in Phys.Rev.B;
alternative e-mail of author is [email protected]
Commensurate dynamic magnetic correlations in La2(Cu,Li)O4
When sufficient numbers of holes are introduced into the two-dimensional CuO2
square lattice, dynamic magnetic correlations become incommensurate with
underlying lattice in all previously investigated La_{2-x}A_xCu_{1-z}B_zO_{4+y}
(A=Sr or Nd, B=Zn) including high T_C superconductors and insulators, and in
bilayered superconducting YBa_2Cu_3O_{6.6} and Bi_2Sr_2CaCu_2O_8. Magnetic
correlations also become incommensurate in structurally related La_2NiO_4 when
doped with Sr or O. We report an exception to this so-far well established
experimental "rule" in La_2Cu_{1-z}Li_{z}O_4 in which magnetic correlations
remain commensurate.Comment: 4 pages, 3 figures, revised version as for publicatio
High transverse momentum eta meson production in p+p, d+Au and Au+Au collisions at sqrt(s_NN) = 200 GeV
Inclusive transverse momentum spectra of eta mesons in the range p_T~2-12
GeV/c have been measured at mid-rapidity (|\eta| < 0,35) by the PHENIX
experiment at RHIC in p+p, d+Au and Au+Au collisions at sqrt(s_NN) = 200 GeV.
The eta mesons are reconstructed through their eta--> \gamma\gamma channel for
the three colliding systems as well as through the eta-->pi^0 pi+ pi- decay
mode in p+p and d+Au collisions. The nuclear modification factor in d+Au
collisions, R_dAu(p_T~1.0-1.1, suggests at most only modest p_T broadening
("Cronin enhancement"). In central Au+Au reactions, the eta yields are
significantly suppressed, with R_AuAu(pT)~0.2. The ratio of eta to pi^0 yields
is approximately constant as a function of p_T for the three colliding systems
in agreement with the high-p_T world average of R_eta/pi^0 \approx 0.5 in
hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions for a wide range
of center-of-mass energies [sqrt(s_NN)~3-1800 GeV] as well as, for high scaled
momentum x_p, in e+e- annihilations at sqrt(s)=91.2 GeV. These results are
consistent with a scenario where high-p_T eta production in nuclear collisions
at RHIC is largely unaffected by initial-state effects, but where light-quark
mesons (pi^0;eta) are equally suppressed due to final-state interactions of the
parent partons in the dense medium produced in Au+Au reactions.Comment: 391 authors, NN pages text, RevTeX4, figures, tables. Submitted to
Physical Review C. Plain text data tables for the points plotted in figures
for this and previous PHENIX publications are (or will be) publicly available
at http://www.phenix.bnl.gov/papers.htm
Inclusive cross section and double helicity asymmetry for pi^0 production in p+p collisions at sqrt(s) = 62.4 GeV
The PHENIX experiment presents results from the RHIC 2006 run with polarized
proton collisions at sqrt(s) = 62.4 GeV for inclusive pi^0 production at
mid-rapidity. Unpolarized cross section results are measured for transverse
momenta p_T = 0.5 to 7 GeV/c. Next-to-leading order perturbative quantum
chromodynamics calculations are compared with the data, and while the
calculations are consistent with the measurements, next-to-leading logarithmic
corrections improve the agreement. Double helicity asymmetries A_LL are
presented for p_T = 1 to 4 GeV/c and probe the higher range of Bjorken_x of the
gluon (x_g) with better statistical precision than our previous measurements at
sqrt(s)=200 GeV. These measurements are sensitive to the gluon polarization in
the proton for 0.06 < x_g < 0.4.Comment: 387 authors from 63 institutions, 10 pages, 6 figures, 1 table.
Submitted to Physical Review D. Plain text data tables for the points plotted
in figures for this and previous PHENIX publications are (or will be)
publicly available at http://www.phenix.bnl.gov/papers.htm
System-Size Independence of Directed Flow Measured at the BNL Relativistic Heavy-Ion Collider
We measure directed flow (ν_1) for charged particles in Au+Au and Cu+Cu collisions at √S_(NN)=200 and 62.4 GeV, as a function of pseudorapidity (η), transverse momentum (p_t), and collision centrality, based on data from the STAR experiment. We find that the directed flow depends on the incident energy but, contrary to all available model implementations, not on the size of the colliding system at a given centrality. We extend the validity of the limiting fragmentation concept to ν_1 in different collision systems, and investigate possible explanations for the observed sign change in ν_1(p_t)
Centrality dependence of charged hadron production in deuteron+gold and nucleon+gold collisions at sqrt(s_NN)=200 GeV
We present transverse momentum (p_T) spectra of charged hadrons measured in
deuteron-gold and nucleon-gold collisions at \sqrts = 200 GeV for four
centrality classes. Nucleon-gold collisions were selected by tagging events in
which a spectator nucleon was observed in one of two forward rapidity
detectors. The spectra and yields were investigated as a function of the number
of binary nucleon-nucleon collisions, \nu, suffered by deuteron nucleons. A
comparison of charged particle yields to those in p+p collisions show that the
yield per nucleon-nucleon collision saturates with \nu for high momentum
particles. We also present the charged hadron to neutral pion ratios as a
function of p_T.Comment: 330 authors, 15 pages text, 16 figures, 3 tables. Submitted to Phys.
Rev. Lett. v2 has minor changes to reflect revisions during review process.
Plain text data tables for the points plotted in figures for this and
previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
Inclusive cross section and single-transverse-spin asymmetry for very forward neutron production in polarized p+p collisions at sqrt(s)=200 GeV
The energy dependence of the single-transverse-spin asymmetry, A_N, and the
cross section for neutron production at very forward angles were measured in
the PHENIX experiment at RHIC for polarized p+p collisions at sqrt(s)=200 GeV.
The neutrons were observed in forward detectors covering an angular range of up
to 2.2 mrad. We report results for neutrons with momentum fraction of x_F=0.45
to 1.0. The energy dependence of the measured cross sections were consistent
with x_F scaling, compared to measurements by an ISR experiment which measured
neutron production in unpolarized p+p collisions at sqrt(s)=30.6--62.7 GeV. The
cross sections for large x_F neutron production for p+p collisions, as well as
those in e+p collisions measured at HERA, are described by a pion exchange
mechanism. The observed forward neutron asymmetries were large, reaching
A_N=-0.08+/-0.02 for x_F=0.8; the measured backward asymmetries, for negative
x_F, were consistent with zero. The observed asymmetry for forward neutron
production is discussed within the pion exchange framework, with interference
between the spin-flip amplitude due to the pion exchange and nonflip amplitudes
from all Reggeon exchanges. Within the pion exchange description, the measured
neutron asymmetry is sensitive to the contribution of other Reggeon exchanges
even for small amplitudes.Comment: 383 authors, 16 pages, 18 figures, 6 tables. Submitted to Phys. Rev.
D. Plain text data tables for the points plotted in figures for this and
previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
Cross sections and double-helicity asymmetries of midrapidity inclusive charged hadrons in p+p collisions at sqrt(s)=62.4 GeV
Unpolarized cross sections and double-helicity asymmetries of
single-inclusive positive and negative charged hadrons at midrapidity from p+p
collisions at sqrt(s)=62.4 GeV are presented. The PHENIX measurements for 1.0 <
p_T < 4.5 GeV/c are consistent with perturbative QCD calculations at
next-to-leading order in the strong coupling constant, alpha_s. Resummed pQCD
calculations including terms with next-to-leading-log accuracy, yielding
reduced theoretical uncertainties, also agree with the data. The
double-helicity asymmetry, sensitive at leading order to the gluon polarization
in a momentum-fraction range of 0.05 ~< x_gluon ~< 0.2, is consistent with
recent global parameterizations disfavoring large gluon polarization.Comment: PHENIX Collaboration. 447 authors, 12 pages, 5 figures, 5 tables.
Submitted to Physical Review
Systematic Measurements of Identified Particle Spectra in pp, d+Au and Au+Au Collisions from STAR
Identified charged particle spectra of , , and
\pbar at mid-rapidity () measured by the \dedx method in the
STAR-TPC are reported for and d+Au collisions at \snn = 200 GeV and for
Au+Au collisions at 62.4 GeV, 130 GeV, and 200 GeV. ... [Shortened for arXiv
list. Full abstract in manuscript.]Comment: 58 pages, 46 figures, 37 table
- …