473 research outputs found
Evidence for nonmonotonic magnetic field penetration in a type-I superconductor
Polarized neutron reflectometry (PNR) provides evidence that nonlocal
electrodynamics governs the magnetic field penetration in an extreme low-k
superconductor. The sample is an indium film with a large elastic mean free
path (11 mkm) deposited on a silicon oxide wafer. It is shown that PNR can
resolve the difference between the reflected neutron spin asymmetries predicted
by the local and nonlocal theories of superconductivity. The experimental data
support the nonlocal theory, which predicts a nonmonotonic decay of the
magnetic field.Comment: 5 pages, 4 figures, LaTex, corrected typos and figure
Orientation of Vortices in a Superconducting Thin-Film: Quantitative Comparison of Spin-Polarized Neutron Reflectivity and Magnetization
We present a quantitative comparison of the magnetization measured by
spin-polarized neutron reflectivity (SPNR) and DC magnetometry on a 1370 \AA\
-thick Nb superconducting film. As a function of magnetic field applied in the
film plane, SPNR exhibits reversible behavior whereas the DC magnetization
shows substantial hysteresis. The difference between these measurements is
attributed to a rotation of vortex magnetic field out of the film plane as the
applied field is reduced. Since SPNR measures only the magnetization parallel
to the film plane whereas DC magnetization is strongly influenced by the
perpendicular component of magnetization when there is a slight sample tilt,
combining the two techniques allows one to distinguish two components of
magnetization in a thin film.Comment: 12 pages, 8 figures, It will be printed in PRB, Oct. 200
Evidence for the Nucleon Resonance from Combined Studies of CLAS Photo- and Electroproduction Data
The analysis of the nine 1-fold differential cross sections for the
photo- and electroproduction reactions
obtained with the CLAS detector at Jefferson Laboratory was carried out with
the goal to establish the contributing resonances in the mass range from
1.6~GeV to 1.8~GeV. In order to describe the photo- and electroproduction data
with -independent resonance masses and hadronic decay widths in the
range below 1.5~GeV, it was found that an state is required
in addition to the already well-established nucleon resonances. This work
demonstrates that the combined studies of photo- and
electroproduction data are vital for the observation of this resonance. The
contributions from the state and the already established
state with a mass of 1.745~GeV are well separated by their
different hadronic decays to the and final states and the
different -evolution of their photo-/electroexcitation amplitudes. The
state is the first recently established baryon resonance for
which the results on the -evolution of the photo-/electrocouplings have
become available. These results are important for the exploration of the nature
of the ``missing'' baryon resonances.Comment: accepted for publication in Phys. Lett.
First Measurement of the Polarization Observable E in the p→ (ɣ→, π+) in Reaction up to 2.25 Gev
First results from the longitudinally polarized frozen-spin target (FROST) program are reported. The double-polarization observable E, for the reaction ɣ→p→→π+n, has been measured using a circularly polarized tagged-photon beam, with energies from 0.35 to 2.37 GeV. The final-state pions were detected with the CEBAF Large Acceptance Spectrometer in Hall B at the Thomas Jefferson National Accelerator Facility. These polarization data agree fairly well with previous partial-wave analyses at low photon energies. Over much of the covered energy range, however, significant deviations are observed, particularly in the high-energy region where high-L multipoles contribute. The data have been included in new multipole analyses resulting in updated nucleon resonance parameters. We report updated fits from the Bonn-Gatchina, Jfilich-Bonn, and SAID groups
Magnetic phases and reorientation transitions in antiferromagnetically coupled multilayers
In antiferromagnetically coupled superlattices grown on (001) faces of cubic
substrates, e.g. based on materials combinations as Co/Cu, Fe/Si, Co/Cr, or
Fe/Cr, the magnetic states evolve under competing influence of bilinear and
biquadratic exchange interactions, surface-enhanced four-fold in-plane
anisotropy, and specific finite-size effects. Using phenomenological
(micromagnetic) theory, a comprehensive survey of the magnetic states and
reorientation transitions has been carried out for multilayer systems with even
number of ferromagnetic sub-layers and magnetizations in the plane. In
two-layer systems (N=2) the phase diagrams in dependence on components of the
applied field in the plane include ``swallow-tail'' type regions of
(metastable) multistate co-existence and a number of continuous and
discontinuous reorientation transitions induced by radial and transversal
components of the applied field. In multilayers (N \ge 4) noncollinear states
are spatially inhomogeneous with magnetization varying across the multilayer
stack. For weak four-fold anisotropy the magnetic states under influence of an
applied field evolve by a complex continuous reorientation into the saturated
state. At higher anisotropy they transform into various inhomogeneous and
asymmetric structures. The discontinuous transitions between the magnetic
states in these two-layers and multilayers are characterized by broad ranges of
multi-phase coexistence of the (metastable) states and give rise to specific
transitional domain structures.Comment: Manuscript 34 pages, 14 figures; submitted for publicatio
A New Measurement of the Radiative Decay Width
High precision measurements of the differential cross sections for
photoproduction at forward angles for two nuclei, C and Pb, have
been performed for incident photon energies of 4.9 - 5.5 GeV to extract the
decay width. The experiment was done at Jefferson
Lab using the Hall B photon tagger and a high-resolution multichannel
calorimeter. The decay width was extracted by
fitting the measured cross sections using recently updated theoretical models
for the process. The resulting value for the decay width is . With the 2.8% total uncertainty, this result is a factor of 2.5 more
precise than the current PDG average of this fundamental quantity and it is
consistent with current theoretical predictions.Comment: 4 pages, 5 figure
A new highly segmented start counter for the CLAS detector
The design, construction and performance of a highly segmented Start Counter are described. The Start Counter is an integral part of the trigger used in photon beam running with CLAS in Hall B at the Thomas Jefferson National Accelerator Facility (TJNAF). The Start Counter is constructed of 24 2.2-mm-thick single-ended scintillation paddles, forming a hermetic hexagon around the target region. This device measures the interaction time of the incoming photon in the target by detecting the outgoing particles. The counter provides complex trigger topologies, shows good efficiency and achieved a time resolution of 350 ps
Beam-target helicity asymmetry for γ→n→→π−p in the N*resonance region
We report the first beam-target double-polarization asymmetries in the γ þ nðpÞ → π− þ pðpÞ reaction
spanning the nucleon resonance region from invariant mass W ¼ 1500 to 2300 MeV. Circularly polarized
photons and longitudinally polarized deuterons in solid hydrogen deuteride (HD) have been used with the
CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. The exclusive final state has been
extracted using three very different analyses that show excellent agreement, and these have been used to
deduce the E polarization observable for an effective neutron target. These results have been incorporated
into new partial wave analyses and have led to significant revisions for several γnN* resonance
photocouplings
- …