412 research outputs found
Calculations of giant magnetoresistance in Fe/Cr trilayers using layer potentials determined from {\it ab-initio} methods
The ab initio full-potential linearized augmented plane-wave method
explicitly designed for the slab geometry was employed to elucidate the
physical origin of the layer potentials for the trilayers nFe/3Cr/nFe(001),
where n is the number of Fe monolayers. The thickness of the transition-metal
ferromagnet has been ranged from up to n=8 while the spacer thickness was
fixed to 3 monolayers. The calculated potentials were inserted in the
Fuchs-Sondheimer formalism in order to calculate the giant magnetoresistance
(GMR) ratio. The predicted GMR ratio was compared with the experiment and the
oscillatory behavior of the GMR as a function of the ferromagnetic layer
thickness was discussed in the context of the layer potentials. The reported
results confirm that the interface monolayers play a dominant role in the
intrinsic GMR.Comment: 17 pages, 7 figures, 3 tables. accepted in J. Phys.: Cond. Matte
Characterization of the autotrophic component in periphyton upon \u3ci\u3eTypha angustifolia\u3c/i\u3e detritus in a freshwater wetland
The autotrophic component of periphyton on Typha angustifolia detritus was characterized in a freshwater wetland during a single growing season. 58 genera of algae and cyanobacteria, representing six divisions, were observed throughout the study period. Although the combined algae-cyanobacteria density from within and outside the Typha stands were significantly affected by both sample date and the combination of date and location, no significant differences occurred in biovolume. Similarly, no clear evidence of successional patterns was observed.
Although few significant interactions were observed, Typha detritus provided a substratum for vast numbers and biomass of periphyton. A combined density for the observed taxa within the Typha stand averaged 134,588 cells cm-2, while the density outside the stand averaged 108,853 cells cm-2. The average total biovolume for the taxa within the Typha stand was 245 x 106 ± 23 x 106 and 136 x 103 ± 314 x 106 um3 cm-2 outside
Selfconsistent calculations of fission barriers in the Fm region
The fission barriers of the nuclei 254Fm, 256Fm, 258Fm, 258No and 260Rf are
investigated in a fully microscopic way up to the scission point. The analysis
is based on the constrained Hartree-Fock-Bogoliubov theory and Gogny's D1S
force. The quadrupole, octupole and hexadecapole moments as well as the number
of nucleons in the neck region are used as constraints. Two fission paths,
corresponding to the bimodal fission, are found. The decrease with isotope mass
of the half-life times of heavy Fm isotopes is also explained.Comment: 29 pages in LaTeX including 14 figure
Misfits in Skyrme-Hartree-Fock
We address very briefly five critical points in the context of the
Skyrme-Hartree-Fock (SHF) scheme: 1) the impossibility to consider it as an
interaction, 2) a possible inconsistency of correlation corrections as, e.g.,
the center-of-mass correction, 3) problems to describe the giant dipole
resonance (GDR) simultaneously in light and heavy nuclei, 4) deficiencies in
the extrapolation of binding energies to super-heavy elements (SHE), and 5) a
yet inappropriate trend in fission life-times when going to the heaviest SHE.
While the first two points have more a formal bias, the other three points have
practical implications and wait for solution.Comment: 9 pages, 4 figure
The Beam Conditions Monitor of the LHCb Experiment
The LHCb experiment at the European Organization for Nuclear Research (CERN)
is dedicated to precision measurements of CP violation and rare decays of B
hadrons. Its most sensitive components are protected by means of a Beam
Conditions Monitor (BCM), based on polycrystalline CVD diamond sensors. Its
configuration, operation and decision logics to issue or remove the beam permit
signal for the Large Hadron Collider (LHC) are described in this paper.Comment: Index Terms: Accelerator measurement systems, CVD, Diamond, Radiation
detector
Fission modes of mercury isotopes
Background: Recent experiments on beta-delayed fission in the mercury-lead
region and the discovery of asym- metric fission in 180 Hg [1] have stimulated
theoretical interest in the mechanism of fission in heavy nuclei. Purpose: We
study fission modes and fusion valleys in 180 Hg and 198 Hg to reveal the role
of shell effects in pre-scission region and explain the experimentally observed
fragment mass asymmetry and its variation with A. Methods: We use the
self-consistent nuclear density functional theory employing Skyrme and Gogny
energy density functionals. Results: The potential energy surfaces in
multi-dimensional space of collective coordinates, including elongation,
triaxiality, reflection-asymmetry, and necking, are calculated for 180 Hg and
198 Hg. The asymmetric fission valleys - well separated from fusion valleys
associated with nearly spherical fragments - are found in in both cases. The
density distributions at scission configurations are studied and related to the
experimentally observed mass splits. Conclusions: The energy density
functionals SkM\ast and D1S give a very consistent description of the fission
process in 180 Hg and 198 Hg. We predict a transition from asymmetric fission
in 180 Hg towards more symmetric distribution of fission fragments in 198 Hg.
For 180 Hg, both models yield 100 Ru/80 Kr as the most probable split. For 198
Hg, the most likely split is 108 Ru/90 Kr in HFB-D1S and 110 Ru/88 Kr in
HFB-SkM\ast.Comment: 6 pages, 5 figures, to be published in Physical Review
Analysis of bulk and surface contributions in the neutron skin of nuclei
The neutron skin thickness of nuclei is a sensitive probe of the nuclear
symmetry energy having multiple implications for nuclear and astrophysical
studies. However, precision measurements of this observable are difficult. The
analysis of the experimental data may imply some assumptions about the bulk or
surface nature of the formation of the neutron skin. Here, we study the bulk or
surface character of neutron skins of nuclei following from calculations with
Gogny, Skyrme, and covariant nuclear mean-field interactions. These
interactions are successful in describing nuclear charge radii and binding
energies but predict different values for neutron skins. We perform the study
by fitting two-parameter Fermi distributions to the calculated self-consistent
neutron and proton densities. We note that the equivalent sharp radius is a
more suitable reference quantity than the half-density radius parameter of the
Fermi distributions to discern between the bulk and surface contributions in
neutron skins. We present calculations for nuclei in the stability valley and
for the isotopic chains of Sn and Pb.Comment: 13 pages, 9 figure
Analysis of bulk and surface contributions in the neutron skin of nuclei
The neutron skin thickness of nuclei is a sensitive probe of the nuclear
symmetry energy having multiple implications for nuclear and astrophysical
studies. However, precision measurements of this observable are difficult. The
analysis of the experimental data may imply some assumptions about the bulk or
surface nature of the formation of the neutron skin. Here, we study the bulk or
surface character of neutron skins of nuclei following from calculations with
Gogny, Skyrme, and covariant nuclear mean-field interactions. These
interactions are successful in describing nuclear charge radii and binding
energies but predict different values for neutron skins. We perform the study
by fitting two-parameter Fermi distributions to the calculated self-consistent
neutron and proton densities. We note that the equivalent sharp radius is a
more suitable reference quantity than the half-density radius parameter of the
Fermi distributions to discern between the bulk and surface contributions in
neutron skins. We present calculations for nuclei in the stability valley and
for the isotopic chains of Sn and Pb.Comment: 13 pages, 9 figure
Fission half-lives of super-heavy nuclei in a microscopic approach
A systematic study of 160 heavy and super-heavy nuclei is performed in the
Hartree-Fock-Bogoliubov approach with the finite range and density dependent
Gogny force with the D1S parameter set. We show calculations in several
approximations: with axially symmetric and reflexion symmetric wave functions,
with axially symmetric and non-reflexion symmetric wave functions and finally
some representative examples with triaxial wave functions are also discussed.
Relevant properties of the ground state and along the fission path are
thoroughly analyzed. Fission barriers, Q-factors and lifetimes with
respect to fission and -decay as well as other observables are
discussed. Larger configuration spaces and more general HFB wave functions as
compared to previous studies provide a very good agreement with the
experimental data.Comment: 26 pages, 15 figure
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