9,062 research outputs found
Saturation from nuclear pion dynamics
We construct an equation-of-state for nuclear matter based on the chiral
Lagrangian. The relevant scales are discussed and an effective chiral power
expansion scheme, which is constructed to work around the nuclear saturation
density, is presented. A realistic equation-of-state is obtained by adjusting
one free parameter, when the leading and subleading terms in the expansion are
included. The saturation mechanism is due to correlations induced by the
one-pion-exchange interaction. Furthermore, we find a substantial deviation
from the Fermi-gas estimate of the quark condensate in nuclear matter already
at the saturation density.Comment: revised version, with minor corrections, 13 pages, 3 Postscript
figure
Antikaon production in nucleon-nucleon reactions near threshold
The antikaon production cross section from nucleon-nucleon reactions near
threshold is studied in a meson exchange model. We include both pion and kaon
exchange, but neglect the interference between the amplitudes. In case of pion
exchange the antikaon production cross section can be expressed in terms of the
antikaon production cross section from a pion-nucleon interaction, which we
take from the experimental data if available. Otherwise, a -resonance
exchange model is introduced to relate the different reaction cross sections.
In case of kaon exchange the antikaon production cross section is related to
the elastic and cross sections, which are again taken from
experimental measurements. We find that the one-meson exchange model gives a
satisfactory fit to the available data for the cross section
at high energies. We compare our predictions for the cross section near
threshold with an earlier empirical parameterization and that from phase space
models.Comment: 16 pages, LaTeX, 5 postscript figures included, submitted to Z. Phys.
Relativistic Hamiltonians in many-body theories
We discuss the description of a many-body nuclear system using Hamiltonians
that contain the nucleon relativistic kinetic energy and potentials with
relativistic corrections. Through the Foldy-Wouthuysen transformation, the
field theoretical problem of interacting nucleons and mesons is mapped to an
equivalent one in terms of relativistic potentials, which are then expanded at
some order in 1/m_N. The formalism is applied to the Hartree problem in nuclear
matter, showing how the results of the relativistic mean field theory can be
recovered over a wide range of densities.Comment: 14 pages, uses REVTeX and epsfig, 3 postscript figures; a postscript
version of the paper is available by anonymous ftp at
ftp://carmen.to.infn.it/pub/depace/papers/951
Magnetic quantization of electronic states in d-wave superconductors
We derive a general quasiclassical approach for long-range magnetic-field
quantization effects in superconductors. The method is applied to superclean
d-wave superconductors in the mixed state. We study the delocalized states with
energies . We find that the energy
spectrum consists of narrow energy bands whose centers are located at the
Landau levels calculated in absence of the vortex potential. We show that
transitions between the states belonging to the different Landau levels give
rise to resonances in the a.c. quasiparticle conductivity and in the a.c.
vortex friction.Comment: 11 pages, no figure
Neutron diffraction study of spin and charge ordering in SrFeO(3-delta)
We report a comprehensive neutron diffraction study of the crystal structure
and magnetic order in a series of single-crystal and powder samples of
SrFeO in the vacancy range . The data
provide detailed insights into the interplay between the oxygen vacancy order
and the magnetic structure of this system. In particular, a crystallographic
analysis of data on Sr8Fe8O23 revealed a structural transition between the
high-temperature tetragonal and a low-temperature monoclinic phase with a
critical temperature T = 75 K, which originates from charge ordering on the Fe
sublattice and is associated with a metal-insulator transition. Our experiments
also revealed a total of seven different magnetic structures of
SrFeO in this range of , only two of which (namely an
incommensurate helix state in SrFeO3 and a commensurate, collinear
antiferromagnetic state in Sr4Fe4O11) had been identified previously. We
present a detailed refinement of some of the magnetic ordering patterns and
discuss the relationship between the magneto-transport properties of
SrFeO samples and their phase composition and magnetic
microstructure.Comment: 37 page
Cortical thickness, surface area and volume measures in Parkinson's disease, multiple system atrophy and progressive supranuclear palsy
OBJECTIVE
Parkinson's disease (PD), Multiple System Atrophy (MSA) and Progressive Supranuclear Palsy (PSP) are neurodegenerative diseases that can be difficult to distinguish clinically. The objective of the current study was to use surface-based analysis techniques to assess cortical thickness, surface area and grey matter volume to identify unique morphological patterns of cortical atrophy in PD, MSA and PSP and to relate these patterns of change to disease duration and clinical features.
METHODS
High resolution 3D T1-weighted MRI volumes were acquired from 14 PD patients, 18 MSA, 14 PSP and 19 healthy control participants. Cortical thickness, surface area and volume analyses were carried out using the automated surface-based analysis package FreeSurfer (version 5.1.0). Measures of disease severity and duration were assessed for correlation with cortical morphometric changes in each clinical group.
RESULTS
Results show that in PSP, widespread cortical thinning and volume loss occurs within the frontal lobe, particularly the superior frontal gyrus. In addition, PSP patients also displayed increased surface area in the pericalcarine. In comparison, PD and MSA did not display significant changes in cortical morphology.
CONCLUSION
These results demonstrate that patients with clinically established PSP exhibit distinct patterns of cortical atrophy, particularly affecting the frontal lobe. These results could be used in the future to develop a useful clinical application of MRI to distinguish PSP patients from PD and MSA patients
Nuclear spins, magnetic moments and quadrupole moments of Cu isotopes from N = 28 to N = 46: probes for core polarization effects
Measurements of the ground-state nuclear spins, magnetic and quadrupole
moments of the copper isotopes from 61Cu up to 75Cu are reported. The
experiments were performed at the ISOLDE facility, using the technique of
collinear laser spectroscopy. The trend in the magnetic moments between the
N=28 and N=50 shell closures is reasonably reproduced by large-scale
shell-model calculations starting from a 56Ni core. The quadrupole moments
reveal a strong polarization of the underlying Ni core when the neutron shell
is opened, which is however strongly reduced at N=40 due to the parity change
between the and orbits. No enhanced core polarization is seen beyond
N=40. Deviations between measured and calculated moments are attributed to the
softness of the 56Ni core and weakening of the Z=28 and N=28 shell gaps.Comment: 13 pagers, 19 figures, accepted by Physical Review
Jahn-Teller versus quantum effects in the spin-orbital material LuVO3
We report on combined neutron and resonant x-ray scattering results,
identifying the nature of the spin-orbital ground state and magnetic
excitations in LuVO3 as driven by the orbital parameter. In particular, we
distinguish between models based on orbital Peierls dimerization, taken as a
signature of quantum effects in orbitals, and Jahn-Teller distortions, in favor
of the latter. In order to solve this long-standing puzzle, polarized neutron
beams were employed as a prerequisite in order to solve details of the magnetic
structure, which allowed quantitative intensity-analysis of extended magnetic
excitation data sets. The results of this detailed study enabled us to draw
definite conclusions about classical vs quantum behavior of orbitals in this
system and to discard the previous claims about quantum effects dominating the
orbital physics of LuVO3 and similar systems.Comment: Phys. Rev. B 91, 161104(R) (2015
Equation of State of Nuclear Matter at high baryon density
A central issue in the theory of astrophysical compact objects and heavy ion
reactions at intermediate and relativistic energies is the Nuclear Equation of
State (EoS). On one hand, the large and expanding set of experimental and
observational data is expected to constrain the behaviour of the nuclear EoS,
especially at density above saturation, where it is directly linked to
fundamental processes which can occur in dense matter. On the other hand,
theoretical predictions for the EoS at high density can be challenged by the
phenomenological findings. In this topical review paper we present the
many-body theory of nuclear matter as developed along different years and with
different methods. Only nucleonic degrees of freedom are considered. We compare
the different methods at formal level, as well as the final EoS calculated
within each one of the considered many-body schemes. The outcome of this
analysis should help in restricting the uncertainty of the theoretical
predictions for the nuclear EoS.Comment: 51 pages, to appear in J. Phys. G as Topical Revie
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