499 research outputs found
Theoretical understanding of the nuclear incompressibility: where do we stand ?
The status of the theoretical research on the compressional modes of finite
nuclei and the incompressibility of nuclear matter, is reviewed. It
is argued that the recent experimental data on the Isoscalar Giant Monopole
Resonance (ISGMR) allow extracting the value of with an uncertainity
of about 12 MeV. Non-relativistic (Skyrme, Gogny) and relativistic mean
field models predict for values which are significantly different
from one another, namely 220-235 and 250-270 MeV
respectively. It is shown that the solution of this puzzle requires a better
determination of the symmetry energy at, and around, saturation. The role
played by the experimental data of the Isoscalar Giant Dipole Resonance (ISGDR)
is also discussed.Comment: To appear in the proceedings of the COMEX1 conference (special issue
of Nucl. Phys. A). Few changes and corrections compared to the previous
version. General conclusion unchange
Dipole states in stable and unstable nuclei
A nuclear structure model based on linear response theory (i.e., Random Phase
Approximation) and which includes pairing correlations and anharmonicities
(coupling with collective vibrations), has been implemented in such a way that
it can be applied on the same footing to magic as well as open-shell nuclei. As
applications, we have chosen to study the dipole excitations both in
well-known, stable isotopes like Pb and Sn as well as in the
neutron-rich, unstable Sn nucleus, by addressing in the latter case the
question about the nature of the low-lying strength. Our results suggest that
the model is reliable and predicts in all cases low-lying strength of non
collective nature.Comment: 16 pages, 6 figures; submitted for publicatio
Exotic modes of excitation in atomic nuclei far from stability
We review recent studies of the evolution of collective excitations in atomic
nuclei far from the valley of -stability. Collective degrees of freedom
govern essential aspects of nuclear structure, and for several decades the
study of collective modes such as rotations and vibrations has played a vital
role in our understanding of complex properties of nuclei. The multipole
response of unstable nuclei and the possible occurrence of new exotic modes of
excitation in weakly-bound nuclear systems, present a rapidly growing field of
research, but only few experimental studies of these phenomena have been
reported so far. Valuable data on the evolution of the low-energy dipole
response in unstable neutron-rich nuclei have been gathered in recent
experiments, but the available information is not sufficient to determine the
nature of observed excitations. Even in stable nuclei various modes of giant
collective oscillations had been predicted by theory years before they were
observed, and for that reason it is very important to perform detailed
theoretical studies of the evolution of collective modes of excitation in
nuclei far from stability. We therefore discuss the modern theoretical tools
that have been developed in recent years for the description of collective
excitations in weakly-bound nuclei. The review focuses on the applications of
these models to studies of the evolution of low-energy dipole modes from stable
nuclei to systems near the particle emission threshold, to analyses of various
isoscalar modes, those for which data are already available, as well as those
that could be observed in future experiments, to a description of
charge-exchange modes and their evolution in neutron-rich nuclei, and to
studies of the role of exotic low-energy modes in astrophysical processes.Comment: 123 pages, 59 figures, submitted to Reports on Progress in Physic
Calculation of stellar electron-capture cross sections on nuclei based on microscopic Skyrme functionals
A fully self-consistent microscopic framework for evaluation of nuclear
weak-interaction rates at finite temperature is introduced, based on Skyrme
functionals. The single-nucleon basis and the corresponding thermal occupation
factors of the initial nuclear state are determined in the finite-temperature
Skyrme Hartree-Fock model, and charge-exchange transitions to excited states
are computed using the finite-temperature RPA. Effective interactions are
implemented self-consistently: both the finite-temperature single-nucleon
Hartree-Fock equations and the matrix equations of RPA are based on the same
Skyrme energy density functional. Using a representative set of Skyrme
functionals, the model is applied in the calculation of stellar
electron-capture cross sections for selected nuclei in the iron mass group and
for neutron-rich Ge isotopes.Comment: 31 pages, 13 figures, submitted to Physical Review
Room Temperature Organic Superconductor?
The electron--phonon coupling in fullerene C28 has been calculated from first
principles. The value of the associated coupling constant lambda/N(0) is found
to be a factor three larger than that associated with C60. Assuming similar
values of the density of levels at the Fermi surface N(0) and of the Coulomb
pseudopotential for C28-based solids as those associated with alkali-doped
fullerides A3C60, one obtains Tc(C28) \approx 8 Tc(C60).Comment: 10 pages, 2 figure
Compression modes in nuclei: microscopic models with Skyrme interactions
The isoscalar giant monopole resonances (ISGMR) and giant dipole resonances
(ISGDR) in medium-heavy nuclei are investigated in the framework of HF+RPA and
HF-BCS+QRPA with Skyrme effective interactions. It is found that pairing has
little effect on these modes. It is also found that the coupling of the RPA
states to 2p-2h configurations results in about (or less than) 1 MeV shifts of
the resonance energies and at the same time gives the correct total widths. For
the ISGMR, comparison with recent data leads to a value of nuclear matter
compression modulus close to 215 MeV. However, a discrepancy between calculated
and measured energies of the ISGDR in Pb is found and remains an open
problem.Comment: To appear in: ``RIKEN Symposium and Workshop on Selected Topics in
Nuclear Collective Excitations'', proceedings of the meeting, RIKEN, Wako
city (Japan), March 20--24, 199
Quasi-particle random phase approximation with quasi-particle-vibration coupling: application to the Gamow-Teller response of the superfluid nucleus Sn
We propose a self-consistent quasi-particle random phase approximation (QRPA)
plus quasi-particle-vibration coupling (QPVC) model with Skyrme interactions to
describe the width and the line shape of giant resonances in open-shell nuclei,
in which the effect of superfluidity should be taken into account in both the
ground state and the excited states. We apply the new model to the Gamow-Teller
resonance in the superfluid nucleus Sn, including both the isoscalar
spin-triplet and the isovector spin-singlet pairing interactions. The strength
distribution in Sn is well reproduced and the underlying microscopic
mechanisms, related to QPVC and also to isoscalar pairing, are analyzed in
detail.Comment: 32 pages, 11 figures, 4 table
The pygmy dipole strength, the neutron radius of Pb and the symmetry energy
The accurate characterization of the nuclear symmetry energy and its density
dependence is one of the outstanding open problems in nuclear physics. A
promising nuclear observable in order to constrain the density dependence of
the symmetry energy at saturation is the neutron skin thickness of medium and
heavy nuclei. Recently, a low-energy peak in the isovector dipole response of
neutron-rich nuclei has been discovered that may be correlated with the neutron
skin thickness. The existence of this correlation is currently under debate due
to our limited experimental knowledge on the microscopic structure of such a
peak. We present a detailed analysis of Skyrme Hartree-Fock (HF) plus random
phase approximation (RPA) predictions for the dipole response in several
neutron-rich nuclei and try to elucidate whether models of common use in
nuclear physics confirm or dismiss its possible connection with the neutron
skin thickness. Finally, we briefly present theoretical results for parity
violating electron scattering on Pb at the conditions of the PREx
experiment and discuss the implications for the neutron skin thickness of
Pb and the slope of the symmetry energy.Comment: Contribution to the 2nd Iberian Nuclear Astrophysics Meeting on
Compact Stars proceeding
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