59 research outputs found
Coupling of Surface and Volume Dipole Oscillations in C-60 Molecules
We first give a short review of the ``local-current approximation'' (LCA),
derived from a general variation principle, which serves as a semiclassical
description of strongly collective excitations in finite fermion systems
starting from their quantum-mechanical mean-field ground state. We illustrate
it for the example of coupled translational and compressional dipole
excitations in metal clusters. We then discuss collective electronic dipole
excitations in C molecules (Buckminster fullerenes). We show that the
coupling of the pure translational mode (``surface plasmon'') with
compressional volume modes in the semiclasscial LCA yields semi-quantitative
agreement with microscopic time-dependent density functional (TDLDA)
calculations, while both theories yield qualitative agreement with the recent
experimental observation of a ``volume plasmon''.Comment: LaTeX, 12 pages, 5 figures (8 *.eps files); Contribution to XIV-th
Nuclear Physics Workshop at Kazimierz Dolny, Poland, Sept. 26-29, 200
Excitation of soft dipole modes in electron scattering
The excitation of soft dipole modes in light nuclei via inelastic electron
scattering is investigated. I show that, under the proposed conditions of the
forthcoming electron-ion colliders, the scattering cross sections have a direct
relation to the scattering by real photons. The advantages of electron
scattering over other electromagnetic probes is explored. The response
functions for direct breakup are studied with few-body models. The dependence
upon final state interactions is discussed. A comparison between direct breakup
and collective models is performed. The results of this investigation are
important for the planned electron-ion colliders at the GSI and RIKEN
facilities.Comment: 23 pages, 8 figures, to be published in Physical Review
Excitations of pygmy dipole resonances in exotic and stable nuclei via Coulomb and nuclear fields
We study heavy-ion inelastic scattering processes in neutron-rich nuclei including the full response to the different multipolarities. Among these we focus in particular on the excitation of low-lying dipole states commonly associated to the pygmy dipole resonance. The multipole response is described within the Hartree-Fock plus RPA formalism with Skyrme interaction. We show how the combined information from reactions processes involving the Coulomb and different mixtures of isoscalar and isovector nuclear interactions can provide a clue to reveal the characteristic features of these states. We have performed calculation for the excitation of 132Sn generated in the reactions with 4He, 40Ca, and 48Ca at several incident energies, as well as for the system 17O +208Pb. Our results suggest that the investigation of the PDR states can be better carried out at low incident energies (below 50 MeV/nucleon). In fact, at these energies the PDR peak is relatively stronger than the giant dipole resonance (GDR) one and the narrow width of the low-lying quadrupole and octupole states should not blur its presence.Ministerio de Ciencia e InnovaciĂłn (España) y FEDER FPA2009-07653 FIS2008-04189Programa Consolider-Ingenio 2010 (España) CSD2007-00042Junta de AndalucĂa P07-FQM-02894 FQM16
Chaotic scattering on surfaces and collisional damping of collective modes
The damping of hot giant dipole resonances is investigated. The contribution
of surface scattering is compared with the contribution from interparticle
collisions. A unified response function is presented which includes surface
damping as well as collisional damping. The surface damping enters the response
via the Lyapunov exponent and the collisional damping via the relaxation time.
The former is calculated for different shape deformations of quadrupole and
octupole type. The surface as well as the collisional contribution each
reproduce almost the experimental value, therefore we propose a proper
weighting between both contributions related to their relative occurrence due
to collision frequencies between particles and of particles with the surface.
We find that for low and high temperatures the collisional contribution
dominates whereas the surface damping is dominant around the temperatures
of the centroid energy.Comment: PRC su
Skyrme-Rpa Description of Dipole Giant Resonance in Heavy and Superheavy Nuclei
The E1(T=1) isovector dipole giant resonance (GDR) in heavy and super-heavy
deformed nuclei is analyzed over a sample of 18 rare-earth nuclei, 4 actinides
and three chains of super-heavy elements (Z=102, 114 and 120). Basis of the
description is self-consistent separable RPA (SRPA) using the Skyrme force
SLy6. The self-consistent model well reproduces the experimental data (energies
and widths) in the rare-earth and actinide region. The trend of the resonance
peak energies follows the estimates from collective models, showing a bias to
the volume mode for the rare-earths isotopes and a mix of volume and surface
modes for actinides and super-heavy elements. The widths of the GDR are mainly
determined by the Landau fragmentation which in turn is found to be strongly
influenced by deformation. A deformation splitting of the GDR can contribute
about one third to the width and about 1 MeV further broadening can be
associated to mechanism beyond the mean-field description (escape, coupling
with complex configurations).Comment: 9 pages, 12 figures, 2 table
Properties of the nuclear medium
We review our knowledge on the properties of the nuclear medium that have
been studied, along many years, on the basis of many-body theory, laboratory
experiments and astrophysical observations. First we consider the realm of
phenomenological laboratory data and astrophysical observations, and the hints
they can give on the characteristics that the nuclear medium should possess.
The analysis is based on phenomenological models, that however have a strong
basis on physical intuition and an impressive success. More microscopic models
are also considered, and it is shown that they are able to give invaluable
information on the nuclear medium, in particular on its Equation of State. The
interplay between laboratory experiments and astrophysical observations are
particularly stressed, and it is shown how their complementarity enriches
enormously our insights into the structure of the nuclear medium. We then
introduce the nucleon-nucleon interaction and the microscopic many-body theory
of nuclear matter, with a critical discussion about the different approaches
and their results. The Landau Fermi Liquid theory is introduced and briefly
discussed. As illustrative example, we discuss neutron matter at very low
density, and it is shown how it can be treated within the many-body theory. A
section is dedicated to the pairing problem. The connection with nuclear
structure is then discussed, on the basis of the Energy Density Functional
method. The possibility to link the physics of exotic nuclei and the
astrophysics of neutron stars is particularly stressed. Finally we discuss the
thermal properties of the nuclear medium, in particular the liquid-gas phase
transition and its connection with the phenomenology on heavy ion reactions and
the cooling evolution of neutron stars. The presentation has been taken for
non-specialists and possibly for non-nuclear physicists.Comment: 90 pages, 29 figures, revised versio
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