463 research outputs found
Extended RPA within a solvable 3 level model
Working within an exactly solvable 3 level model, we discuss am extension of
the
Random Phase Approximation (RPA) based on a boson formalism. A boson
Hamiltonian is defined via a mapping procedure and its expansion truncated at
four-boson terms. RPA-type equations are then constructed and solved
iteratively. The new solutions gain in stability with respect to the RPA ones.
We perform diagonalizations of the boson Hamiltonian in spaces containing up to
four-phonon components. Approximate spectra exhibit an improved quality with
increasing the size of these multiphonon spaces. Special attention is addressed
to the problem of the anharmonicity of the spectrum.Comment: 5 figure
Effect of large neutron excess on the dipole response in the region of the Giant Dipole Resonance
The evolution of the Dipole Response in nuclei with strong neutron excess is
studied in the Hartree-Fock plus Random Phase Approximation with Skyrme forces.
We find that the neutron excess increases the fragmentation of the isovector
Giant Dipole Resonance, while pushing the centroid of the distribution to lower
energies beyond the mass dependence predicted by the collective models. The
radial separation of proton and neutron densities associated with a large
neutron excess leads to non vanishing isoscalar transition densities to the GDR
states, which are therefore predicted to be excited also by isoscalar nuclear
probes. The evolution of the isoscalar compression dipole mode as a function of
the neutron excess is finally studied. We find that the large neutron excess
leads to a strong concentration of the strength associated with the isoscalar
dipole operator , that mainly originates from uncorrelated
excitations of the neutrons of the skin.Comment: 11 pages 8 figures, use elsart.sty and graphics packag
Le malattie batteriche emergenti o riemergenti delle colture agrarie in Italia e nel bacino del Mediterraneo
Vengono descritte alcune malattie batteriche emergenti, o riemergenti, che potrebbero diventare un serio fattore limitante le produzioni ortofrutticole italiane e mediterranee
Extension of random-phase approximation preserving energy weighted sum rules: an application to a 3-level Lipkin model
A limitation common to all extensions of random-phase approximation including
only particle-hole configurations is that they violate to some extent the
energy weighted sum rules. Considering one such extension, the improved RPA
(IRPA), already used to study the electronic properties of metallic clusters,
we show how it can be generalized in order to eliminate this drawback. This is
achieved by enlarging the configuration space, including also elementary
excitations corresponding to the annihilation of a particle (hole) and the
creation of another particle (hole) on the correlated ground state. The
approach is tested within a solvable 3-level model.Comment: 2 figure
Second random-phase approximation with the Gogny force. First applications
We present the first applications of the second random-phase-approximation
model with the finite-range Gogny interaction. We discuss the advantages of
using such an interaction in this type of calculations where 2 particle-2 hole
configurations are included. The results found in the present work confirm the
well known general features of the second random-phase approximation spectra:
we find a large shift, several MeV, of the response centroids to lower energies
with respect to the corresponding random-phase-approximation values. As known,
these results indicate that the effects of the 1 particle-1 hole/2 particle-2
hole and 2 particle-2 hole/2 particle-2 hole couplings are important. It has
been found that the changes of the strength distributions with respect to the
standard random-phase-approximation results are particularly large in the
present case. This important effect is due to some large neutron-proton matrix
elements of the interaction and indicates that these matrix elements (which do
not contribute in the mean-field calculations employed in the conventional fit
procedures of the force parameters) should be carefully constrained to perform
calculation
Quenching of excitations in the double giant resonance
The electromagnetic excitation of the two-phonon isovector giant dipole
resonance in relativistic projectiles incident on heavy targets can be proceed
via several intermediate one-phonon giant resonance states. In two step
electric dipole transitions the population of , , and
two-phonon states are possible. We calculate the amplitude distribution of
excitations with an RPA formalism, and use it to calculate the
electromagnetic excitation of two-phonon states in second order perturbation
theory and coupled-channels. We show that a conspiracy between angular momentum
coupling and the strength of the electromagnetic fields suppresses
contributions of states to the total cross sections.Comment: 10 pages, 2 Postscript figures available upon reques
Many-body correlations in a multistep variational approach
We discuss a multistep variational approach for the study of many-body
correlations. The approach is developed in a boson formalism (bosons
representing particle-hole excitations) and based on an iterative sequence of
diagonalizations in subspaces of the full boson space. Purpose of these
diagonalizations is that of searching for the best approximation of the ground
state of the system. The procedure also leads us to define a set of excited
states and, at the same time, of operators which generate these states as a
result of their action on the ground state. We examine the cases in which these
operators carry one-particle one-hole and up to two-particle two-hole
excitations. We also explore the possibility of associating bosons to
Tamm-Dancoff excitations and of describing the spectrum in terms of only a
selected group of these. Tests within an exactly solvable three-level model are
provided.Comment: 24 pages, 6 figures, to appear in Phys. Rev.
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