97 research outputs found
Proton-neutron pairing in N=Z nuclei: quartetting versus pair condensation
The isoscalar proton-neutron pairing and isovector pairing, including both
isovector proton-neutron pairing and like-particle pairing, are treated in a
formalism which conserves exactly the particle number and the isospin. The
formalism is designed for self-conjugate (N=Z) systems of nucleons moving in
axially deformed mean fields and interacting through the most general isovector
and isoscalar pairing interactions. The ground state of these systems is
described by a superposition of two types of condensates, i.e., condensates of
isovector quartets, built by two isovector pairs coupled to the total isospin
T=0, and condensates of isoscalar proton-neutron pairs. The comparison with the
exact solutions of realistic isovector-isoscalar pairing Hamiltonians shows
that this ansatz for the ground state is able to describe with high precision
the pairing correlation energies. It is also shown that, at variance with the
majority of Hartree-Fock-Bogoliubov calculations, in the present formalism the
isovector and isoscalar pairing correlations coexist for any pairing
interactions. The competition between the isovector and isoscalar
proton-neutron pairing correlations is studied for N=Z nuclei with the valence
nucleons moving in the and shells and in the major shell above
Sn. We find that in these nuclei the isovector pairing prevail over the
isoscalar pairing, especially for heavier nuclei. However, the isoscalar
proton-neutron correlations are significant in all nuclei and they always
coexist with the isovector pairing correlations.Comment: 12 pages, 1 figur
Subtraction method in the second random--phase approximation: first applications with a Skyrme energy functional
We make use of a subtraction procedure, introduced to overcome
double--counting problems in beyond--mean--field theories, in the second
random--phase--approximation (SRPA) for the first time. This procedure
guarantees the stability of SRPA (so that all excitation energies are real). We
show that the method fits perfectly into nuclear density--functional theory. We
illustrate applications to the monopole and quadrupole response and to
low--lying and states in the nucleus O. We show that the
subtraction procedure leads to: (i) results that are weakly cutoff dependent;
(ii) a considerable reduction of the SRPA downwards shift with respect to the
random--phase approximation (RPA) spectra (systematically found in all previous
applications). This implementation of the SRPA model will allow a reliable
analysis of the effects of 2 particle--2 hole configurations () on the
excitation spectra of medium--mass and heavy nuclei.Comment: 1 tex, 16 figure
Deformation effects on the coexistence between neutron-proton and particle like pairing in N=Z medium mass nuclei
A model combining self-consistent mean-field and shell-model techniques is
used to study the competition between particle like and proton-neutron pairing
correlations in fp-shell even-even self-conjugate nuclei. Results obtained
using constant two-body pairing interactions as well as more sophisticated
interactions are presented and discussed. The standard BCS calculations are
systematically compared with more refined approaches including correlation
effects beyond the independent quasi-particle approach. The competition between
proton-neutron correlations in the isoscalar and isovector channels is also
analyzed, as well as their dependence on the deformation properties. Besides
the expected role of the spin-orbit interaction and particle number
conservation, it is shown that deformation leads to a reduction of the pairing
correlations. This reduction originates from the change of the single-particle
spectrum and from a quenching of the residual pairing matrix elements. The
competition between isoscalar and isovector pairing in the deuteron transfer is
finally addressed. Although a strong dependence the isovector pairing
correlations with respect to nuclear deformation is observed, they always
dominate over the isoscalar ones.Comment: 16 pages, 14 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
Coexistence of quartets and pairs in even-even nuclei
We analyse the structure of the ground states of even-even nuclei with
nucleons moving in the same major shell and interacting via realistic two-body
forces of shell-model type. We express the ground states of these nuclei as a
product of a quartet term, which represents the subsystem, and a pair
condensate built with the excess neutrons. The accuracy of this approximation
is discussed for nuclei with valence nucleons in the and major
shells.Comment: 9 pages, 4 figure
Low-lying dipole response in the stable Ca nuclei with the second random-phase approximation
Low-energy dipole excitations are analyzed for the stable isotopes Ca
and Ca in the framework of the Skyrme-second random-phase approximation.
The corresponding random-phase approximation calculations provide a negligible
strength distribution for both nuclei in the energy region from 5 to 10 MeV.
The inclusion and the coupling of 2 particle-2 hole configurations in the
second random-phase approximation lead to an appreciable dipole response at low
energies for the neutron-rich nucleus Ca. The presence of a neutron skin
in the nucleus Ca would suggest the interpretation of the low-lying
response in terms of a pygmy excitation. The composition of the excitation
modes (content of 1 particle-1 hole and 2 particle-2 hole configurations),
their transition densities and their collectivity (number and coherence of the
different contributions) are analyzed. This analysis indicates that, in
general, these excitations cannot be clearly interpreted in terms of
oscillations of the neutron skin against the core with the exception of the
peak with the largest value, which is located at 9.09 MeV. For this
peak the neutron transition density dominates and the neutron and proton
transition densities oscillate out of phase in the internal part of the nucleus
leading to a strong mixing of isoscalar and isovector components. Therefore,
this state shows some features usually associated to pygmy resonances
Excitation of Pygmy Dipole Resonance in neutron-rich nuclei via Coulomb and nuclear fields
We study the nature of the low-lying dipole strength in neutron-rich nuclei,
often associated to the Pygmy Dipole Resonance. The states are described within
the Hartree-Fock plus RPA formalism, using different parametrizations of the
Skyrme interaction. We show how the information from combined 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.Comment: 9 Pages, 8 figures, contribution to International Symposium On
Nuclear Physics, December 8-12, 2009,Bhabha Atomic Research Centre, Mumbai,
Indi
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 Innovación (España) y FEDER FPA2009-07653 FIS2008-04189Programa Consolider-Ingenio 2010 (España) CSD2007-00042Junta de Andalucía P07-FQM-02894 FQM16
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