No abstract

No abstract

No abstract

Isoscalar (T=0,J=1) and isovector (T=1,J=0) pairing correlations in the ground state of self-conjugate nuclei are treated in terms of alpha-like quartets built by two protons and two neutrons coupled to total isospin T=0 and total angular momentum J=0. Quartets are constructed dynamically via an iterative variational procedure and the ground state is represented as a product of such quartets. It is shown that the quartet formalism describes accurately the ground state energies of realistic isovector plus isoscalar pairing Hamiltonians in nuclei with valence particles outside the 16O, 40Ca and 100Sn cores. Within the quartet formalism we analyse the competition between isovector and isoscalar pairing correlations and find that for nuclei with the valence nucleons above the cores 40Ca and 100Sn the isovector correlations account for the largest fraction of the total pairing correlations. This is not the case for sd-shell nuclei for which isoscalar correlations prevail. Contrary to many mean-field studies, isovector and isoscalar pairing correlations mix significantly in the quartet approach.Comment: 13 page

No abstract

Low-energy spectra of 4n nuclei are described with high accuracy in terms of four-body correlated structures ("quartets"). The states of all N ≥ Z nuclei belonging to the A = 24 isobaric chain are represented as a superposition of two-quartet states, with quartets being characterized by isospin T and angular momentum J. These quartets are assumed to be those describing the lowest states in 20 Ne (Tz=0), 20 F (Tz=1) and 20 O (Tz=2). We find that the spectrum of the self-conjugate nucleus 24 Mg can be well reproduced in terms of T =0 quartets only and that, among these, the J=0 quartet plays by far the leading role in the structure of the ground state. The same conclusion is drawn in the case of the three-quartet N = Z nucleus 28 Si. As an application of the quartet formalism to nuclei not confined to the sd shell, we provide a description of the low-lying spectrum of the proton-rich 92 Pd. The results achieved indicate that, in 4n nuclei, four-body degrees of freedom are more important and more general than usually expected. In nuclear physics, four-body correlated structures are usually associated with α-clustering. α-clustering is known to be a relevant phenomenon in light N = Z nuclei especially at excitation energies close to the α emission threshold. A common theoretical approach to α-clustering is represented by the α-cluster model [1]. According to this model, the nucleus consists of a N = Z core to which some α clusters are appended. These clusters are tightly bound and spatially localized structures of two neutrons and two protons. The α-cluster model exhibits a striking contrast with the standard shell model picture in which protons and neutrons are described instead as weakly interacting quasiparticles in a mean-field. However, these two pictures are expected to coexist at low excitation energies where, due to the Pauli blocking which acts stronger than in the states close to the α emission threshold (e.g., see the case of the Hoyle state [2]), the α-clustering is expected to manifest itself mainly as four-body correlations in the configuration space. It is thus commonly supposed that correlated four-body structures ("quartets") play a major role in the ground and excited sates of N = Z nuclei. However, to our knowledge, this supposition has never been supported by compelling calculations. In this letter, by using a simple microscopic quartet model, we will show how the low-energy spectra of 4n nuclei can be indeed described in terms of quartets with an accuracy comparable with state-of-the-art shell model calculations. We shall prove that this is the case not only for self-conjugate nuclei but also for 4n nuclei with N = Z. For the latter nuclei the low-lying states will be expressed in terms of quartets built not only by two protons and two neutrons but also by one proton and three neutrons as well as by four neutrons. This fact indicates that the four-body degrees of freedom are important also for configurations which are different from the α-like ones.Microscopic quartet models have a long history ...

Starting from a simplified model, treating proton two-particle two-hole excitations in interaction with low-lying quadrupole vibrational configurations, we calculate the low-lying levels in even-even "" Cd. %e also present configuration mixing calculations within the framework of the interacting boson model. Both approaches give a good description of the quintuplet of levels occurring below E =1.5 MeV in "'" Cd (energy spectra and a detailed account of E2 and EO decay properties). Finally, we point out some similarities between both approaches and. try to interpret the interacting boson model mixing parameters in terms of shell-model quantities (two-body matrix elements and single-particle energies). NUCLEAR STRUCTURE Collective excitations in "2*" Cd; shellmodel intruder states; interacting boson model configuration mixing.

No abstract