581 research outputs found
Quartet structure of nuclei in a boson formalism: the case of Si
The structure of the nucleus Si is studied by resorting to an
IBM-type formalism with and bosons representing isospin and
angular momentum and quartets, respectively. quartets are
four-body correlated structures formed by two protons and two neutrons. The
microscopic nature of the quartet bosons, meant as images of the fermionic
quartets, is investigated by making use of a mapping procedure and is supported
by the close resemblance between the phenomenological and microscopically
derived Hamiltonians. The ground state band and two low-lying side bands, a
and a band, together with all known transitions and
quadrupole moments associated with these states are well reproduced by the
model. An analysis of the potential energy surface places Si, only known
case so far, at the critical point of the U(5)-
transition of the IBM structural diagram.Comment: To appear in Physics Letters
Quartet correlations in N=Z nuclei induced by realistic two-body interactions
Two variational quartet models previously employed in a treatment of pairing
forces are extended to the case of a general two-body interaction. One model
approximates the nuclear states as a condensate of identical quartets with
angular momentum and isospin while the other let these quartets to
be all different from each other. With these models we investigate the role of
alpha-like quartet correlations both in the ground state and in the lowest
, excited states of even-even nuclei in the -shell. We
show that the ground state correlations of these nuclei can be described to a
good extent in terms of a condensate of alpha-like quartets. This turns out to
be especially the case for the nucleus S for which the overlap between
this condensate and the shell model wave function is found close to one. In the
same nucleus, a similar overlap is found also in the case of the first excited
state. No clear correspondence is observed instead between the second
excited states of the quartet models and the shell model eigenstates in all the
cases examined.Comment: 10 pages, to appear in EPJ
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
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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