6,883 research outputs found
Dynamical symmetry of isobaric analog 0+ states in medium mass nuclei
An algebraic sp(4) shell model is introduced to achieve a deeper
understanding and interpretation of the properties of pairing-governed 0+
states in medium mass atomic nuclei. The theory, which embodies the simplicity
of a dynamical symmetry approach to nuclear structure, is shown to reproduce
the excitation spectra and fine structure effects driven by proton-neutron
interactions and isovector pairing correlations across a broad range of nuclei.Comment: 7 pages, 5 figure
New effective interaction for -shell nuclei and its implications for the stability of the ==28 closed core
The effective interaction GXPF1 for shell-model calculations in the full
shell is tested in detail from various viewpoints such as binding energies,
electro-magnetic moments and transitions, and excitation spectra. The
semi-magic structure is successfully described for or Z=28 nuclei,
Mn, Fe, Co and Ni, suggesting the
existence of significant core-excitations in low-lying non-yrast states as well
as in high-spin yrast states. The results of odd-odd nuclei, Co
and Cu, also confirm the reliability of GXPF1 interaction in the isospin
dependent properties. Studies of shape coexistence suggest an advantage of
Monte Carlo Shell Model over conventional calculations in cases where
full-space calculations still remain too large to be practical.Comment: 29pages, 26figures, to be published in Physical Review
Description of single and double analog states in the f7/2 shell: The Ti isotopes
The excitation energies of single analog states in even-odd Ti isotopes and
double analog states in even-even Ti isotopes are microscopically described in
a single j-shell formalism. A projection procedure for generalized BCS states
has been used. As an alternative description a particle-core formalism is
presented. The latter picture provides a two-parameter expression for
excitation energies, which describes fairly well the data in four odd and three
even isotopes of Ti.Comment: 14 pages,7 figures, 2 tables. To appear in Phys. Rev.
Pairing and alpha-like quartet condensation in N=Z nuclei
We discuss the treatment of isovector pairing by an alpha-like quartet
condensate which conserves exactly the particle number, the spin and the
isospin. The results show that the quartet condensate describes accurately the
isovector pairing correlations in the ground state of systems with an equal
number of protons and neutronsComment: 4 pages, to appear in Journal of Physics: Conference Serie
Nuclear symmetry energy and the r-mode instability of neutron stars
We analyze the role of the symmetry energy slope parameter on the {\it
r}-mode instability of neutron stars. Our study is performed using both
microscopic and phenomenological approaches of the nuclear equation of state.
The microscopic ones include the Brueckner--Hartree--Fock approximation, the
well known variational equation of state of Akmal, Pandharipande and Ravenhall,
and a parametrization of recent Auxiliary Field Diffusion Monte Carlo
calculations. For the phenomenological approaches, we use several Skyrme forces
and relativisic mean field models. Our results show that the {\it r}-mode
instability region is smaller for those models which give larger values of .
The reason is that both bulk () and shear () viscosities increase
with and, therefore, the damping of the mode is more efficient for the
models with larger . We show also that the dependence of both viscosities on
can be described at each density by simple power-laws of the type
and . Using the measured spin
frequency and the estimated core temperature of the pulsar in the low-mass
X-ray binary 4U 1608-52, we conclude that observational data seem to favor
values of larger than MeV if this object is assumed to be outside
the instability region, its radius is in the range () km, and
its mass (). Outside this range it is not possible to
draw any conclusion on from this pulsar.Comment: 10 pages, 6 figures. Version published in Physical Review
Boson mappings and four-particle correlations in algebraic neutron-proton pairing models
Neutron-proton pairing correlations are studied within the context of two
solvable models, one based on the algebra SO(5) and the other on the algebra
SO(8). Boson-mapping techniques are applied to these models and shown to
provide a convenient methodological tool both for solving such problems and for
gaining useful insight into general features of pairing. We first focus on the
SO(5) model, which involves generalized T=1 pairing. Neither boson mean-field
methods nor fermion-pair approximations are able to describe in detail
neutron-proton pairing in this model. The analysis suggests, however, that the
boson Hamiltonian obtained from a mapping of the fermion Hamiltonian contains a
pairing force between bosons, pointing to the importance of boson-boson (or
equivalently four-fermion) correlations with isospin T=0 and spin S=0. These
correlations are investigated by carrying out a second boson mapping. Closed
forms for the fermion wave functions are given in terms of the fermion-pair
operators. Similar techniques are applied -- albeit in less detail -- to the
SO(8) model, involving a competition between T=1 and T=0 pairing. Conclusions
similar to those of the SO(5) analysis are reached regarding the importance of
four-particle correlations in systems involving neutron-proton pairing.Comment: 31 pages, Latex, 3 Postscript figures, uses epsf.sty, submitted to
Physical Review
General pairing interactions and pair truncation approximations for fermions in a single-j shell
We investigate Hamiltonians with attractive interactions between pairs of
fermions coupled to angular momentum J. We show that pairs with spin J are
reasonable building blocks for the low-lying states. For systems with only a J
= Jmax pairing interaction, eigenvalues are found to be approximately integers
for a large array of states, in particular for those with total angular momenta
I le 2j. For I=0 eigenstates of four fermions in a single-j shell we show that
there is only one non-zero eigenvalue. We address these observations using the
nucleon pair approximation of the shell model and relate our results with a
number of currently interesting problems.Comment: a latex text file and 2 figures, to be publishe
Classification of states of single- fermions with -pairing interaction
In this paper we show that a system of three fermions is exactly solvable for
the case of a single- in the presence of an angular momentum- pairing
interaction. On the basis of the solutions for this system, we obtain new sum
rules for six- symbols. It is also found that the "non-integer" eigenvalues
of three fermions with angular momentum around the maximum appear as
"non-integer" eigenvalues of four fermions when is around (or larger than)
and the Hamiltonian contains only an interaction between pairs of
fermions coupled to spin . This pattern is also found in
five and six fermion systems. A boson system with spin exhibits a similar
pattern.Comment: to be published in Physical Review
Contribution of the massive photon decay channel to neutrino cooling of neutron stars
We consider massive photon decay reactions via intermediate states of
electron-electron-holes and proton-proton-holes into neutrino-antineutrino
pairs in the course of neutron star cooling. These reactions may become
operative in hot neutron stars in the region of proton pairing where the photon
due to the Higgs-Meissner effect acquires an effective mass that
is small compared to the corresponding plasma frequency. The contribution of
these reactions to neutrino emissivity is calculated; it varies with the
temperature and the photon mass as
for . Estimates show that these processes appear as extra
efficient cooling channels of neutron stars at temperatures K.Comment: accepted to publication in Zh. Eksp. Teor. Fiz. (JETP
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