880 research outputs found
Optimizing phonon space in the phonon-coupling model
We present a new scheme to select the most relevant phonons in the
phonon-coupling model, named here time-blocking approximation (TBA). The new
criterion, based on the phonon-nucleon coupling strengths rather than on
values, is more selective and thus produces much smaller phonon spaces
in TBA. This is beneficial in two respects: first, it curbs down the
computational cost, and second, it reduces the danger of double counting in the
expansion basis of TBA. We use here TBA in a form where the coupling strength
is regularized to keep the given Hartree-Fock ground state stable. The scheme
is implemented in an RPA and TBA code based on the Skyrme energy functional. We
first explore carefully the cutoff dependence with the new criterion and can
work out a natural (optimal) cutoff parameter. Then we use the freshly
developed and tested scheme to a survey of giant resonances and low-lying
collective states in six doubly magic nuclei looking also on the dependence of
the results when varying the Skyrme parametrization.Comment: 9 figures, 3 table
Self-consistency in the phonon space of the particle-phonon coupling model
In the paper the non-linear generalization of the time blocking approximation
(TBA) is presented. The TBA is one of the versions of the extended random-phase
approximation (RPA) developed within the Green-function method and the
particle-phonon coupling model. In the generalized version of the TBA the
self-consistency principle is extended onto the phonon space of the model. The
numerical examples show that this non-linear version of the TBA leads to the
convergence of the results with respect to enlarging the phonon space of the
model.Comment: 12 pages, 10 figures, 1 tabl
Landau-Migdal vs. Skyrme
The magnitude and density-dependence of the non-spin dependent Landau-Migdal
parameters are derived from Skyrme energy functionals and compared with the
phenomenological ones. We perform RPA calculations with various approximations
for the Landau-Migdal particle-hole interaction and compare them with the
results obtained with the full Skyrme interaction. For the first time the next
to leading order in the Landau-Migdal approach is considered in nuclear
structure calculations.Comment: Dedicated to the memory of G.E. Brow
Multi-quark components in baryons
A brief review on some recent progresses in our understanding of multi-quark
components in baryons is presented. The multi-quark components in baryons seem
to be mainly in colored quark cluster configurations rather than in ``meson
cloud'' configurations or in the form of a sea of quark-antiquark pairs. The
colored quark cluster multi-quark picture gives a natural explanation of
empirical indications for a positive strangeness magnetic moment of the
proton and the longstanding mass-reverse problem of S11(1535) and \mu_s$ of the proton is given.Comment: Contribution to the International Conference on QCD and Hadronic
Physics, June 16-20, 2005, Beijin
A nonlinear classical model for the decay widths of Isoscalar Giant Monopole Resonances
The decay of the Isoscalar Giant Monopole Resonance (ISGMR) in nuclei is
studied by means of a nonlinear classical model consisting of several
noninteracting nucleons (particles) moving in a potential well with an
oscillating nuclear surface (wall). The motion of the nuclear surface is
described by means of a collective variable which appears explicitly in the
Hamiltonian as an additional degree of freedom. The total energy of the system
is therefore conserved. Although the particles do not directly interact with
each other, their motions are indirectly coupled by means of their interaction
with the moving nuclear surface. We consider as free parameters in this model
the degree of collectivity and the fraction of nucleons that participate to the
decay of the collective excitation. Specifically, we have calculated the decay
width of the ISGMR in the spherical nuclei , ,
and . Despite its simplicity and its purely
classical nature, the model reproduces the trend of the experimental data which
show that with increasing mass number the decay width decreases. Moreover the
experimental results (with the exception of ) can be well fitted
using appropriate values for the free parameters mentioned above. It is also
found that these values allow for a good description of the experimentally
measured and decay widths. In addition, we give
a prediction for the decay width of the exotic isotope for which
there is experimental interest. The agreement of our results with the
corresponding experimental data for medium-heavy nuclei is dictated by the
underlying classical mechanics i.e. the behaviour of the maximum Lyapunov
exponent as a function of the system size
A microscopic investigation of the transition form factor in the region of collective multipole excitations of stable and unstable nuclei
We have used a self-consistent Skyrme-Hartree-Fock plus Continuum-RPA model
to study the low-multipole response of stable and neutron/proton-rich Ni and Sn
isotopes. We focus on the momentum-transfer dependence of the strength
distribution, as it provides information on the structure of excited nuclear
states and in particular on the variations of the transition form factor (TFF)
with the energy. Our results show, among other things, that the TFF may show
significant energy dependence in the region of the isoscalar giant monopole
resonance and that the TFF corresponding to the threshold strength in the case
of neutron-rich nuclei is different compared to the one corresponding to the
respective giant resonance. Perspectives are given for more detailed future
investigations.Comment: 13 pages, incl. 9 figures; to appear in J.Phys.G,
http://www.iop.org/EJ/jphys
Low-energy M1 excitations in Pb and the spin channel of the Skyrme energy-density functional
We investigate the spin dependent part of the Skyrme energy-density
functional, in particular its impact on the residual particle-hole interaction
in self-consistent calculations of excitations. Test cases are the low-energy
M1 excitations in Pb treated within the self-consistent random-phase
approximation based on the Skyrme energy-density functional. We investigate
different parametrizations of the functionals to find out which parameters of
the functional have strongest correlations with M1 properties. We explore a
simple method of the modification of the spin-related parameters which delivers
a better description of M1 excitations while basically maintaining the good
description of ground state properties.Comment: 15 pages, 7 figure
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