171 research outputs found
Difference between stable and exotic nuclei: medium polarization effects
The bare NN-potential, parametrized so as to reproduce the nuclear phase
shifts leads to a sizable Cooper pair binding energy in nuclei along the
stability valley. It is a much debated matter whether this value accounts for
the "empirical" value of the pairing gap or whether a similarly important
contribution arises from the exchange of collective vibrations between Cooper
pair partners. In keeping with the fact that two-particle transfer reactions
are the specific probe of pairing in nuclei, and that exotic halo nuclei like
11Li are extremely polarizable, we find that the recent studied reaction,
namely 11Li+p -> 9Li+t, provides direct evidence of phonon mediated pairing in
nuclei
Vortices in Bose-Einstein condensates - finite-size effects and the thermodynamic limit
For a weakly-interacting Bose gas rotating in a harmonic trap we relate the
yrast states of small systems (that can be treated exactly) to the
thermodynamic limit (derived within the mean-field approximation). For a few
dozens of atoms, the yrast line shows distinct quasi-periodic oscillations with
increasing angular momentum that originate from the internal structure of the
exact many-body states. These finite-size effects disappear in the
thermodynamic limit, where the Gross-Pitaevskii approximation provides the
exact energy to leading order in the number of particles N. However, the exact
yrast states reveal significant structure not captured by the mean-field
approximation: Even in the limit of large N, the corresponding mean-field
solution accounts for only a fraction of the total weight of the exact quantum
state.Comment: Phys Rev A, in pres
Nilsson diagrams for light neutron-rich nuclei with weakly-bound neutrons
Using Woods-Saxon potentials and the eigenphase formalism for one-particle
resonances, one-particle bound and resonant levels for neutrons as a function
of quadrupole deformation are presented, which are supposed to be useful for
the interpretation of spectroscopic properties of some light neutron-rich
nuclei with weakly-bound neutrons. Compared with Nilsson diagrams in text books
which are constructed using modified oscillator potentials, we point out a
systematic change of the shell structure in connection with both weakly-bound
and resonant one-particle levels related to small orbital angular momenta
. Then, it is seen that weakly-bound neutrons in nuclei such as
C and Mg may prefer to being deformed as a result of
Jahn-Teller effect, due to the near degeneracy of the 1d-2s
levels and the 1f-2p levels in the spherical potential,
respectively. Furthermore, the absence of some one-particle resonant levels
compared with the Nilsson diagrams in text books is illustrated.Comment: 12 pages, 5 figure
Seniority conservation and seniority violation in the g_{9/2} shell
The g_{9/2} shell of identical particles is the first one for which one can
have seniority-mixing effects. We consider three interactions: a delta
interaction that conserves seniority, a quadrupole-quadrupole (QQ) interaction
that does not, and a third one consisting of two-body matrix elements taken
from experiment (98Cd) that also leads to some seniority mixing. We deal with
proton holes relative to a Z=50,N=50 core. One surprising result is that, for a
four-particle system with total angular momentum I=4, there is one state with
seniority v=4 that is an eigenstate of any two-body interaction--seniority
conserving or not. The other two states are mixtures of v=2 and v=4 for the
seniority-mixing interactions. The same thing holds true for I=6. Another point
of interest is that the splittings E(I_{max})-E(I_{min}) are the same for three
and five particles with a seniority conserving interaction (a well known
result), but are equal and opposite for a QQ interaction. We also fit the
spectra with a combination of the delta and QQ interactions. The Z=40,N=40 core
plus g_{9/2} neutrons (Zr isotopes) is also considered, although it is
recognized that the core is deformed.Comment: 19 pages, 9 figures; RevTeX4. We have corrected the SDI values in
Table1 and Fig.1; in Sect.VII we have included an explanation of Fig.3
through triaxiality; we have added comments of Figs.10-12 in Sect.IX; we have
removed Figs.7-
Rotating Bose-Einstein condensates: Closing the gap between exact and mean-field solutions
When a Bose-Einstein condensed cloud of atoms is given some angular momentum,
it forms vortices arranged in structures with a discrete rotational symmetry.
For these vortex states, the Hilbert space of the exact solution separates into
a "primary" space related to the mean-field Gross-Pitaevskii solution and a
"complementary" space including the corrections beyond mean-field. Considering
a weakly-interacting Bose-Einstein condensate of harmonically-trapped atoms, we
demonstrate how this separation can be used to close the conceptual gap between
exact solutions for systems with only a few atoms and the thermodynamic limit
for which the mean-field is the correct leading-order approximation. Although
we illustrate this approach for the case of weak interactions, it is expected
to be more generally valid.Comment: 8 pages, 5 figure
Direct observation of the glue pairing the halo of the nucleus 11Li
With the help of a unified description of the nuclear structure and of the
direct reaction mechanism we show that a recent 1H(11Li,9Li)3H experiment
provides, for the first time in nuclear physics, direct evidence of phonon
mediated pairing.Comment: 9 pages, 4 figures. Major change
The Onset of Chaos with a Quadrupole--Quadrupole Interaction
The transition from order to chaos in atomic nuclei has been studied
analytically and numerically using a quadrupole--quadrupole residual
interaction. This interaction leads to chaotic behaviour, but the critical
energy MeV, corresponding to the onset of chaos, is higher
than that of the experimental one.Comment: 14 pages, 5 figures (available upon request to the authors), LaTex,
DFPD/93/TH/73, to be published in Nuovo Cimento
The Yrast Spectra of Weakly Interacting Bose-Einstein Condensates
The low energy quantal spectrum is considered as a function of the total
angular momentum for a system of weakly interacting bosonic atoms held together
by an external isotropic harmonic potential. It is found that besides the usual
condensation into the lowest state of the oscillator, the system exhibits two
additional kinds of condensate and associated thermodynamic phase transitions.
These new phenomena are derived from the degrees of freedom of "partition
space" which describes the multitude of different ways in which the angular
momentum can be distributed among the atoms while remaining all the time in the
lowest state of the oscillator
Weakly Interacting Bose-Einstein Condensates Under Rotation: Mean-field versus Exact Solutions
We consider a weakly-interacting, harmonically-trapped Bose-Einstein
condensed gas under rotation and investigate the connection between the
energies obtained from mean-field calculations and from exact diagonalizations
in a subspace of degenerate states. From the latter we derive an approximation
scheme valid in the thermodynamic limit of many particles. Mean-field results
are shown to emerge as the correct leading-order approximation to exact
calculations in the same subspace.Comment: 4 pages, RevTex, submitted to PR
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