20 research outputs found
On the description of two-particle transfer in superfluid systems
Exact results of pair transfer probabilities for the Richardson model with
equidistant or random level spacing are presented. The results are then
compared either to particle-particle random phase approximation (ppRPA) in the
normal phase or quasi-particle random phase approximation (QRPA) in the
superfluid phase. We show that both ppRPA and QRPA are globally well
reproducing the exact case although some differences are seen in the superfluid
case. In particular the QRPA overestimates the pair transfer probabilities to
excited states in the vicinity of the normal-superfluid phase transition, which
might explain the difficult in detecting collective pairing phenomena as for
example the Giant Pairing Vibration. The shortcoming of QRPA can be traced back
to the breaking of particle number that is used to incorporate pairing. A
method, based on direct diagonalization of the Hamiltonian in the space of two
quasi-particle projected onto good particle number is shown to improve the
description of pair transfer probabilities in superfluid systems.Comment: 9 pages, 7 figure
Quantal corrections to mean-field dynamics including pairing
Extending the stochastic mean-field model by including pairing, an approach
is proposed for describing evolutions of complex many-body systems in terms of
an ensemble of Time-Dependent Hartree-Fock Bogoliubov trajectories which is
determined by incorporating fluctuations in the initial state. Non-linear
evolution of the initial fluctuations provides an approximate description of
quantal correlations and fluctuations of collective observables. Since the
initial-state fluctuations break the particle-number symmetry, the dynamical
description in which pairing correlations play a crucial role is greatly
improved as compare to the mean-field evolution. The approach is illustrated
for a system of particles governed by a pairing Hamiltonian.Comment: 5 pages, 2 figures, To appear in Phys. Rev. C (Rapid communication
Pairing and specific heat in hot nuclei
The thermodynamics of pairing phase-transition in nuclei is studied in the
canonical ensemble and treating the pairing correlations in a
finite-temperature variation after projection BCS approach (FT-VAP). Due to the
restoration of particle number conservation, the pairing gap and the specific
heat calculated in the FT-VAP approach vary smoothly with the temperature,
indicating a gradual transition from the superfluid to the normal phase, as
expected in finite systems. We have checked that the predictions of the FT-VAP
approach are very accurate when compared to the results obtained by an exact
diagonalization of the pairing Hamiltonian. The influence of pairing
correlations on specific heat is analysed for the isotopes Dy and
Yb. It is shown that the FT-VAP approach, applied with a level
density provided by mean field calculations and supplemented, at high energies,
by the level density of the back-shifted Fermi gas model, can approximate
reasonably well the main properties of specific heat extracted from
experimental data. However, the detailed shape of the calculated specific heat
is rather sensitive to the assumption made for the mean field.Comment: 10 pages, 12 figure
Thermodynamics of small superconductors with fixed particle number
The Variation After Projection approach is applied for the first time to the
pairing hamiltonian to describe the thermodynamics of small systems with fixed
particle number. The minimization of the free energy is made by a direct
diagonalization of the entropy. The Variation After Projection applied at
finite temperature provides a perfect reproduction of the exact canonical
properties of odd or even systems from very low to high temperature.Comment: 4 pages, 3 figure
Nuclear incompressibility from spherical and deformed nuclei
We present an analysis based on the deformed Quasi Particle Random Phase Approximation, on top of a deformed Hartree-Fock-Bogoliubov description of the ground state, aimed at studying the isoscalar monopole and quadrupole response in a deformed nucleus. This analysis is motivated by the need of understanding the coupling between the two modes and how it might affect the extraction of the nuclear incompressibility from the monopole distribution. After discussing this motivation, we present the main ingredients of our theoretical framework, and we show some results obtained with the SLy4 and SkM* interactions for the nucleus 24Mg
Multiphonon excitations and pygmy resonances in tin isotopes
We study, within a semiclassical coupled-channels approach, the possible effects of anharmonicities and non linearities on the excitation of the so-called pygmy resonances in several Sn isotopes and using two different Skyrme interactions. In the energy region of the pygmy resonances, there are a few low-lying multiphonon states. The question is whether they may contribute to the observed peak. Calculations show that the inelastic cross sections in the relevant energy region have an increase that varies from 3% to 21% depending on the isotope and on the kind of Skyrme force used. At the same time, we have studied the nature of the pygmy resonance state by means of a new criterion to establish whether this state can be considered as a collective one.Ministerio de Educación y Ciencia y FEDER FIS2008-04189 FPA2006-13807-C02-01Programa Consolider-Ingenio 2010 CSD2007-00042Junta de Andalucía P07-FQM-02894 FQM16
Pygmy resonances in Sn isotopes within a microscopic multiphonon approach
We study the pygmy resonances in Sn isotopes within a microscopic multiphonon approach which has been successfully applied to heavy ion reactions in recent years. In the energy region of the pygmy resonances there are a few low lying multiphonon states. The question is whether they may contribute to the observed peak. Calculations show that the inelastic cross sections in the relevant energy region have a conisderable increase depending on the isotope and on the kind of Skyrme force used
Recent applications of the subtracted SRPA approximation
The Second Random Phase Approximation (SRPA) is a natural extension of the Random Phase Approximation obtained by introducing more general excitation operators where two particle-two hole configurations, in addition to the one particle-one hole ones, are considered. Only in the last years, large-scale SRPA calculations, without usually employed approximations have been performed. The SRPA model corrected by a subtraction procedure designed to cure double counting issues and the related instabilities has been recently implemented and applied in the study of different physical cases. We report here on some of the most recent results obtained by using this model. In particular, results on the dipole strength 48Ca and on a systematic study of the isoscalar giant quadrupole resonance in spherical nuclei will be shown and discussed
Interplay between proton-neutron pairing and deformation in self-conjugated medium mass nuclei
We employ a model combining self-consistent mean-field and shell model techniques to study the competition between particle-like and proton-neutron pairing correlations in fp-shell even-even self-conjugate nuclei. Deformation effects are realistically and microscopically described. The resulting approach can give a precise description of pairing correlations and eventually treat the coexistence of different condensate formed of pairs with different total spin/ isospin. The standard BCS calculations are systematically compared with approaches including correlation effects beyond the independent quasi-particle picture. The competition between proton-neutron correlations in the isoscalar and isovector channels is also analyzed, as well as their dependence on the deformation properties