arXiv subjects

L. M. Robledo

Publications and source records attributed to L. M. Robledo.

125 records · Page 7Linked to original sources

arXiv2003

Thermal shape fluctuation effects in the description of hot nuclei

The behavior of several nuclear properties with temperature is analyzed within the framework of the Finite Temperature Hartree-Fock-Bogoliubov (FTHFB) theory with the Gogny force and large configuration spaces. Thermal shape fluctuations in the quadrupole degree of freedom, around the mean field solution, are taken into account with the Landau prescription. As representative examples the nuclei $^{164}$Er, $^{152}$Dy and $^{192}$Hg are studied. Numerical results for the superfluid to normal and deformed to spherical shape transitions are presented. We found a substantial effect of the fluctuations on the average value of several observables. In particular, we get a decrease in the critical temperature ($T_c$) for the shape transition as compared with the plain FTHFB prediction as well as a washing out of the shape transition signatures. The new values of $T_c$ are closer to the ones found in Strutinsky calculations and with the Pairing Plus Quadrupole model Hamiltonian.

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arXiv2003

Quadrupole collectivity of neutron-rich Neon isotopes

The angular momentum projected Generator Coordinate Method, with the quadrupole moment as collective coordinate and the Gogny force (D1S) as the effective interaction, is used to describe the properties of the ground state and low-lying excited states of the even-even Neon isotopes $^{20-34}$Ne, that is, from the stability valley up to the drip-line. It is found that the ground state of the N=20 nucleus $^{30}$Ne is deformed but to a lesser extent than the N=20 isotope of the Magnesium. In the calculations, the isotope $^{32}$Ne is at the drip-line in good agreement with other theoretical predictions. On the other hand, rather good agreement with experimental data for many observables is obtained.

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arXiv2002

Mean-field Based Approaches to Pairing Correlations in Atomic Nuclei

The evolution of the pairing correlations from closed shell to middle shell nuclei is analyzed with a Finite Range Density Dependent interaction in the Sn isotopes. As theoretical approaches we use the Hartree-Fock-Bogoliubov, the Lipkin-Nogami, their particle number projected counterparts and the full variation after particle number projection method. We find that whereas all approaches succeed rather well in the description of the total energy they differ significantly in the pairing correlation content of the wave functions. The description of the evolution from the weak to the strong pairing regime is also approach dependent, specially at shell closure.

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arXiv2002

Correlations beyond the mean field in Magnesium isotopes: Angular momentum projection and configuration mixing

The quadrupole deformation properties of the ground and low-lying excited states of the even-even Magnesium isotopes with N ranging from 8 to 28 have been studied in the framework of the angular momentum projected generator coordinate method with the Gogny force. It is shown that the N=8 neutron magic number is preserved (in a dynamical sense) in \nuc{20}{Mg} leading to a spherical ground state. For the magic numbers N=20 and N=28 this is not the case and prolate deformed ground states are obtained. The method yields values of the two neutron separation energies which are in much better agreement with experiment than those obtained at the mean field level. It is also obtained that \nuc{40}{Mg} is at the neutron dripline. Concerning the results for the excitation energies of the $2^+$ excited states and their transition probabilities to the ground state we observe a good agreement with the available experimental data. On the theoretical side, we also present a detailed justification of the prescription used for the density dependent part of the interaction in our beyond-mean-field calculations.

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arXiv2002

Selfconsistent calculations of fission barriers in the Fm region

The fission barriers of the nuclei 254Fm, 256Fm, 258Fm, 258No and 260Rf are investigated in a fully microscopic way up to the scission point. The analysis is based on the constrained Hartree-Fock-Bogoliubov theory and Gogny's D1S force. The quadrupole, octupole and hexadecapole moments as well as the number of nucleons in the neck region are used as constraints. Two fission paths, corresponding to the bimodal fission, are found. The decrease with isotope mass of the half-life times of heavy Fm isotopes is also explained.

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arXiv2001

Particle number projection with effective forces

The particle number projection method is formulated for density dependent forces and in particular for the finite range Gogny force. Detailed formula for the projected energy and its gradient are provided. The problems arising from the neglection of any exchange term, which may lead to divergences, are throughly discussed and the possible inaccuracies estimated. Numericala results for the projection after variation method are shown for the nucleus 164Er and for the projection before variation approach for the nuclei 48-50Cr. We also confirm the Coulomb antipairing effect found in mean field theories.

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arXiv2001

Quadrupole collectivity in Si isotopes around N=20

The angular momentum projected Generator Coordinate Method using the quadrupole moment as collective coordinate and the Gogny force as the effective interaction is used to describe the properties of the ground state and low-lying excited states of the neutron rich light nuclei 32,34,36Si. It is found that the ground state of the nucleus 34Si is spherical. However, this is not only due to the N=20 shell closure as the ground state of 34Si contains a significant amount of the intruder f7/2 neutron orbital. On the other hand, rather good agreement with experimental data for many observables is obtained.

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arXiv2000

Moments of Inertia of Nuclei in the Rare Earth Region: A Relativistic versus Non-Relativistic Investigation

A parameter free investigation of the moments of inertia of ground state rotational bands in well deformed rare-earth nuclei is carried out using Cranked Relativistic Hartree-Bogoliubov (CRHB) and non-relativistic Cranked Hartree-Fock-Bogoliubov (CHFB) theories. In CRHB theory, the relativistic fields are determined by the non-linear Lagrangian with the NL1 force and the pairing interaction by the central part of finite range Gogny D1S force. In CHFB theory, the properties in particle-hole and particle-particle channels are defined solely by Gogny D1S forces. Using an approximate particle number projection before variation by means of the Lipkin Nogami method improves the agreement with the experimental data, especially in CRHB theory. The effect of the particle number projection on the moments of inertia and pairing energies is larger in relativistic than in non-relativistic theory.

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arXiv2000

Coulomb exchange and pairing contributions in nuclear Hartree-Fock-Bogoliubov calculations with the Gogny force

We present exact Hartree-Fock-Bogoliubov calculations with the finite range density dependent Gogny force using a triaxial basis. For the first time, all contributions to the Pairing and Fock Fields arising from the Gogny and Coulomb interactions as well as the two-body correction of the kinetic energy have been calculated in this basis. We analyze the relevance of these terms in different regions of the periodic table at zero and high angular momentum. The validity of commonly used approximations that neglect different terms in the variational equations is also checked. We find a decrease of the proton pairing energies mainly due to a Coulomb antipairing effect.

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arXiv2000

Properties of the predicted super-deformed band in ^{32}S

Properties like the excitation energy with respect to the ground state, moments of inertia, B(E2) transition probabilities and stability against quadrupole fluctuations at low spin of the predicted superdeformed band of ^{32}S are studied with the Gogny force D1S using the angular momentum projected generator coordinate method for the axially symmetric quadrupole moment. The Self Consistent Cranking method is also used to describe the superdeformed rotational band. In addition, properties of some collective normal deformed states are discussed.

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arXiv2000

Description of quadrupole collectivity in N ~ 20 nuclei with techniques beyond the mean field

Properties of the ground and several collective excited states of the light nuclei ^{30,32,34}Mg are described in the framework of the angular momentum projected Generator Coordinate Method using the quadrupole moment as collective coordinate and the Gogny force as the effective interaction. The calculated excitation energies and B(E2) transition probabilities agree reasonably well with experiment. The results clearly indicate that both the restoration of the rotational symmetry and the quadrupole dynamics are key ingredients for the description of the properties of the above mentioned nuclei.

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arXiv2000

Angular momentum projected analysis of Quadrupole Collectivity in \protect(^{30,32,34}Mg\protect) and \protect(^{32,34,36,38}Si\protect) with the Gogny interaction

A microscopic angular momentum projection after variation is used to describe quadrupole collectivity in (^{30,32,34}Mg) and (^{32,34,36,38}Si). The Hartree-Fock-Bogoliubov states obtained in the quadrupole constrained mean field approach are taken as intrinsic states for the projection. Excitation energies of the first (2^{+}) states and the (B(E2,0^{+}\to 2^{+})) transition probabilities are given. A reasonable agreement with available experimental data is obtained. It is also shown that the mean field picture of those nuclei is strongly modified by the projection.

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arXiv1999

Approximate particle number projection with density dependent forces: Superdeformed bands in the A=150 and A=190 regions

We derive the equations for approximate particle number projection based on mean field wave functions with finite range density dependent forces. As an application ground bands of even-A superdeformed nuclei in the A=150 and A=190 regions are calculated with the Gogny force. We discuss nuclear properties such as quadrupole moments, moments of inertia and quasiparticle spectra, among others, as a function of the angular momentum. We obtain a good overall description.

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arXiv1999

A variational approach to approximate particle number projection with effective forces

Kamlah's second order method for approximate particle number projection is applied for the first time to variational calculations with effective forces. High spin states of normal and superdeformed nuclei have been calculated with the finite range density dependent Gogny force for several nuclei. Advantages and drawbacks of the Kamlah second order method as compared to the Lipkin-Nogami recipe are thoroughly discussed. We find that the Lipkin-Nogami prescription occasionally may fail to find the right energy minimum in the strong pairing regime and that Kamlah's second order approach, though providing better results than the LN one, may break down in some limiting situations.

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arXiv1996

Backbending in 50Cr

The collective yrast band and the high spin states of the nucleus 50Cr are studied using the spherical shell model and the HFB method. The two descriptions lead to nearly the same values for the relevant observables. A first backbending is predicted at I=10\hbar corresponding to a collective to non-collective transition. At I=16\hbar a second backbending occurs, associated to a configuration change that can also be interpreted as an spherical to triaxial transition.

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arXiv1995

Intrinsic vs. laboratory frame description of the deformed nucleus 48Cr

The collective yrast band of the nucleus $^{48}$Cr is studied using the spherical shell model and the HFB method. Both approaches produce basically the same axially symmetric intrinsic state up to the - accurately reproduced - observed backbending. Agreement between both calculations extends to most observables. The only significant discrepancy comes from the static moments of inertia and can be attributed to the need of a more refined treatment of pairing correlations in the HFB calculation.

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arXiv1995

Approximate particle number projection for finite range density dependent forces

The Lipkin-Nogami method is generalized to deal with finite range density dependent forces. New expressions are derived and realistic calculations with the Gogny force are performed for the nuclei $^{164}$Er and $^{168}$Er. The sharp phase transition predicted by the mean field approximation is washed out by the Lipkin-Nogami approach; a much better agreement with the experimental data is reached with the new approach than with the Hartree-Fock_Bogoliubov one, specially at high spins.

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