R. Niembro

On the sufficient conditions for the pseudospin symmetry in relativistic models

Abstract The reliability of the sufficient conditions for the pseudospin symmetry proposed for atomic nuclei has been analyzed, at a quantitative level, in the framework of the relativistic Hartree approximation (some effects of the exchange terms are also investigated). The contribution to the single-particle energy of the different terms appearing in the equation for the small component F of the Dirac spinor is calculated for the neutron pseudospin doublet states in the 40Ca and 208Pb nuclei. The F wave function is numerically calculated for the hypothetical case in which the symmetry is supposed to be exact.

Asymmetric nuclear matter in the relativistic approach

Abstract Bulk properties of asymmetric nuclear matter are studied in the Dirac-Hartree-Fock approximation. The model includes isoscalar (σ, ω) and isovector (π, ρ) mesons. The variations of binding energy, equilibrium density and compression modulus with increasing neutron excess are discussed and compared with predictions of non-relativistic Hartree-Fock calculations. The dependence of nucleon self-energies, effective masses and single-particle spectra on the neutron excess is also examined.

Description of nuclear systems within the relativistic Hartree–Fock method with zero-range self-interactions of the scalar field

An exact method is suggested to treat the nonlinear self-interactions (NLSI) in the relativistic Hartree–Fock (RHF) approach for nuclear systems. We consider here the NLSI constructed from the relativistic scalar nucleon densities including products of six and eight fermion fields. This type of NLSI corresponds to the zero-range limit of the standard cubic and quartic self-interactions of the scalar field. The method to treat the NLSI uses the Fierz transformation, which enables one to express the exchange (Fock) components in terms of the direct (Hartree) ones. The method is applied to nuclear matter and finite nuclei. It is shown that, in the RHF formalism, the NLSI, which are explicitly isovector-independent, generate scalar, vector and tensor nucleon self-energies with a strong isovector dependence. This strong isovector structure of the self-energies is due to the exchange terms of the RHF method. Calculations are carried out with a parametrization containing five free parameters. The model allows a description of both types of systems compatible with experimental data.

Pion tensor force and nuclear binding energy in the relativistic Hartree-Fock formalism

The binding energies of several isotopic families are studied within the relativistic Hartree-Fock approximation with the pseudovector coupling for the πN vertex, to find out a suitable strength for the effective pion tensor force (EPTF). An approximation for determining separately the contributions of the central and tensor forces generated by pion is considered. The results for heavy nuclei indicate that a realistic strength for the EPTF is smaller than a half of that appearing in the OPEP. This conclusion also applies to the results for the single-particle energies. Besides, it has been found that there is a genuine relativistic contribution of the EPTF in nuclear matter which is small but significant.

Isotopic dependence of the nuclear charge radii and binding energies in the relativistic Hartree-Fock formalism

Relativistic nonlinear models based on the Hartree and Hartree-Fock approximations, including the σ, ω, π, and ρ mesons, are worked out to explore the behavior of the nuclear charge radii and the binding energies of several isotopic chains. We find a correlation between the magnitude of the anomalous kink effect (KE) in the Pb isotopic family and the compressibility modulus (K) of nuclear matter. The KE appears to be sensitive, in particular, to the mechanisms which control the K value. The influence of the symmetry energy on the Ca isotopic chain is also studied. The behavior of the charge radii of single-particle states for some special cases and its repercussion on the nuclear charge radius is analyzed. The effect of pairing correlations on the models improves considerably the quality of the results in both binding energy and KE.

Nuclear relativistic hartree-fock approximation with effective pions

The effect of an interaction between the σ- and π-meson fields, as well as that of the mixture of pseudoscalar (PS) and pseudovector (PV) couplings for the πN vertex on the shell structure of finite nuclei are analyzed in the framework of different nonlinear nuclearmodels based on the relativistic Hartree-Fock approximation. For the case of a dominant πN PV coupling, it is found that if the σ-π interaction generates an effective pion mass increasing with the nuclear density, the unrealistic effect of pions on the shell structure can be strongly reduced, keeping, roughly, the contribution of pions to the total binding energy. The πN PS coupling increases slightly the spin-orbit splittings and the binding energy of the single-particle levels.

The pseudospin symmetry in atomic nuclei

The grounds on which the nuclear pseudospin symmetry (PSS) is supposed to be based are analysed within the relativistic mean-field framework. A comparative analysis of the mechanisms responsible for the breaking of the spin and pseudospin symmetries, which clarifies the different nature of these symmetries, is made. A non-relativistic explanation of the PSS, based on the effect of the spin-orbit interaction, is also sketched.

On the relativistic origin of the kink effect in the chain of Pb isotopes

Abstract We investigate the origin of the kink effect (KE) in the relativistic mean field theory by transforming the single-particle Dirac equation into a Schrodinger-like equation. It is found that relativistic self-consistent effects as well as contributions coming from the ρ meson determine the actual structure of the KE. However, the spin–orbit force generated by the ρ meson has no significant influence on the KE.

Investigations of the scalar field self-interactions generated by a chiral model in the Hartree-Fock approach to nuclear structure

We have investigated the role of the exchange effects in a chiral theory (including self-interactions of the scalar field) in the framework of the self-consistent Hartree-Fock approximation to nuclear matter and finite nuclei. The properties of normal solutions are found to be in better agreement with the experiment than in the Hartree case. The stability of abnormal solutions decreases as a consequence of the Fock terms.

Nuclear relativistic Hartree-Fock approximation with weakened pion tensor force

Properties linked to the single-particle energies, as nuclear spectra, spin-orbit splittings and shell gaps are investigated in the framework of the relativistic Hartree-Fock approximation with pseudovector coupling for the πN vertex. The role of an effective mass of pions moving in the nuclear medium and its relationship with the strength of pion tensor force is discussed. A simple method to reduce the contribution of this tensor force that considerably improves the single-particle spectrum of nuclei is proposed.