Triaxiality and shape coexistence in Germanium isotopes

The ground-state deformations of the Ge isotopes are investigated in the framework of Gogny-Hartree-Fock-Bogoliubov (HFB) and Skyrme Hartree-Fock plus pairing in the BCS approximation. Five different Skyrme parametrizations are used to explore the influence of different effective masses and spin-orbit models. There is generally good agreement for binding energies and deformations (total quadrupole moment, triaxiality) with experimental data where available (i.e., in the valley of stability). All calculations agree in predicting a strong tendency for triaxial shapes in the Ge isotopes with only a few exceptions due to neutron (sub-)shell closures. The frequent occurrence of energetically very close shape isomers indicates that the underlying deformation energy landscape is very soft. The general triaxial softness of the Ge isotopes is demonstrated in the fully triaxial potential energy surface. The differences between the forces play an increasing role with increasing neutron number. This concerns particularly the influence of the spin-orbit model, which has a visible effect on the trend of binding energies towards the drip line. Different effective mass plays an important role in predicting the quadrupole and triaxial deformations. The pairing strength only weakly affects binding energies and total quadrupole deformations, but considerably influences triaxiality.Comment: 9 page

Criteria for nonlinear parameters of relativistic mean field models

Based on the properties of the critical and the actual effective masses of sigma and omega mesons, criteria to estimate the values of the isoscalar nonlinear terms of the standard relativistic mean field model that reproduce stable equations of state in respect to particle hole excitation at high densities are derived. The relation between nuclear matter stability and the symmetric nuclear matter properties are shown. The criteria are used to analyze in a more systematic way the high-density longitudinal and transverse instabilities of some parameter sets of relativistic mean field models. The critical role of the vector and vector-scalar nonlinear terms is also discussed quantitatively.Comment: 21 pages, 10 figures, 4 tables. Accepted for Publication in Physical review

Consequences of the center-of-mass correction in nuclear mean-field models

We study the influence of the scheme for the correction for spurious center-of-mass motion on the fit of effective interactions for self-consistent nuclear mean-field calculations. We find that interactions with very simple center-of-mass correction have significantly larger surface coefficients than interactions for which the center-of-mass correction was calculated for the actual many-body state during the fit. The reason for that is that the effective interaction has to counteract the wrong trends with nucleon number of all simplified schemes for center-of-mass correction which puts a wrong trend with mass number into the effective interaction itself. The effect becomes clearly visible when looking at the deformation energy of largely deformed systems, e.g. superdeformed states or fission barriers of heavy nuclei.Comment: 12 pages LATeX, needs EPJ style files, 5 eps figures, accepted for publication in Eur. Phys. J.

Pairing gaps from nuclear mean-field models

We discuss the pairing gap, a measure for nuclear pairing correlations, in chains of spherical, semi-magic nuclei in the framework of self-consistent nuclear mean-field models. The equations for the conventional BCS model and the approximate projection-before-variation Lipkin-Nogami method are formulated in terms of local density functionals for the effective interaction. We calculate the Lipkin-Nogami corrections of both the mean-field energy and the pairing energy. Various definitions of the pairing gap are discussed as three-point, four-point and five-point mass-difference formulae, averaged matrix elements of the pairing potential, and single-quasiparticle energies. Experimental values for the pairing gap are compared with calculations employing both a delta pairing force and a density-dependent delta interaction in the BCS and Lipkin-Nogami model. Odd-mass nuclei are calculated in the spherical blocking approximation which neglects part of the the core polarization in the odd nucleus. We find that the five-point mass difference formula gives a very robust description of the odd-even staggering, other approximations for the gap may differ from that up to 30% for certain nuclei.Comment: 17 pages, 8 figures. Accepted for publication in EPJ

Entropy Production in Collisions of Relativistic Heavy Ions -- a signal for Quark-Gluon Plasma phase transition?

Entropy production in the compression stage of heavy ion collisions is discussed within three distinct macroscopic models (i.e. generalized RHTA, geometrical overlap model and three-fluid hydrodynamics). We find that within these models \sim 80% or more of the experimentally observed final-state entropy is created in the early stage. It is thus likely followed by a nearly isentropic expansion. We employ an equation of state with a first-order phase transition. For low net baryon density, the entropy density exhibits a jump at the phase boundary. However, the excitation function of the specific entropy per net baryon, S/A, does not reflect this jump. This is due to the fact that for final states (of the compression) in the mixed phase, the baryon density \rho_B increases with \sqrt{s}, but not the temperature T. Calculations within the three-fluid model show that a large fraction of the entropy is produced by nuclear shockwaves in the projectile and target. With increasing beam energy, this fraction of S/A decreases. At \sqrt{s}=20 AGeV it is on the order of the entropy of the newly produced particles around midrapidity. Hadron ratios are calculated for the entropy values produced initially at beam energies from 2 to 200 AGeV.Comment: 17 pages, 8 figures, uses epsfig.sty; Submitted to Nucl.Phys.