Asymmetric nuclear matter and its instabilities

Abstract

In order to investigate the instabilities in asymmetric nuclear matter described within relativistic mean field hadron models, we build the spinodals. We have used relativistic models both with constant and density dependent couplings at zero and finite temperatures. We have seen that the main differences in the spinodals occur at finite temperature and large isospin asymmetry close to the boundary of the instability regions.It is important to test the relativistic models often used in the literature at finite temperature and different densities, since these models can describe the ground state of both stable and unstable nuclei [1] as well as the properties of neutron stars and supernovae [2], once the couplings are conveniently adjusted.Asymmetric nuclear matter (ANM) may present instabilities and phase transitions. This is a topic of great interest nowadays. The instabilities present in ANM may manifest themselves as an isospin distillation or fractionation [3]. Multifragmentation also takes place when the system enters the spinodal region through nucleation or through spinodal decomposition. The correlation between spinodal decomposition and negative heat capacity evidences the fact that the spinodal decomposition is the dynamics underlying the liquid gas phase transition [4]. Also in neutron stars the underlying asymmetry of the equation of state which is used to describe it is an important source of investigation. It has recently been proposed that there is a relationship between the neutron skin of heavy nuclei and the properties of neutron star crusts, namely the thicker the neutron skin of a heavy nucleus the thinner the solid crust of a neutron star.Moreover, standard relativistic mean field interactions show limitations for describing nuclei close to the stability line. This is due to the fact that the isovector channel is poorly constrained by experimental data. An example is the systematic overestimate of the neutron skins [5]. Some of these limitations are overcome by quantum hadrodynamical models with density dependent meson-nucleon couplings (which we refer to as TW) [6,7], which have been used with success to describe both nuclear matter and finite nuclei.We may ask whether the recent improvements of the RMF models, both through the inclusion of density dependent meson-nucleon couplings and/or the δ scalar-isovector meson, present different features at subnuclear densities of nuclear asymmetric matter which could have consequences for the properties of the inner crust of neutron stars or in multifragmentation or isospin fractionation reactions. The parametrizations of these models take generally into account saturation properties of nuclear matter and properties of stable nuclei. Extension of the model for very asymmetric nuclear matter or to finite temperatures may show different behaviors. The simplest test between the models is a comparison of the regions of uniform unstable matter.We start from the lagrangian density of the relativistic TW model [6] whereThe parameters of the model are: the ...