Spinodal in asymmetric nuclear matter

The phase diagram of nuclear matter is quite rich; it shows such phenomena as phase-transitions, formation of condensates, clustering, etc. >From the analysis of the spinodal instability, one can learn about the region of liquid-gas coexistence in nuclear matter at low densities and finite isospin asymmetries. In a recent paper, we have shown that asymmetric nuclear matter at sub-nuclear densities should undergo only one type of instability. The associated order parameter is dominated by the isoscalar density and so the transition is of liquid-gas type. The instability goes in the direction of a restoration of the isospin symmetry leading to a fractionation phenomenon. The nuclear interaction is very similar to the Van der Waals potential which acts between molecules. For this reason, below saturation density, the nuclear interaction is also expected to lead to a liquid-gas phase transition. Recently, a converging ensemble of experimental signals seems to have established the phase transition. One is the spinodal decomposition which considers volume instabilities (domain of negative incompressibility). One expects that the system which enters such a forbidden region will favorably breakup into nearly equally-sized 'primitive' fragments in relation to the wavelengths of the most unstable modes. How the simple picture is modified by the asymmetry charge and whether we can expect new signals related to the collision of very asymmetric nuclei are the questions which the author finally poses. (author)