Phase transition and nucleon as soliton in the Nambu-Jona-Lasinio model   at finite temperature and density
  We study some bulk thermodynamical characteristics, meson properties and the nucleon as a baryon number one soliton in a hot quark matter in the NJL model as well as in a hot nucleon matter in a hybrid NJL model in which the Dirac sea of quarks is combined with a Fermi sea of nucleons. In both cases, working in mean-field approximation, we find a chiral phase transition from Goldstone to Wigner phase. At finite density the chiral order parameter and the constituent quark mass have a non-monotonic temperature dependence - at finite temperatures not close to the critical one they are less affected than in the cold matter. Whereas the quark matter is rather soft against thermal fluctuations and the corresponding chiral phase transition is smooth, the nucleon matter is much stiffer and the chiral phase transition is very sharp. The thermodynamical variables show large discontinuities which is an indication for a first order phase transition. We solve the B=1 solitonic sector of the NJL model in the presence of an external hot quark and nucleon medium. In the hot medium at intermediate temperature the soliton is more bound and less swelled than in the case of a cold matter. At some critical temperature, which for the nucleon matter coincides with the critical temperature for the chiral phase transition, we find no more a localized solution. According to this model scenario one should expect a sharp phase transition from the nucleon to the quark matter. 
J. Berger and Chr.-V. Christov