  The properties of critical parameters and phase diagram structure of
liquid-gas phase transition are investigated in thermal symmetric and
asymmetric nuclear matter with the covariant density functional (CDF) theory.
Although uncertainty remains in predicting the critical parameters such as the
critical temperature and pressure from various CDF functionals, several
correlations are explored numerically and verified to be approximately linear
between them. These correlations become worse when nuclear matter is more
isospin asymmetric, resulting mainly from the effects induced by symmetry
energy. By looking over the isospin dependence of the critical temperature, the
role of the symmetry energy in LG transition properties of asymmetric matter is
realized. The change of critical temperature with isospin asymmetry is found to
be correlated well with and as a consequence could be constrained by the
density slope of symmetry energy at saturation density. Then, the structure of
phase diagram of thermal nuclear matter is analyzed carefully. It is revealed
that the contribution from symmetry energy dominates the size of liquid-gas
phase coexistence area. Moreover, the specific pattern of the phase diagram
could be determined by the critical temperature at non-zero isospin asymmetry,
illustrated from the correlations of the temperature with pressures at several
characteristic points, paving the possible way to further explore the structure
of liquid-gas phase diagram of thermal nuclear matter.

Nuclear Theory