  By means of critical behaviors of the dynamical fermion mass in four-fermion
interaction models, we have shown by explicit calculations that when T=0 the
particle density will have a discontinuous jumping across the critical chemical
potential $\mu_c$ in 2D and 3D Gross-Neveu (GN) model and these physically
explain the first order feature of corresponding symmetry restoring phase
transitions. For second order phase transitions in 3D GN model when $T\to 0$
and in 4D Nambu-Jona-Lasinio (NJL) model when T=0, it has been proven that the
particle density itself will be continuous across $\mu_c$ but its derivative
over the chemical potential $\mu$ will have a discontinuous jumping. The
results give a physical explanation of implications of the tricritical point
$(T,\mu)=(0,\mu_c)$ in 3D GN model. The discussions also show effectiveness of
the critical analysis approach of phase transitions.
