Isospin Lattice-Gas Model and Liquid-Gas Phase Transition in Asymmetric Nuclear Matter

An isospin lattice-gas model, which is a spin-1 Ising model, is employed to investigate the liquid-gas phase transition in asymmetric nuclear matter. We consider nuclear matter as a lattice where each lattice site can be either empty or occupied by a proton or a neutron, with a nearest-neighbor interaction among the nucleons. With the Bragg-Williams mean field approximation, we calculate various thermodynamic properties of nuclear matter for different densities and different proton-neutron asymmetry parameter $s$. Our model exhibits liquid-gas phase transition below a critical temperature $T_c$, and predicts a monotonic decreasing of $T_c$ as the magnitude of $s$ is increased. The dependence of the nuclear matter isotherms on the asymmetry parameter $s$ is discussed.