PARAMETRIC SOLUTION OF THE PHASE-DIAGRAM OF A MEAN-FIELD ISING-MODEL EXHIBITING A TRICRITICAL POINT

A mean-field approach to an Ising lattice with two-spin (pair) and fou r-spin (quartet) interactions is presented as an instructional example of a system exhibiting a tricritical point. In the absence of an exte rnal magnetic field, it is shown that the phase diagram of this system presents second- and first-order transition lines in accordance with the relative strength of the pair and quartet interaction constants. T hese lines meet at a tricritical point. A parametric solution of the m etastability (spinodal) and coexistence curves is reported.