STUDY OF CRITICAL PROPERTIES OF THE POTTS-MODEL BY THE MODIFIED VARIATIONAL CUMULANT EXPANSION METHOD

The modified variational cumulant expansion (VCE) method valid for the whole temperature range and for arbitrary fields is developed to stud y the critical properties of the general q-state Potts model used to s timulate various magnetic systems. The series expansion is generated d irectly in terms of the Potts Hamiltonian. The free energy and the int ernal energy are calculated up to the third-order cumulant terms for t he d-dimensional hypercubic q-state Potts model. The expression for th e critical temperature T-c is derived in a closed form which allows to determine T-c up to arbitrary order cumulant terms. The modified VCE method eliminates the unphysical first-order phase transition at the m ean-field critical point in the internal energy of the Potts model whi ch plagues the conventional VCE method.