COHERENT-ANOMALY ANALYSIS WITH CLUSTER VARIATION METHOD FOR 2-DIMENSIONAL ISING-MODEL WITH NEAREST-NEIGHBOR AND NEXT-NEAREST-NEIGHBOR INTERACTIONS

The spin-pair correlation functions in the Ising model with nearest-neighbor interaction J > 0 and next-nearest-neighbor interaction J' on the square lattice are obtained in three approximations of the simplified cluster variation method. We estimate the critical indices etanu, gamma and nu, by applying the coherent-anomaly method to the obtained spin-pair correlation functions. We conclude that the system belongs to the Onsager universality class when J'/J > - 0.5 and does not when J'/J < - 0.5.