Progress of Theoretical Physics Supplement
Coherent-Anomaly Analysis with Cluster Variation Method for Two-Dimensional Ising Model with Nearest-Neighbor and Next-Nearest-Neighbor Interactions
Kazuyuki Tanaka, Tsuyoshi Horiguchi, Tohru Morita
Abstract
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 ην, γ and ν, 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.