Nearest neighbor ising model with added local 4-spin interaction
The effects of adding a local 4-spin term to the nearest neighbor Ising model are studied. The relevant parameter isx, the ratio of the four- to the two-spin interaction strength. We have evaluated the first six terms of the high temperature susceptibility expansion and found them to be consistent with the value 1.75 of the critical exponent γ. The critical exponent α is shown to remain zero to first order inx, provided that the free energy is analytic inx. We also demonstrate the equivalence of the model examined to an elastic spin system with pure two-spin interactions.