Complex-Temperature Singularities of Ising Models

We report new results on complex-temperature properties of Ising models. These include studies of the s = 1/2 model on triangular, honeycomb, kagomé, 3·12 ², and 4·8 ² lattices. We elucidate the complex- T phase diagrams of the higher-spin 2D Ising models, using calculations of partition function zeros. Finally, we investigate the 2D Ising model in an external magnetic field, mapping the complex- T phase diagram and exploring various singularities therein. For the case βH = iπ/2, we give exact results on the phase diagram and obtain susceptibility exponents γ' at various singularities from low-temperature series analyses.