Two-Dimensional Ising Model with Crossing and Four-Spin Interactions and a Magnetic i(rc[2) kT

where T is the temperature. This solution, which has since been rederived from a variety of different approaches, (2~) exhibits a second-order phase transition occuring at infinite temperature. This leads to the occurrence of a zero-temperature phase transition in a fully frustrated Ising model (5'7) in the dual space. It is also known that this solution of the nearest-neighbor model yields information on monomer correlations in the dimer problem. 2 In this paper we show that the phase transition occuring in the twodimensional Ising model at the pure imaginary field (1) behaves differently