Location of zeros in the complex temperature plane: Absence of Lee-Yang theorem.

In extensions of the Yang-Lee theory to the complex temperature plane, it has often been assumed that the zeros of partition function would again fall in lines rather than in areas. Though this happens to be true for the isotropic Ising model, we point out that there is in general no basis for this assumption. We discuss the location of the zeros of the partition function of the anisotropic Ising model in the complex temperature plane and show that they indeed always fall in areas.