Complex-temperature singularities in Potts models on the square lattice.

We report some results on the complex-temperature ~CT! singularities of q-state Potts models on the square lattice. We concentrate on the problematic region Re(a),0 ~where a5e) in which CT zeros of the partition function are sensitive to finite lattice artifacts. From analyses of low-temperature series expansions for 3<q<8, we establish the existence, in this region, of complex-conjugate CT singularities at which the magnetization and susceptibility diverge. From calculations of zeros of the partition function, we obtain evidence consistent with the inference that these singularities occur at endpoints ae ,ae * of arcs protruding into the ~complex-temperature extension of the! ferromagnetic phase. Exponents for these singularities are determined; e.g., for q53, we find be520.125(1), consistent with be521/8. By duality, these results also imply associated arcs extending into the ~CT extension of the! symmetric paramagnetic phase. Analytic expressions are suggested for the positions of some of these singularities; e.g., for q55, our finding is consistent with the exact value ae ,ae*52(217i). Further discussions of complex-temperature phase diagrams are given. @S1063-651X~96!09412-3#