Phase Transitions and Distributions of Zeros of the Partition Functions for an Antiferromagnetic Ising Model and for a Hard Core Lattice Gas

Zero distributions of the partition function for a Ising model of 6×6 square lattice with the first ( J 1 ) and the second ( J 2 ) neighbor interactions and loci of the zeros of that for 4×∞ lattice are obtained. From the results of the present and previous studies, the followings are concluded. The locus of the Ising model at low temperature is a unit circle when J 1 >0 and J 1 +2 J 2 >0; dual circles when J 1 <0 and J 2 >0, (or J 1 >0 and J 2 <0) and in the region of the first order transition; dual loops with cusps when J 1 <0, J 2 >0 and in the region of the second order transition; four-fold loops with negative real tail when J 1 <0 and J 2 >0. Loci of zeros for the hard core lattice gas of 6×∞ lattice with and without the second neighbor interaction are also obtained. The locus of the hard core lattice gas with attraction is nearly a circle below the tricritical temperature, and a loop with a cusp above the tricritical temperature. Apparent contradictions to the Yang-Lee theory observed in earlier stages disappear.