Fat Fisher zeroes

We show that it is possible to determine the locus of Fisher zeroes in the thermodynamic limit for the Ising model on planar (“fat”) φ⁴ random graphs and their dual quadrangulations by matching up the real part of the high and low temperature branches of the expression for the free energy. The form of this expression for the free energy also means that series expansion results for the zeroes may be obtained with rather less effort than might appear necessary at first sight by simply reverting the series expansion of a function g(z) which appears in the solution and taking a logarithm.Unlike regular 2D lattices where numerous unphysical critical points exist with non-standard exponents, the Ising model on planar φ⁴ graphs displays only the physical transition at c=exp(−2β)=1/4 and a mirror transition at c=−1/4 both with KPZ/DDK exponents (α=−1, β=1/2, γ=2). The relation between the φ⁴ locus and that of the dual quadrangulations is akin to that between the (regular) triangular and honeycomb lattices since there is no self-duality.