arXiv1994
We investigate the complex-temperature singularities of the susceptibility of the 2D Ising model on a square lattice. From an analysis of low-temperature series expansions, we find evidence that as one approaches the point $u=u_s=-1$ (where $u=e^{-4K}$) from within the complex extensions of the FM or AFM phases, the susceptibility has a divergent singularity of the form $χ\sim A_s'(1+u)^{-γ_s'}$ with exponent $γ_s'=3/2$. The critical amplitude $A_s'$ is calculated. Other critical exponents are found to be $α_s'=α_s=0$ and $β_s=1/4$, so that the scaling relation $α_s'+2β_s+γ_s'=2$ is satisfied. However, using exact results for $β_s$ on the square, triangular, and honeycomb lattices, we show that universality is violated at this singularity: $β_s$ is lattice-dependent. Finally, from an analysis of spin-spin correlation functions, we demonstrate that the correlation length and hence susceptibility are finite as one approaches the point $u=-1$ from within the symmetric phase. This is confirmed by an explicit study of high-temperature series expansions.