COMPLEX-TEMPERATURE SINGULARITIES OF THE SUSCEPTIBILITY IN THE D=2 ISING-MODEL

We report new results on complex-temperature singularities of the susc eptibility (chi) of the 2D Ising model. We give a unified treatment of both the physical critical point and the singularity at u = u(s) = -1 , which yields the relation y(s)' = 2(y' - 1). Using exact results, we show that universality is violated at the singularity u = -1 and disc uss the reasons. We also show that certain exponent relations are viol ated at this point. The behavior of(chi) is analyzed as u = -1 is appr oached from the symmetric PM phase and from the FM and AFM phases.