SUSCEPTIBILITY SINGULARITIES AT 1ST-ORDER PHASE-TRANSITIONS

The bubble picture for pair-spin correlation functions is applied to two-dimensional Ising-like models, at subcritical temperatures, in an external field h. from which we show, via the fluctuation sum, that the susceptibility chi(h) has an essential singularity at h=0. This is studied further using a solid-on-solid bubble model for (a) a restricted ensemble corresponding to metastability, where h=0 is found to be the limit point of an infinite number of poles of chi along the negative real axis, and (b) an unrestricted ensemble, where a Yang-Lee circle theorem is found.