NEW CRITICAL-POINT FOR 2-DIMENSIONAL XY-TYPE MODELS

A new critical point is established through Monte Carlo simulations for a class of two dimensional XY models with a modified nearest-neighbor spin interaction. The critical exponents found are nu almost-equal-to 1.4, beta almost-equal-to 0.6, and an alpha < 0, consistent with hyperscaling. The phase transition is interpreted as a vortex-unbinding transition. Comparisons are made with the phase diagram of the two dimensional Coulomb gas and with the spin-1/2 XY model.