19-VERTEX VERSION OF THE FULLY FRUSTRATED XY MODEL

We investigate a 19-vertex version of the two-dimensional fully frustr ated XY (FFXY) model. We construct Yang-Baxter equations for this mode l and show that there is no solution. Therefore we have chosen a numer ical approach based on the transfer matrix. The results show that a co upled XY Ising model is in the same universality class as the FFXY mod el. We find that the phase coupling over an Ising wall is irrelevant a t criticality. This leads to a correction of earlier determinations of the dimension x(h,Is) of the Ising disorder operator. We find x(h,Is ) = 0.123(5) and a conformal anomaly c = 1.55(5). These results are c onsistent with the hypothesis that the FFXY model behaves as a superpo sition of an Ising model and an XY model. However, the dimensions asso ciated with the energy, x(t) = 0.77(3), and with the XY magnetization X(h,XY) almost-equal-to 0.17, refute this hypothesis.