MIXED-SPIN ISING-MODEL ON THE UNION JACK LATTICE

We show that the mixed-spin Ising model on the union jack lattice with S = 1/2 and S = 1 spin operators placed on different sublattices is equivalent to the eight-vertex model. On a two-dimensional manifold in a parameter space, the model corresponds to the symmetric eight-vertex model and thus is solvable. We locate exactly a coexistence surface between two ordered phases of the mixed-spin model. This surface is bounded by a bicritical line with continuously changing exponents.