Tricritical point of the J1-J2 Ising model on a hyperbolic lattice

The ferromagnetic-paramagnetic phase transition of the two-dimensional frustrated Ising model on a hyperbolic lattice is investigated by use of the corner transfer matrix renormalization group method. The model contains a ferromagnetic nearest-neighbor interaction J(1) and a competing antiferromagnetic interaction J(2). A mean-field-like second-order phase transition is observed when the ratio kappa=J(2)/J(1) is less than 0.203. In the region 0.203 < 1/4, the spontaneous magnetization is discontinuous at the transition temperature. Such tricritical behavior suggests that the phase transitions on hyperbolic lattices need not always be mean-field-like.