Tricritical behavior of nonequilibrium Ising spins in fluctuating environments.

We investigate the phase transitions in a coupled system of Ising spins and a fluctuating network. Each spin interacts with q neighbors through links of the rewiring network. The Ising spins and the network are in thermal contact with the heat baths at temperatures T_{S} and T_{L}, respectively, so the whole system is driven out of equilibrium for T_{S}≠T_{L}. The model is a generalization of the q-neighbor Ising model [A. Jędrzejewski et al., Phys. Rev. E 92, 052105 (2015)PLEEE81539-375510.1103/PhysRevE.92.052105], which corresponds to the limiting case of T_{L}=∞. Despite the mean-field nature of the interaction, the q-neighbor Ising model was shown to display a discontinuous phase transition for q≥4. Setting up the rate equations for the magnetization and the energy density, we obtain the phase diagram in the T_{S}-T_{L} parameter space. The phase diagram consists of a ferromagnetic phase and a paramagnetic phase. The two phases are separated by a continuous phase transition belonging to the mean-field universality class or by a discontinuous phase transition with an intervening coexistence phase. The equilibrium system with T_{S}=T_{L} falls into the former case while the q-neighbor Ising model falls into the latter case. At the tricritical point, the system exhibits the mean-field tricritical behavior. Our model demonstrates a possibility that a continuous phase transition turns into a discontinuous transition by a nonequilibrium driving. Heat flow induced by the temperature difference between two heat baths is also studied.