Phase transition in potts model with invisible states

We study phase transition in the ferromagnetic Potts model with invisible states that are added as redundant states by mean-field calculation and Monte Carlo simulation. Invisible states affect the entropy and free energy, although they do not contribute to the internal energy. A second-order phase transition takes place at finite temperature in the standard q-state ferromagnetic Potts model on two-dimensional lattice for q = 2,3, and 4. However, our present model on two-dimensional lattice undergoes a first-order phase transition with spontaneous q-fold symmetry breaking (q = 2,3, and 4) due to entropy effect of invisible states. The model is fundamental for the analysis of a first-order phase transition with spontaneous discrete symmetry breaking.