Quasi-Equilibrium Nonadditivity

The possibility that a short-range interacting system exhibits nonadditivity is investigated. After the discussion on the precise definition of additivity and its consequence, we show that it is possible when the system is in a quasi-equilibrium state by examining the specific model, in which the spin degrees of freedom are coupled to the motional degrees of freedom and which exhibits a type of structural phase transitions. In this model, the interactions are local but the system evolves under the global constraint, i.e., the triangular lattice structure, which effectively plays the role of long-range interactions and causes the nonadditivity. The triangular lattice structure is just a metastable structure, and hence this nonadditivity is not an equilibrium property but a property in a quasi-equilibrium state.