Thermodynamics of extensive but nonadditive systems: modified Gibbs-Duhem equation in the dipolar gas

Thermodynamic properties of extensive but nonadditive systems are investigated. The precise definitions of additivity and extensivity are presented, and we will see that additivity derives several important properties including the shape-independence of the thermodynamic functions, the concavity of the entropy, and the equivalence of ensembles. In nonadditive systems, some of the above properties can be violated. It is pointed out that the shape-dependence of the entropy density in a nonadditive system results in the violation of the Gibbs-Duhem equation. As an example, the dipolar gas is numerically studied and the violation of the Gibbs-Duhem equation is confirmed. The predicted violation of the Gibbs-Duhem equation should be observable in cold-atom experiment with polarized dipolar gas.