Symmetries of microcanonical entropy surfaces

Symmetry properties of the microcanonical entropy surface as a function of the energy and the order parameter are deduced from the invariance group of the Hamiltonian of the physical system. The consequences of these symmetries for the microcanonical order parameter in the high-energy and in the low-energy phases are investigated. In particular the breaking of the symmetry of the microcanonical entropy in the low-energy regime is considered. The general statements are corroborated by investigations of various examples of classical spin systems.