Microcanonical entropy of the spherical model with nearest-neighbour interactions

For the spherical model with nearest-neighbour interactions, the microcanonical entropy s(epsilon, m) is computed analytically in the thermodynamic limit for all accessible values of the energy epsilon and the magnetization m per spin. The entropy function is found to be concave (albeit not strictly concave), implying that the microcanonical and the canonical ensembles are equivalent, despite the long-range nature of the spherical constraint that the spins have to obey. Two transition lines are identified in the (epsilon, m)-plane, separating a paramagnetic phase from a ferromagnetic and an antiferromagnetic one. The resulting microcanonical phase diagram is compared to the more familiar canonical one.