Physics – Condensed Matter – Statistical Mechanics
Scientific paper
2011-05-31
J. Stat. Mech. (2011) P08024
Physics
Condensed Matter
Statistical Mechanics
17 pages, 19 figures, revised version
Scientific paper
10.1088/1742-5468/2011/08/P08024
We study the problem of ensemble equivalence in spin systems with short-range interactions under the existence of a first-order phase transition. The spherical model with nonlinear nearest-neighbour interactions is solved exactly both for canonical and microcanonical ensembles. The result reveals apparent ensemble inequivalence at the first-order transition point in the sense that the microcanonical entropy is non-concave as a function of the energy and consequently the specific heat is negative. In order to resolve the paradox, we show that an unconventional saddle point should be chosen in the microcanonical calculation that represents a phase separation. The XY model with non-linear interactions is also studied by microcanonical Monte Carlo simulations in two dimensions to see how this model behaves in comparison with the spherical model.
Martin-Mayor Victor
Nishimori Hidetoshi
Takahashi Kazutaka
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