Inequivalence of ensembles in a system with long-range Interactions - art.no. 030601

We study the global phase diagram of the Infinite-range Blume-Emery-Griffit hs model both in the canonical and in the microcanonical ensembles. The can onical phase diagram shows first-order and continuous transition lines sepa rated by a tricritical point. We find that below the tricritical point, whe n the canonical transition is first order, the phase diagrams of the two en sembles disagree. In this region the microcanonical ensemble exhibits energ y ranges with negative specific heat and temperature jumps at transition en ergies. These results can be extended to weakly decaying nonintegrable inte ractions.