Ensemble dependence of the critical behavior of a system with long-range interaction and quenched randomness

We propose a hybrid model governed by the Blume-Emery-Griffiths (BEG) Hamiltonian with a mean-fieldlike interaction, where the spins are randomly quenched such that some of them are "pure" Ising and the others admit the BEG set of states. It is found, by varying the concentration of the Ising spins, that the model displays different phase portraits in concentration-temperature parameter space, within the canonical and the microcanonical ensembles. Phenomenological indications that these portraits are rich and rather unusual are provided.