title: Phase transitions in nonextensive spin systems.
abstract: The spherical spin model with infinite-range ferromagnetic interactions is investigated analytically in the framework of nonextensive thermostatics generalizing the Boltzmann-Gibbs statistical mechanics. We show that for repulsive correlations, a weak-ferromagnetic phase develops. There is a tricritical point separating para-, weak-ferro, and ferro regimes. The transition from paramagnetic to weak-ferromagnetic phase is an unusual first-order phase transition in which a discontinuity of the averaged order parameter appears, even for finite number of spins. This result puts in a different way the question of the stability of critical phenomena with respect to the long-ranged correlations.
authors.affiliation: Laboratoire de Physique des Solides - CNRS, Bâtiment 510, Université Paris-Sud, Centre d'Orsay, F-91405 Orsay, France.
authors.forename: Robert
authors.initials: R
authors.lastname: Botet
authors.forename: Marek
authors.initials: M
authors.lastname: Płoszajczak
authors.forename: Jorge A
authors.initials: JA
authors.lastname: González