Ensemble inequivalence in a mean-field XY model with ferromagnetic and nematic couplings

Arkady Pikovsky 1, 2 Shamik Gupta 3 Tarcisio N. Teles 4 Fernanda P. C. Benetti 4 Renato Pakter 4 Yan Levin 4 Stefano Ruffo 5

Physical Review E, American Physical Society, 2014, pp.062141

We explore ensemble inequivalence in long-range interacting systems by studying an XY model of classical spins with ferromagnetic and nematic coupling. We demonstrate the inequivalence by mapping the microcanonical phase diagram onto the canonical one, and also by doing the inverse mapping. We show that the equilibrium phase diagrams within the two ensembles strongly disagree within the regions of first-order transitions, exhibiting interesting features like temperature jumps. In particular, we discuss the coexistence and forbidden regions of different macroscopic states in both the phase diagrams.

  • 1. UNIPOTSDAM - Department of Physics and Astronomie, Univ. Potsdam
  • 2. Department of Control Theory
  • 3. LPTMS - Laboratoire de Physique Théorique et Modèles Statistiques
  • 4. IFUFRGS - Instituto de Física
  • 5. Dipartimento di Fisica e Astronomia and CSDC