The Anisotropic Four-State Clock Model in the Presence of Random Fields
  A four-state clock ferromagnetic model is studied in the presence of different configurations of anisotropies and random fields. The model is considered in the limit of infinite-range interactions, for which the mean-field approach becomes exact. Both representations of Cartesian spin components and two Ising variables are used, in terms of which the physical properties and phase diagrams are discussed. The random fields follow bimodal probability distributions and the richest criticality is found when the fields, applied in the two Ising systems, are not correlated. The phase diagrams present new interesting topologies, with a wide variety of critical points,which are expected to be useful in describing different complex phenomena. 
Octavio Daniel Rodriguez Salmon and Fernando Dantas Nobre
cond-mat.stat-mech