Laura Messio : Order by disorder and phase transitions

Although classical models may seem trivial compared to the quantum models with which we are familiar, they still hold many mysteries, as for example the well-known spin ice models.
We are interested in less degenerate models, where the order-by-disorder phenomenon lifts the degeneracy at infinitesimal temperatures and leads to exotic symmetry breaking and emergent order parameters. Motivated by the physical properties of Vesignieite BaCu3V2O8(OH)2, we study the example of the J1-J3 Heisenberg model on the kagome lattice, with J3>>|J1|. The nature of the ground state and the possible phase transitions are investigated through analytical calculations and parallel tempering Monte Carlo simulations. Entropic effects favor collinear configurations and lead to an emergent q=4 Potts parameter and to a finite temperature phase transition.
The effect of magnetic vacancies on the order and on the transition is explored: this quenched disorder completely destroys the q=4 Potts order to favor a chiral phase.