On universality in ageing ferromagnets

This work is a contribution to the study of universality in out-of-equilibrium lattice models undergoing a second-order phase transition at equilibrium. The experimental protocol that we have chosen is the following: the system is prepared in its high-temperature phase and then quenched at the critical temperature T c . We investigated by means of Monte Carlo simulations two quantities that are believed to take universal values: the exponent λ/ z obtained from the decay of autocorrelation functions, and the asymptotic value of the fluctuation–dissipation ratio X ( t , s ). This protocol was applied to the Ising model, the 3-state clock model and the 4-state Potts model on square, triangular and honeycomb lattices and to the Ashkin–Teller model at the point belonging at equilibrium to the 3-state Potts model universality class and to a multispin Ising model and the Baxter–Wu model both belonging to the 4-state Potts model universality class at equilibrium.