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 lambda/z obtained from the decay of autocorrelation functions, and the asymptotic value X-infinity of the fluctuation - dissipation ratio X(t, s). This protocol was applied to the Isingm 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 Isingm model and the Baxter - Wu model both belonging to the 4-state Potts model universality class at equilibrium.