Persistence exponent in superantiferromagnetic quenching

We measure the persistence exponent in a phase separating two-dimensional spin system with non-conserved dynamics quenched in a region with four coexisting stripe phases. The system is an Ising model with nearest neighbor, next-to-the-nearest neighbor and plaquette interactions. Due to the particular nature of the ground states, the order parameter is defined in terms of blocks of spins. Our estimate of the persistence exponent, θ=0.42, differs from those of the two-dimensional Ising and four state Potts models. Our procedure allows the study of persistence properties also at finite temperature T: our results are compatible with the hypothesis that θ does not depend on T below the critical point.