Universality of the island density exponent in growth models of monomer and dimer chains in two dimensions

Using Monte Carlo simulations, the scaling of the island density exponent f or the aggregation of particles as chains of monomers and dimers is examine d. We found that particles can perform a one- or two-dimensional diffusion depending on the details of the models and on the values of the coverage an d diffusion anisotropy. Hence a nonuniversal behavior for the island densit y exponent chi can be obtained. Eventually, in the asymptotic regime, the o ne-dimensional exponent (chi=1/4) is always recovered because the formed on e-dimensional islands (chains) pray the role of obstacles to the diffusing particles which are thus forced to perform a one-dimensional diffusion.