Coarsening Process in One-Dimensional Surface Growth Models

Surface growth models may give rise to unstable growth with mound formation whose tipical linear size L increases in time. In one dimensional systems coarsening is generally driven by an attractive interaction between domain walls or kinks. This picture applies to growth models for which the largest surface slope remains constant in time (model B): coarsening is known to be logarithmic in the absence of noise (L(t)=log t) and to follow a power law (L(t)=t{1/3}) when noise is present. If surface slope increases indefinitely, the deterministic equation looks like a modified Cahn-Hilliard equation: here we study the late stage of coarsening through a linear stability analysis of the stationary periodic configurations and through a direct numerical integration. Analytical and numerical results agree with regard to the conclusion that steepening of mounds makes deterministic coarsening faster: if alpha is the exponent describing the steepening of the maximal slope M of mounds (Malpha = L) we find that L(t)=t n: n is equal to 1/4 for 1<alpha<2 and it decreases from 1/4 to 1/5 for alpha>2, according to n=alpha/(5*alpha -2). On the other side, the numerical solution of the corresponding stochastic equation clearly shows that in the presence of shot noise steepening of mounds makes coarsening slower than in model B: L(t)=t^{1/4}, irrespectively of alpha. Finally, the presence of a symmetry breaking term is shown not to modify the coarsening law of model alpha=1, both in the absence and in the presence of noise.