Bimodal island-size distributions in submonolayer growth

We present a simple model based on the existence of exchange processes that explains the bimodal island-size distributions in submonolayer growth recently reported in the literature. The model can reproduce both the temperature dependence observed in the case of Co/Cu(100) and Ni/Au(111) and the coverage dependence measured for Fe/Au(100). We show also that, under the conditions where the model is applicable, none of the standard scaling relationships is preserved.