Step ). Fractal islands are formed when an adatom arrives at an edge, but relaxation to a more favorable position is hindered. Looking at the diffusion along the steps on Mg(101 ! 1), it is noticeable that depending on the directions, the barriers are very heterogeneous. On the one hand, there is barrierless diffusion The reason might be due to different coordinations of the diffusing atom in the initial/final sites (hollow vs. fcc-like sites).