It was noted that the Schwoebel barrier which was calculated  ab initio  for downward self-diffusion across A-type steps on (111) was equal to only 0.02eV. This challenged current understanding of epitaxy on clean and O pre-covered Pt(111). Its smallness was difficult to reconcile with the suggestion that O acted as a surfactant by assisting interlayer transport. Geometrical arguments were presented which explained why the Schwoebel barrier at B-type steps was more than an order of magnitude higher than the barrier at A-type steps.

P.J.Feibelman: Physical Review Letters, 1998, 81[1], 168-71