ZENO-OF-ELEA PARADOX

Crystal imperfections are generally created during the growth, and on the surface. We describe an instability which can occur during growth by molecular beam epitaxy. It is due to the fact that atoms which have not yet been incorporated (adatoms) diffuse more easily to an upward terrace edge than to a downward step (''Schwoebel effect'). We study a continuous model in which we suggest that, in principle, the instabil ity always takes place, but after a time which can be very long if the Schwoebel effect is weak. The basic reason of the instability is rela ted to Zenon's paradoxon.