Tip induced motion of adatoms on metal surfaces

From total energy calculations we show that for certain tip-adatom separati ons the activation barrier for the adatom to move towards the tip disappear s and the adatom experiences an attractive force in the direction of the ti p. For a Cu adatom at a (100) microfaceted step on Cu(111) this happens at a lateral separation of about one lattice constant, in agreement with recen t experimental findings. Simultaneously, the activation barrier in the dire ction away from the tip increases significantly. The details of the changes in the potential energy surface induced by the tip are found to depend on the characteristics of the tip apex and its height above the adatom.