Tip–surface transfer of adatoms in AFMrSTM: effect of quantum oscillations

An adiabatic theory of spontaneous adatom transfer between a surface and an AFMrSTM tip has been developed under the assumption that the dwell time of the adsorbate at the adsorption site is small compared to the phonon and electron-hole relaxation times. The adsorbate is assumed to be tunneling across the potential barrier from a ground state at the sample to that at the tip. A simple analytical expression has been obtained for the ground-level energy splitting, which governs the dynamics of the coherent tunneling. The transition rate is shown to depend strongly on the adatom mass, binding energy and separation distance between the adsorption sites. It is found that, at typical AFMrSTM tip velocities and separation distances, an adatom may be involved in a process of consecutive transitions from the sample to the tip and vice versa. It is shown that coherent tunneling is a dominant mechanism of adatom transfer at temperatures below 10 K for insulators and semiconductors and below 0.1 K in the case of metals. The total transfer probability as a function of time may exhibit quantum oscillations whose period is very sensitive to variation of physical parameters. q1998 Elsevier Science B.V. All rights reserved.