TIP-SURFACE TRANSFER OF ADATOMS IN AFM STM - EFFECT OF QUANTUM OSCILLATIONS/

An adiabatic theory of spontaneous adatom transfer between a surface a nd an AFM/STM tip has been developed under the assumption that the dwe ll time of the adsorbate at the adsorption site is small compared to t he phonon and electron-hole relaxation times. The adsorbate is assumed to be tunneling across the potential barrier from a ground state at t he sample to that at the tip. A simple analytical expression has been obtained for the ground-level energy splitting, which governs the dyna mics of the coherent tunneling. The transition rate is shown to depend strongly on the adatom mass, binding energy and separation distance b etween the adsorption sites. It is found that, at typical AFM/STM tip velocities and separation distances, an adatom may be involved in a pr ocess of consecutive transitions from the sample to the tip and vice v ersa. It is shown that coherent tunneling is a dominant mechanism of a datom transfer at temperatures below 10 K for insulators and semicondu ctors and below 0.1 K in the case of metals. The total transfer probab ility as a function of lime may exhibit quantum oscillations whose per iod is very sensitive to variation of physical parameters. (C) 1998 El sevier Science B.V. All rights reserved.