Quantum coherence in surface-tip transfer of adatoms in AFM/STM

An adiabatic theory has been developed to describe spontaneous transfer of adatoms from a surface to a tip in atomic force microscopy and scanning tunneling microscopy (AFM/STM). It is shown that, at sufficiently low temperatures, the in influence of phonon and electron-bore excitations on the transfer process may become so small that coherent transitions of an adatom between the surface and the tip are possible. The adatom tunnels back and forth between the surface and the tip with a definite period that depends on the surface-tip separation. The effect is mainly due to a unique opportunity to vary the tunneling amplitude of the adsorbate by controlling the tip-surface separation distance. Two features contribute to the quantum coherence phenomenon being even more pronounced as compared to that of interstitial hopping in the bulk: (a) the electron density of states at the Fermi level: and (b) the lattice deformations associated with the presence of the adsorbate outside the surface are noticeably smaller than for an interstitial in the bulk. As a result, electron and phonon polaron effects, respectively, which. reduce the mobility of the adatom with respect to surface-tip transition, are substantially suppressed at all temperatures. For typical AFM/STM tip velocities and separation distances, the coherent transition rate of chemisorbed hydrogen atoms at temperatures below 10 K for insulators and semiconductors, and below 0.1 K for metals, may not only be comparable with but also exceed the thermally activated (incoherent) transfer rates at room temperature. [S0163-1829(98)08108-9].