(submitted to Phys. Rev. B, Feb. 23, 1996; revised Aug 26, 1996)
Shiwu Gao, M. Persson and B. I. Lundqvist
Dep. of Applied Phys.,
CTH/GU, Göteborg
We present and discuss in detail a theory for atom transfer (or bond-breaking) by the tip of a scanning tunneling microscope that was first outlined by us [Solid State Comm. {\bf 84} 271 (1992)]. The theory is applied to the ``Eigler atomic switch''. In this theory the bond is broken by overcoming the associated potential barrier thanks to gain in energy from the tunneling electrons. The barrier crossing is described by a truncated harmonic oscillator and the inelastic electron tunneling is modelled by a simple resonance model for the electronic structure. The rate of atom transfer is shown to be Arrhenius-like with a vibrational temperature set by the inelastic tunneling rate. Characteristic features of this mechanism include a cross-over from current-driven to thermally activated bond-breaking with decreasing applied voltage and a power-law dependence of the bond-breaking rate with the applied voltage, the latter in agreement with experimental findings. We have also identified a ``wind-force'' in the resonance model for tunneling, which in some cases may give an important current dependent contribution to the potential energy surface. The general features of our theory should have relevance for many other electronically driven surface processes.