Noncontact Atomic Force Microscopy by Seizo Morita Franz J. Giessibl Ernst Meyer & Roland Wiesendanger

Author:Seizo Morita, Franz J. Giessibl, Ernst Meyer & Roland Wiesendanger
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

13.4.3.1 Defect and Symmetry Induced Modification of Energy Barriers

Although our simulations for the pristine surface offer an explanation for the absence of the “three-in-a-row” structure in our experiments, they do not provide an explanation for how the phason pair is created, and why successful manipulations are regularly observed, as a substantial barrier must be crossed to transit from the “three-in-a-row” structure. In order to answer this question we performed an additional series of NEB simulations. These explored the energy landscape with the tip at close approach for a series of surface slabs with a two dimer vacancy (2DV) type defect, or a small sub-surface expansion or compression introduced in order to simulate the effects of long range strain in the surface. In these simulations (Fig. 13.8b) we observe that the introduction of defects significantly changes the barrier to transit to the phason pair structure, with an increase in the barrier for the 2DV type defect (110 meV), and a reduction in the barrier for the compressive and expansion induced strain (60 and 45 meV respectively). Although none of the simulations presented here resulted in a complete barrier collapse, we suggest that on the real surface the strain field is such that in some locations the barrier for transition is reduced due to the complex strain fields generated by defects, step edges, and sub-surface defects, creating regions where phason pair creation is possible, while in others the defects strongly pin the dimers and prevent manipulation [8, 9, 17]. This correlates well with observations that in some regions of the surface we observe a large number of “failed flips ” where the dimer returns to its original configuration as the tip retracts [17].

One subtle effect that has not been discussed, and is not captured in our simplified surface slabs, is that of symmetry breaking. A casual inspection of the model for phason pair creation on the pristine surface (e.g. Fig. 13.2) reveals an interesting paradox—why does the dimer above the target dimer relax, rather than the dimer below (or vice versa)? On a defect free surface with a symmetrically positioned tip there should be no preference for either one to respond over the other. Therefore, some factor, either the strain field at the surface, or the position of the tip, must break this symmetry in order for the system to reproducibly relax one way or the other. Indeed, in some experiments [17, 46] we have observed a weak variation in the outcome of phason pair creation experiments. However, the long range nature of the surface effects that can influence the response of the dimer presents a significant challenge for future simulations, as accurate modelling will require extremely large numbers of atoms in the simulation cell.

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