Solitonic mechanisms of dissipation in adsorbed atomic mono-layers

In this work we investigate the possibility of a dissipation topography sensitive to local bonding defects (solitons or kinks) of adatoms monolayers. We carry out this study in a very idealized scheme: the standard Frenkel-Kontorova model, with a harmonic chain representing the layer of atoms adsorbed on a crystalline surface, interacting with a periodic potential representing the solid substrate. In experiments dissipation is mapped through the tip of an Atomic Force Microscope (AFM), which we simulate by means of a Gaussian-shaped oscillating potential. We find evidence of two distinct dissipative regimes, low and a high tip frequency, both showing a pronounced dissipation contrast for the kinks with respect to in-register adatoms. The high visibility of the kinks is explained by the resonant behavior of the power dissipated by the tip as a function of the local atomic vibration frequency, which is strongly sensitive to the rigidity of the local bonding. For weak adatom-adatom interaction, kinks are strongly localized and pinned to the substrate; under these conditions we obtain stationary reproducible dissipation maps. In contrast, when the adatom-adatom interaction exceeds the corrugation of the adatom-substrate interaction, kinks become extended weakly pinned objects which can be dragged along by the tip: correspondingly the dissipation maps acquire a history-dependent quality. Advisor: Dr. Nicola Manini Co-Advisor: Prof. Giuseppe Santoro