FRICTIONAL FORCE BETWEEN A SHARP ASPERITY AND A SURFACE STEP

We report a detailed study of the frictional force between the tip of a scanning force microscope and a step on a crystalline surface. Exper iments on surfaces of freshly cleaved graphite reveal different contri butions to the lateral force at steps with distinctly different depend encies on normal load and scan direction. The different contributions can be attributed to topography-induced tip twisting and an increased dissipative force due to tile Schwoebel barrier at the steps. The latt er contribution is strongly reduced when near-surface step dislocation s are imaged.