  Experiments performed by friction force microscopy at atomic-scale surface
steps on graphite, MoS$_2$, and NaCl in ambient conditions are presented. Both
step-down and step-up scans exhibit higher frictional forces at the edge, but
distinguish in their load dependence: While the additional frictional force due
to the step edge increases linearly with load if the tip has to jump a step up,
it remains constant for downward jumps. This phenomena represents a universal
effect that can be explained in terms of a modified Prandtl-Tomlinson model
featuring a Schoebel-Ehrlich barrier at steps.
