Sliding friction of solid xenon monolayers and bilayers on Ag(111)

We have measured the force required to slide one- and two-atom-thick solid films of xenon along a Ag(111) surface, and observe that the friction associated with the bilayer is approximately 25% greater than that associated with the monolayer. Our result sheds new light on the nature of atomic-scale frictional energy dissipation mechanisms, and demonstrates how localized to an interface such mechanisms can be.