Interfacial Friction of Helium Films Adsorbed on Graphite

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Friction is a common force, but is poorly understood from a microscopic point of view. Recently, experimental observations of atomic-scale friction were reported using newly developed techniques, e.g., the atomic force microscope (AFM), and the quartz-crystal microbalance (QCM). And the peculiar friction nature has been observed in an atomic scale.Thus motivated, we investigated the interfacial friction of helium films adsorbed on graphite. In the present experiments, the QCM technique was adopted for the measurements of this friction. When the film slips, a rapid increase in the resonant frequency is observed.We found the slippage of nonsuperfluid ^4He films under a certain condition at low temperatures, i.e., the decrease in the interfacial friction. In addition, it was found that the temperature at which the films start to slip decreases stepwise around the second layer completion. Above 33 atoms/nm^2, we observed both of the slippage and the superfluidity, which means that the inert layers underneath the superfluid film slip. The present experiments revealed the properties of the interfacial friction of helium films on graphite.