Study on the Interfacial Friction of Helium Films

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Friction is a common force, but is poorly understood from a microscopic point of view. Recently, experimental observations of an atomic-scale friction were reported using newly developed techniques, e.g., the atomic force microscope (AMF). And a peculiar friction nature has been observed.Thus motivated, we investigated the interfacial friction of helium films adsorbed on Grafoil (exfoliated graphite). In the present experiments, the quartz-crystal microbalance (QCM) technique was adopted for the measurement of this friction. In the technique, when the interfacial friction of the films is decreased and the film is decoupled from the lateral oscillation, an increase in the resonant frequency of the crystal is observed.We found that nonsuperfluid ^4He films partially decouple from the lateral oscillation, and the degree of decoupling increases below Ts. From the amplitude dependence of this increase, we showed that the frictional force between Grafoil substrate and ^4He film is almost proportional to the square root of the sliding velocity. However, the atomic mechanism of this nonlinear friction is still an open question. In addition, the increase in decoupling below Ts is closely related to the layer structure of ^4He film, making this a subject for future study.