Slippage 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 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. Recently, Krim and her colleagues measured the mechanical response of noble-gas films on a noble-metal substrate at 77K using the QCM technique. On the other hand, One may expect that since the attraction force of helium atom is so weak, physisorbed helium films slide easily on a substrate.Thus motivated, we investigated the slippage of both superfluid and nonsuperfluid helium films on a two-dimensional porous material, hectrite. In the present experiments, the ultrasonic technique was adopted for the measurements of the mechanical response of these films. When the film slips, a rapid increase in sound velocity and an attenuation peak are observed.We found that nonsuperfluid helium films slip from the oscillating substrate under certain conditions. Up to the coverage at which the fluid state appears at absolute zero, nonsuperfluid ^3He and ^4He films slipped from the oscillation at low temperatures, and this slippage was inhibited as the coverage increases further. In addition, it was found that from the temperature dependence of the slippage, a thermally activated process plays an important role in the frictional force of this system.