Interfacial Friction of Helium Films Adsorbed on Graphite

摩擦力是一种常见的力，但从微观的角度来看，人们对它的理解却很少。最近，使用新开发的技术报道了原子尺度摩擦的实验观察，例如，原子力显微镜（AFM）和石英晶体微量天平（QCM）。在原子尺度上观察到了这种特殊的摩擦性质，因此，我们研究了石墨表面吸附氦膜的界面摩擦。在本实验中，采用QCM技术测量这种摩擦。当薄膜发生滑移时，共振频率会迅速增加，我们发现非超流^4He薄膜在低温下的一定条件下会发生滑移，界面摩擦力的降低。此外，发现膜开始滑动的温度在第二层完成附近逐步降低。在高于33原子/nm^2时，我们观察到滑移和超流性，这意味着超流膜下面的惰性层滑移。实验揭示了石墨表面氦膜的界面摩擦特性。

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.