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，人们对它的理解很少。近年来，利用原子力显微镜（AFM）和石英晶体微天平（QCM）等新技术对原子尺度摩擦进行了实验观察。这种特殊的摩擦性质已经在原子尺度上被观察到。因此，我们研究了石墨吸附氦膜的界面摩擦。在本实验中，采用了QCM技术来测量这种摩擦。当薄膜滑动时，观察到谐振频率的快速增加。我们发现非超流体^4He薄膜在一定的低温条件下发生了滑移，即界面摩擦力的减小。此外，我们还发现，在第二层完成前后，薄膜开始滑动的温度逐步降低。在33个原子/nm^2以上，我们观察到滑移和超流动性，这意味着超流体膜下的惰性层滑动。本实验揭示了氦膜在石墨上的界面摩擦特性。

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.