Study on the Interfacial Friction of Helium Films

摩擦力是一种常见的力，但从微观角度对它的理解很少。最近，使用新发展的技术，如原子力显微镜(AMF)，报道了原子级摩擦的实验观测。并观察到了一种特殊的摩擦性质。因此，我们研究了氦膜吸附在石墨表面的界面摩擦。在本实验中，采用了石英晶体微天平(QCM)技术来测量这种摩擦力。在该技术中，当薄膜的界面摩擦减小，薄膜与横向振荡解耦时，观察到晶体的共振频率增加，发现非超流的4He薄膜与横向振荡部分解耦，并且解耦程度在ts以下增加。从这一增加的幅度依赖关系，我们表明，玻璃衬底与~4He薄膜之间的摩擦力几乎与滑动速度的平方根成正比。然而，这种非线性摩擦的原子机制仍然是一个悬而未决的问题。此外，T_s以下退耦的增加与^4He薄膜的层状结构密切相关，这是一个有待进一步研究的课题。

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.