Physics of Friction

摩擦是最常见的物理现象之一，自古以来就被研究过。然而，摩擦的基本机制至今仍不清楚。摩擦研究之所以出现这种局面，是因为摩擦是一种复杂的现象，它表现出各种材料的性质、形式、表面状态、润滑等方面的不同。近年来各种实验技术的发展，如摩擦力显微镜和计算机，使我们能够在良好的控制条件下研究摩擦，这在过去几十年是不可能的。在此基础上，采用分子动力学模拟的方法研究了干净石墨衬底与干净石墨薄片之间的摩擦。我们成功地再现了原子尺度的周期性粘滑运动和低摩擦系数，这两者都在实验中观察到。我们首先阐明了石墨低摩擦系数的机理，即石墨片中两个亚晶格间相互作用的消除。ii型超导体中磁涡流的钉住和脱羽和密度波是固体中典型的摩擦现象，它们的研究为上述固体表面摩擦的研究提供了启示。新发现的超导体MgB2由于其在金属化合物中Tc最高和存在两个超导间隙而引起了人们的广泛关注。研究了该体系的磁通流电阻率，明确了实验中观察到的行为是两个不同磁场依赖的超导间隙的表现。许多著作研究了以钉扎作为固体摩擦现象理论和数值模型的弹性流形模型。然而，不幸的是，该模型没有考虑到塑性的影响，但塑性在实际系统中可以发挥重要作用。对允许塑性变形的模型进行了详细的数值研究，明确了塑性流态与动态固态之间的动态相变。跃迁的临界外部驱动力随观测时间的增加而增大，在无限观测时间时发散。因此我们可以得出结论，动态固相只有在有限的观测时间内是稳定的。少

Friction is one of the most familiar physical phenomena and has been investigated from ancient age. The fundamental mechanism of friction is, however, not clear still now. This situation of the investigation of friction results from the fact that friction is complicated phenomena and shows various aspects by the property of each material, form, surface state, lubrication and etc. Recent development of various experimental technique, such as frictional force microscope and computer enable us to study of friction under well controlled condition, which is impossible before the last few decades. Stimulated by these experiments we investigated the friction between clean graphite substrate and clean graphite flake by molecular dynamics simulation. We succeeded to reproduce atomic scale periodic stick slip motion and low frictional coefficients, both of which are observed in experiments. We first clarified the mechanism of low frictional coefficient of graphite as the cancellation of interlay … More er interaction between two sublattices in the flake.Pinning and depinning of magnetic vortex in type-II superconductors and density waves are typical frictional phenomena in solids and their investigation shed light on the study of friction at solid surface mentioned above. Newly discovered superconductor, MgB2, attract much attention recently due to highest Tc in metallic compounds and the existence of two superconducting gaps. We investigated flux flow resistivity of this system and made clear that its behavior observed in experiment is the manifestation of two superconducting gaps with different magnetic field dependence.Many works investigated elastic manifold model with pinning as the theoretical and numerical model of frictional phenomena in solids. Unfortunately the effects of plasticity is, however, not taken into account in this model, but the plasticity can play an important effects in real systems. We studied numerically the model which allows plastic deformation in detail and made clear the dynamical phase transition between the plastic flow regime and dynamic solid regime. The critical external driving force of the transition increases with observation time and diverges at infinite observation time. So we can conclude that the dynamical solid phase is stable only within finite observation time. Less