Dependence of maximum static friction on waiting time using dynamics of vortices in superconductors

Vortices in high-Tc superconductors were investigated in terms of the microscopic friction. The I-V characteristics and the transient response of driven vortices were measured in La2-xSrxCuO4 thin films with different Sr concentrations and samples with columnar defects. We found the remarkable dependence of the maximum static friction force on the waiting time at low temperatures. The result suggests that the competition between the flux creep by the thermal fluctuation and the pinning yields a characteristic time scale to stabilize vortices. This relaxation phenomenon is very similar to the so-called boundary lubrication, which occurs under the existence of a thin lubricant film between interfaces. Such a strong dependence changed into a weak logarithmic dependence at higher temperatures. These results imply that the dynamics of vortices can be used for the model not only of the dry friction but also of the lubricated friction. Also, the dependence of the kinetic friction force on the sliding velocity showed the non-Amontons-Coulomb-like behavior even for the lubricated friction. Thus, we confirmed that the Amontons-Coulomb's law does not realize when the thermal fluctuation is sufficiently large