Recently, Riedo et al. have measured the friction force on a nanoscale tip sliding on the mica surface as a function of externally-applied load and the sliding velocity, by means of a friction force microscope (FFM) [1]. For a low velocity range, they have observed the monotonic increase of the average friction force, , with the sliding velocity, V. Beyond the critical velocity, a practically horizontal "plateau" of has been observed. Furthermore, they externally activated the tip by applying lateral oscillations to the cantilever at different frequencies during its sliding. The resonant reduction of the friction force has been observed only for a low velocity.
In this study, we explore the mechanism of atomic-scale friction of FFM with the dynamical simulation in the one-dimensional Tomlinson model at a finite temperature. We have assumed the values of parameters suggested by the experiment [1]. The calculated velocity (V) dependence of the average friction reproduces the observed saturation tendency at high scanning velocities. Such saturation is due to the induction of higher slip motions [2] as the scan velocity increases. When the support point of the cantilever is oscillated at constant amplitude during its sliding, two types of resonant reduction have been distinguished in low velocity range. The first is the synchronous enhancement of slip motion, which becomes effective at the external frequency close to the sliding frequency V/a0, where a0 is the lattice constant. The second is the oscillation excitation of a tip atom at the stick point, which becomes effective at the external frequency close to the oscillation frequency.
[1] E. Riedo et al.,Phys. Rev. Lett. 91, 084502 (2003)
[2] J. Nakamura, S. Wakunami, and A. Natori, Phys. Rev. B 72, 235415 (2005); ibid Phys. Rev. B 73, 169901 (2006) |