Modulation of contact resonance frequency accompanying atomic-scale stick-slip in friction force microscopy

Novel phenomena accompanying atomic-scale friction are studied on NaCl(001) by the combination of quasistatic lateral force measurements with dynamic measurements of contact resonance frequencies. For loads up to a few nN the flexural resonance is tracked by a phase-locked-loop by the use of small oscillation amplitude (50 pm). The contact resonance varies during the stick stages, which demonstrates that the dynamic measurement provides additional information about small changes of the stressed contact. Improved sensitivity is also observed across atomic-scale defects which are clearly observed in the contact frequency channel. The low lateral contact stiffness inferred from the observed torsional resonance agrees well with that deduced from the quasistatic measurements and strongly suggests that the contact is atomic-sized.