Non-contact Friction and Relaxational Dynamics of Surface Defects

摘要

Motion of cantilever near sample surfaces exhibits additional friction evenbefore two bodies come into mechanical contact. Called non-contact friction(NCF), this friction is of great practical importance to the ultrasensitiveforce detection measurements. Observed large NCF of a micron-scale cantileverfound anomalously large damping that exceeds theoretical predictions by 8-11orders of magnitude. This finding points to contribution beyond fluctuatingelectromagnetic fields within van der Waals approach. Recent experimentsreported by Saitoh et al. (Phys. Rev. Lett. 105, 236103 (2010)) also foundnontrivial distance dependence of NCF. Motivated by these observations, wepropose a mechanism based on the coupling of cantilever to the relaxationdynamics of surface defects. We assume that the surface defects couple to thecantilever tip via spin-spin coupling and their spin relaxation dynamics givesrise to the backaction terms and modifies both the friction coefficient and thespring constant. We explain the magnitude, as well as the distance dependenceof the friction due to these backaction terms. Reasonable agreement is foundwith the experiments.