All posts by modphysfrict

Charge-density-wave surface phase slips and noncontact nanofriction

F. Pellegrini, G.E. Santoro, and E. Tosatti, Phys. Rev. B 89, 245416 (2014).

Abstract:

Bulk electrical dissipation caused by charge-density-wave (CDW) depinning and sliding is a classic subject. We present a local, nanoscale mechanism describing the occurrence of mechanical dissipation peaks in the dynamics of an atomic force microscope tip oscillating above the surface of a CDW material. Local surface 2π slips of the CDW phase are predicted to take place, giving rise to mechanical hysteresis and large dissipation at discrete tip surface distances. The results of our static and dynamic numerical simulations are believed to be relevant to recent experiments on NbSe2; other candidate systems in which similar effects should be observable are also discussed.

 

Soliton dynamics in a solid lubricant during sliding friction

A. Vigentini, B. Van Hattem, E, Diato, P. Ponzellini, T. Meledina, A. Vanossi, G.E. Santoro, E. Tosatti, and N. Manini, Phys. Rev. B 89, 094301 , (2014).

Abstract:

Recent highly idealized model studies of lubricated nanofriction for two crystalline sliding surfaces with an interposed thin solid crystalline lubricant layer showed that the overall relative velocity of the lubricant vlub/vslider depends only on the ratio of the lattice spacings, and retains a strictly constant value even when system parameters are varied within a wide range. This peculiar “quantized” dynamical locking was understood as due to the sliding-induced motion of misfit dislocations, or soliton structures. So far the practical relevance of this concept to realistic sliding three-dimensional crystals has not been demonstrated. In this work, by means of classical molecular dynamics simulations and theoretical considerations, we realize a realistic three-dimensional crystal-lubricant-crystal geometry. Results show that the flux of lubricant particles associated with the advancing soliton lines gives rise here too to a quantized-velocity ratio. Moreover, depending on the interface lattice spacing mismatch, both forward and backward quantized motion of the lubricant is predicted. The persistence under realistic conditions of the dynamically pinned state and quantized sliding is further investigated by varying sliding speed, temperature, load, and lubricant film thickness. The possibilities of experimental observation of quantized sliding are also discussed.

Giant frictional dissipation peaks and charge-density-wave slips at the NbSe2 surface

M. Langer, M. Kisiel, R. Pawlak, F. Pellegrini, G.E. Santoro, R. Buzio, A. Gerbi, G. Balakrishnan, A. Baratoff, E. Tosatti, and E. Meyer, Nature Materials 13, 173 (2014).

Abstract:

Understanding nanoscale friction and dissipation is central to nanotechnology. The recent detection of the electronic-friction drop caused by the onset of superconductivity in Nb by means of an ultrasensitive non-contact pendulum atomic force microscope (AFM) raised hopes that a wider variety of mechanical-dissipation mechanisms become accessible. Here, we report a multiplet of AFM dissipation peaks arising a few nanometres above the surface of NbSe2—a layered compound exhibiting an incommensurate charge-density wave (CDW). Each peak appears at a well-defined tip–surface interaction force of the order of a nanonewton, and persists up to 70 K, where the short-range order of CDWs is known to disappear. Comparison of the measurements with a theoretical model suggests that the peaks are associated with local, tip-induced 2π phase slips of the CDW, and that dissipation maxima arise from hysteretic behaviour of the CDW phase as the tip oscillates at specific distances where sharp local slips occur.