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

Understanding nanoscale friction and dissipation is central to nanotechnology(1-4). The recent detection of the electronicfriction drop caused by the onset of superconductivity in Nb (ref. 5) 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 chargedensity 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 2 pi 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.