CDW slips and giant frictional dissipation peaks at the NbSe$_2$ surface

TL;DR

This study reveals multiple dissipation peaks near NbSe$_2$ surface linked to tip-induced phase slips in charge-density waves, advancing understanding of nanoscale friction and electronic dissipation mechanisms.

Contribution

It demonstrates that AFM can detect and analyze local 2π phase slips in charge-density waves, providing new insights into nanoscale dissipation phenomena.

Findings

Multiple dissipation peaks observed at specific tip-surface forces

Dissipation peaks persist up to 70K, correlating with CDW short-range order

Theoretical model links peaks to tip-induced local 2π CDW phase slips

Abstract

Accessing, controlling and understanding nanoscale friction and dissipation is a crucial issue in nanotechnology, where moving elements are central. Recently, ultra-sensitive noncontact pendulum Atomic Force Microscope (AFM) succeeded in detecting the electronic friction drop caused by the onset of superconductivity in Nb, raising hopes that a wider variety of mechanisms of mechanical dissipation arising from electron organization into different collective phenomena will become accessible through this unconventional surface probe. Among them, the driven phase dynamics of charge-density-waves (CDWs) represents an outstanding challenge as a source of dissipation. Here we report a striking multiplet of AFM dissipation peaks arising at nanometer distances above the surface of NbSe$_2$ - a layered compound exhibiting an incommensurate CDW. Each peak appears at a well defined tip-surface interaction force of the order of a nN, and persists until T=70K where CDW short-range order is known to disappear. A theoretical model is presented showing that the peaks are connected to tip-induced local 2$\pi$ CDW phase slips. Under the attractive potential of the approaching tip, the local CDW surface phase landscape deforms continuously until a series of 2$\pi$ jumps occur between different values of the local phase. As the tip oscillates to and fro, each slip gives rise to a hysteresis cycle, appearing at a selected distance, the dissipation corresponding to "pumping" in and out a local slip in the surface CDW phase of NbSe$_2$.