The vibrating tip of an atomic force microscope in the vicinity of a charge density wave (charge density wave, CDW) on the surface of a layered structure of niobium and selenium atoms (yellow and blue spheres). Near the top is on the surface of a single CDW phase slip process visible. Image: Univ. of BaselFriction is often seen as disadvantageous phenomenon, which leads to wear and cause energy losses. Conversely, however, may be disadvantageous too little friction, for example, while running on ice or when driving on wet road.

The understanding of frictional effects is therefore of great importance - also in nanotechnology, where friction is to be controlled in the nanometer range. A contribution such as friction works in microscopic dimensions, now makes a new study by researchers at the Univ. of Basel, the Univ. of Warwick, the CNR-SPIN Institute in Genoa and the International Centre for Theoretical Physics (ICTP) in Trieste.

For their experiment, the scientists left to the Basel experimental physicist Prof. Ernst Meyer nanometer-fine tip of an atomic force microscope on the surface of a layer structure of niobium and selenium atoms vibrate. They used this connection due to their unique electronic properties, in particular formed therein at extremely low temperatures so-called charge-density waves. The electrons are not evenly distributed as in a metal, but it forms areas where the electron density between high and low ranges.

Energy losses in the vicinity of charge density waves

in the vicinity of such charge density waves, the researchers registered very high energy losses between the surface and the tip of the atomic force microscope, even at relatively large distances of several atomic diameters. "The drop in energy was so strong, as if the tip suddenly caught in a viscous fluid," Meyer describes the effect of friction.

This energy loss could observe the researchers only at temperatures below 70 Kelvin (-203 C). Since charge density waves do not exist at higher temperatures, they interpreted this as evidence that frictional forces between measuring needle tip and charge density waves cause the energy losses.

The theoretical model shows that the high energy loss are caused by a series of local phase shifts in the charge density waves. This phenomenon newly discovered could have practical importance for nanotechnology, especially now that the friction effect can be modulated as a function of distance and tension.

Giant frictional dissipation peaks and charge-density-wave slips at the NbSe 2 surface

Source: Univ. of Basel