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Suppression of electronic friction on Nb-films below the critical temperature
10 January 2011
Is friction dominated by electrons or by lattice vibrations? A nano-contact experiment shows that on a Nb surface friction drops by a factor of three when crossing the superconductivity transition, showing that it has essentially an electronic nature in the metallic state, whereas the phononic contribution dominates in the superconducting state.
Certainly there exist only few physical phenomena attracting people's attention more than friction. 500 years after the first studies of Leonardo da Vinci in the field of sliding friction, scientists still work actively on this topic, employing more sophisticated techniques nowadays capable to probe atomic scale phenomena. The insight into the atomic world gives the opportunity to study the microscopic origins of friction, which, even today, are poorly known. In particular, is the friction between bodies which are moving with respect to one another, caused by atomic or electronic excitations in the sample? The answer to this question was given by scientists from the group of Ernst Meyer working in the Physics Department of the University of Basel. The results of the experiment are published in the January 9th issue of Nature Materials (Marcin Kisiel, Enrico Gnecco, Laurent Marot, Urs Gysin, Simon Rast, Ernst Meyer, Nature Materialas, January 9th,DOI 10.1038/NMAT2936).
In the experiment, atomic force sensors were used, oscillating like a tiny pendulum across the niobium (Nb) sample with a few nanometers (1nm=10-9m) amplitude. The frictional forces were measured at different temperatures, across the superconducting transition of Nb. The relative electronic and phononic contributions to the friction could be distinguished, because electrons bound in Cooper pairs cannot contribute to friction. Temperature dependent friction measurements across the critical temperature of Nb films, reveal a reduction of dissipation in the superconductive state compared to the metal state by a factor of 3. Therefore, the electronically induced friction is the main energy dissipation channel above the transition temperature. This observation is also supported by the dependence of friction force on the probe-sample distance and on the probe-sample voltage.