Suppression of electronic friction on Nb films in the superconducting state

Investigations on the origins of friction are still scarce and controversial(1-4). In particular, the contributions of electronic and phononic excitations are poorly known(5-11). A direct way to distinguish between them is to work across the superconducting phase transition(7-12). Here, non-contact friction(13-16) on a Nb film is studied across the critical temperature T-C using a highly sensitive cantilever oscillating in the pendulum geometry in ultrahigh vacuum. The friction coefficient Gamma is reduced by a factor of three when the sample enters the superconducting state. The temperature decay of Gamma is found to be in good agreement with the Bardeen-Cooper-Schrieffer theory(12,17-19), meaning that friction has an electronic nature in the metallic state, whereas phononic friction dominates in the superconducting state. This is supported by the dependence of friction on the probe-sample distance d and on the bias voltage V. Gamma is found to be proportional to d(-1) and V-2 in the metallic state, whereas Gamma similar to d(-4) and Gamma similar to V-4 in the superconducting state. Therefore, phononic friction becomes the main dissipation channel below the critical temperature(16).