Suppression of electronic friction on Nb films in the superconducting state

Abstract

Investigations on the origins of friction are still scarce and controversial. In particular, the contributions of electronic and phononic excitations are poorly known. A direct way to distinguish between them is to work across the superconducting phase transition. Here, non-contact friction on a Nb film is studied across the critical temperature TC using a highly sensitive cantilever oscillating in the pendulum geometry in ultrahigh vacuum. The friction coefficient is reduced by a factor of three when the sample enters the superconducting state. The temperature decay of is found to be in good agreement with the Bardeen Cooper Schrieffer theory, 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 probesample distance d and on the bias voltage V. is found to be proportional to d'1 and V2 in the metallic state, whereas '4 and V4 in the superconducting state. Therefore, phononic friction becomes the main dissipation channel below the critical temperature. © 2011 Macmillan Publishers Limited. All rights reserved.