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 Γ is reduced by a factor of three when the sample enters the superconducting state. The temperature decay of Γ 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 probe-sample distance d and on the bias voltage V. Γ is found to be proportional to d-1 and V2 in the metallic state, whereas Γ∼d-4 and Γ∼V4 in the superconducting state. Therefore, phononic friction becomes the main dissipation channel below the critical temperature.

Velocity dependence of kinetic friction in the Prandtl-Tomlinson model

“…This ad hoc damping, which can be motivated and derived from quite rigorous linear response treatments, reflects the direct coupling of the point particle to phonons and potentially electronic degrees of freedom of the substrate. 16,20,21 Most theories focus on the overdamped limit (e.g., by choosing the unit of time to be 1/γ and then sending m to 0), partly because this facilitates the solution of analytical theories. Conversely, AFM tips do not seem to be overdamped.…”

Current trends in the physics of nanoscale friction

“…More polarizable adsorbates like nitrogen, krypton and xenon are instead found to be completely pinned to lead below 10 K [129]. A convincing confirmation of electronic friction and of its suppression in the superconducting state is provided by non-contact friction measurements on niobium films carried out across the critical temperature using a highly sensitive cantilever oscillating in the pendulum noncontact geometry in ultrahigh vacuum [130]. The friction coefficient drops by a factor between 2 and 3 when the sample enters the superconducting state (see Fig.…”

Friction Modification by Shifting of Phonon Energy Dissipation in Solid Atoms

“…Nevertheless, the details of the nonequilibrium atomic process are still vague because the friction interface is difficult to access experimentally. Recent sophisticated nanoscale experiments by atomic force microscopy (AFM) have shed light on the energy dissipation process [9][10][11][12][13][14][15][16]. For example, Saitoh et al measured the noncontact friction between solids and an AFM probe under high vacuum and observed a significant maximum value at a distance of a few nanometers between them [10].…”

“…Cannara et al performed a unique AFM experiment to compare the frictions between silicon surfaces with hydrogen and deuterium absorbates and suggested that the friction exhibits an isotopic effect due to the modification of phonon energy dissipation; however, a molecular dynamics (MD) simulation did not support their suggested mechanism [11,12]. Some researchers have studied the superconducting transition of a metal sample to distinguish the electronic contribution to the kinetic friction [10,[13][14][15]. Such a modern view that comprehensively addresses the friction as a nonequilibrium atomic phenomenon has identified remarkable features of the friction, but these investigations are still few in number.…”