  In a recent paper [Phys. Rev. Lett. 80 (1998) 1690], Krim and coworkers have
observed that the friction force, acting on a thin physisorbed layer of N_2
sliding on a lead film, abruptly decreases by a factor of ~2 when the lead film
is cooled below its superconductivity transition temperature. We discuss the
possible mechanisms for the abruptness of the sliding friction drop, and also
discuss the relevance of these results to the problem of electronic friction.
