Frigid running.

From the days of generating a blaze by rubbing wood on wood to the modern-day tribulations of lubricating computer disk drives rotating at tremendous rates, friction has been both a blessing and a curse. The basic rule of friction--that it depends on the strength of the force pressing two surfaces together but not the contact area between them--was discovered centuries ago. Only recently have researchers begun to understand the origin of friction on an atomic scale (SN: 4/15/95, p. 239).

Now, Jacqueline Krim and her colleagues at Northeastern University in Boston have measured the friction that arises when a solid nitrogen slab only a few molecules thick slides across a lead surface at temperatures above and below the transition temperature at which lead becomes a superconductor. They observed that friction drops abruptly when lead enters its superconducting state at temperatures below 7.2 kelvins.

Reported in the Feb. 23 Physical Review Letters, the results provide the first direct experimental evidence that interactions between electrons may contribute to friction.

Theorists had previously suggested that a sort of electronic friction could arise between the free electrons of a metal and those associated with atoms of another material. When a metal turns into a superconductor, free electrons in the superconductor begin to pair up and travel through the material without resistance, or loss of energy. That pairing process--which makes the electrons unavailable for producing friction--generally occurs gradually. Hence, Krim and her colleagues expected to see a modest decline in friction. Instead, they observed a sudden, large decrease.

"The number of superconducting electrons does not increase abruptly at the transition, so the new phenomenon we are observing may involve other effects," the researchers note. "It remains open for theoretical interpretation."