Powdered platinum sheds all resistance.

At temperatures near absolute zero, most metals become superconductors--substances that offer no resistance to electric current. However, a handful of metals has refused to transform into superconductors, no matter how cold researchers make them. Ironically, these holdouts include elements that rank among the best conductors at ordinary temperatures, such as gold, silver, copper, and platinum.

Investigators in Germany now report finally observing superconductivity in platinum--when they weren't looking for it. Their unexpected success came about because they happened to be experimenting with platinum in powdered form. Their aim was to explore how platinum's magnetic properties might assist in the cooling of liquid helium.

"Beginning these magnetic measurements, we just found this superconductivity," says Thomas Herrmannsdorfer. He, Reinhard Konig, and Alexander Schindler, all of the University of Bayreuth, report their discovery in the May 31 PHYSICAL REVIEW LETTERS.

The finding "is important because it leads to further understanding of superconductivity in general and of the relationship between superconductivity and magnetism," comments M. Brian Maple of the University of California, San Diego.

In previous attempts to make platinum superconduct, researchers simply cooled the metal to the lowest possible temperatures. In 1997, Herrmannsdorfer and other colleagues failed to spur superconductivity even when they chilled solid pieces of platinum to approximately 1.5 microkelvins.

By contrast, cylindrical blocks of compressed powder began superconducting at roughly 1,000 times that temperature. Because the latter temperature is still close to absolute zero, platinum has little appeal as a commercial superconductor, the scientists say.

The Bayreuth team is now testing other powdered metals. Powdered silver cooled to 100 microkelvins has shown no switch to superconductivity. The group also plans to test platinum powder pressed into thin films and wires.

"We have ideas how this [transformation] can occur in powder while not in bulk platinum," Herrmannsdorfer says. Even the purest platinum contains a smattering of impurities such as iron, he notes. Scientists have long suspected that magnetic interactions among such atoms might prevent superconductivity. Pulverizing the metal may break up those interactions, Herrmannsdorfer speculates.

Conversion to a powder might also increase the number of vibration modes in the metal's crystal lattice, he says. In other materials, vibrations are known to facilitate superconductivity. Because the ratio of surface area to volume soars when materials are powdered, surface vibrations absent in the bulk metal become abundant in the powder, the Bayreuth researchers suggest.

Finally, the most tantalizing but least likely explanation for the superconductivity, according to Herrmannsdorfer, is that magnetically enhanced electron pairing is taking place (SN: 6/7/97, p. 351). If so, the new finding could shed light on a mechanism that researchers suspect occurs in cuprates, materials of enormous scientific and commercial interest that superconduct electricity at temperatures up to 133 kelvins (SN: 8/15/98, p. 111).

Although the evidence so far favors the less provocative reasons for superconductivity in platinum powder, "it doesn't rule out the possibility there is something more exotic happening," Maple says.