Revisiting intermetallic superconductors.

Since 1986, copper oxide compounds have dominated the superconductivity fast-track, with scientists making headlines last year by reporting transition temperatures near 250 kelvins -- about [23 degrees C] (SN: 12/18 & 25/93, p.405).

But copper oxides are not the only superconductors worthy of study.. In 1953, researchers first saw the zero-resistance phenomenon in a multimetal compound based on niobium, designated A15. In 1974, a group of chemists achieved superconductivity at a temperature of 23.2 kelvins in a niobium-based thin film a breakthrough that has gone unchallenged for nearly 20 years.

Now, superconductivity researchers are refocusing attention on such "intermetallic" compounds. In the Jan. 13 and Jan. 20 NATURE, researchers at AT&T Bell laboratories in Murray Hill, N.J., led by physicist Robert J. Cava, describe several new families of intermetallic compounds, made from combinations of ordinary and rare earth metals, that achieve superconductivity at elevated temperatures. In the Jan. 10 PHYSICAL REVIEW LETTERS, R. Nagarajan, a physicist at the Tata Institute of Fundamental Research in Bombay, India, and his colleagues describe superconductivity in a nickel alloy.

Wondering why other such superconductors hadn't turned up before, Cava realized "people had just stopped looking."

These superconductors are distinguished by their use of three or four metals in combination. Cava's team describes one quaternary intermetallic compound - based on the elements yttrium, palladium, boron, and carbon - in which they achieved superconductivity at 23 kelvins. In another family of compounds, based on nickel, boron, carbon, and a fourth element from a group of rare earth metals, they saw no electrical resistance at 16.6 kelvins. Indeed, Nagarajan and his colleagues also confirmed superconducting transition temperatures at 12.5 and 13.5 kelvins in a related family of nickel-based alloys.

"We believe that the yttrium-palladium boride carbide superconductor will prove to be the first of a new family of high-temperature [superconducting] intermetallic compounds, and we suggest that boride carbides (and borides) represent one road to high-temperature [superconductors] that is worthy of exploration," the Cava group writes.

While recognizing that nickel raises the temperature at which superconductivity occurs, Cava and his co-workers aren| certain why. They note that the layeredcrystal structure of the intermetallic compounds "is reminiscent of those of the copper oxide superconductors" Yet whether they operate by the same electrical mechanism, the physicists say, "remains to be seen."

Of interest, too, notes Nagarajan's group, is carbon's role in facilitating superconductivity. In one particular compound - containing yttrium, niobium, and boron - the researchers note that adding carbon "leads to a dramatic enhancement of superconducting properties," seemingly by modifying its "superstructure." But that mechanism, too, defies a complete explanation.

"The idea of putting four metal elements together to make a superconductor is new," says Cava. "People have tried two and three, but not four. Sometimes to improve results, you have to let nature stabilize competing factors, and the best way to do that is to put more elements into the pot and see what nature cooks up."

"The other obstacle to overcome is the prevailing idea that nature always wants to keep things simple. That's not always so. If anything, what copper oxide shows us is that the simplest compound is not necessarily the best."