Run-of-the-mill compound becomes superstar

Science News, March 3, 2001 by P. Weiss

Physicists in Japan have found superconductivity at a surprisingly high temperature in the simple, readily available metallic compound magnesium diboride.

The discovery that the material can carry electric current with no resistance, reported in the March 1 NATURE by Jun Akimitsu and his coworkers at Aoyama-Gakuin University in Tokyo, has startled scientists. Investigators worldwide are racing to measure the chemical's properties. One group has even formed the compound into tiny wires.

Scientists are also testing ways of boosting the compound's so-called critical temperature; below which superconductivity kicks in. The Japanese team measured it at 39 kelvins.

Researchers have long dreamed of superconductors that would operate at room temperature, or 300 K. Such substances would revolutionize industry and other sectors of society by slashing the energy required to run machines while bettering their performance.

Conventional superconductors, such as certain metal alloys, are used in commercial magnetic resonance imaging (MRI) machines and in research magnets. Decades of research have yielded no conventional superconductors with a critical temperature higher than 23 K.

Since the late 1980s, the discovery of so-called ceramic high-temperature superconductors has diverted attention from conventional compounds. Some of the ceramic materials proved to have critical temperatures higher than 160 K, fanning hopes that room-temperature superconductivity was within reach.

But it wasn't. What's more, the materials have turned out to be complex, difficult to work with, and expensive. Only now are ceramic superconductors showing up in equipment prototypes, such as power cables and motors (SN: 11/18/00, p. 330). Also, conventional theory can't explain their superconductivity.

Akimitsu first described his group's findings regarding magnesium diboride on Jan. 10 at a scientific meeting in Sendai, Japan. Initially, researchers wondered if the material might be displaying a new type of superconductivity. However, mounting evidence suggests that the compound--although breaking records--obeys conventional theory.

Evidence for this is described in the Feb. 26 PHYSICAL REVIEW LETTERS by Sergey L. Bud'ko and his colleagues at Iowa State University in Ames and the Department of Energy's Ames Laboratory there. The scientists, who formed magnesium diboride wires, observed the isotope effect typical of conventional superconductors. When they made magnesium diboride containing a light form, or isotope, of boron, its critical temperature rose 1 K.

The prospect that magnesium diboride may be the first of a line of inexpensive, easily processed superconductors with yet higher critical temperatures is fueling the excitement. Finding one such material that superconducts at 77 K, the temperature at which cooling with relatively cheap liquid nitrogen is possible, would be a huge advance, scientists say.

That mark is probably still out of reach, comments Paul M. Grant of the Electric Power Research Institute in Palo Alto, Calif. He notes that theorists more than 30 years ago predicted a 40 K maximum for conventional superconductors.

Nonetheless, new reports on magnesium diboride are pouring in to scientific journals and the on-line physics preprint server on the Internet (http://xxx.lanl.gov). In one preprint, Robert J. Cava and his colleagues at Princeton University describe a failed attempt to raise the compound's critical temperature by adding aluminum (http://xxx.lanl.gov/abs/cond-mat/0102262).

Prospects for finding superconducting cousins of magnesium diboride remain bright, researchers say. "Never in the history of superconductivity has there been a single, solitary example of a superconductor that works in a [unique] way," Cava claims. "There's always more than one."