s a scientific problem, magnesium diboride turned out to be a pretty easy nut to crack.

In January 2001, Dr. Jun Akimitsu of Aoyama Gakuin University in Tokyo set off a flurry of research with the announcement that the substance is a superconductor, able to convey electricity with virtually no resistance. That was unexpected because magnesium diboride is a simple, readily available metallic compound. (No one had thought to check it for superconductivity before.) And it remains a superconductor at temperatures about 29 degrees warmer than any other metal.

Advertisement

"We're red-faced we didn't predict magnesium diboride," said Dr. Marvin L. Cohen, a professor of physics at the University of California at Berkeley.

Now, just a year and a half later, scientists have an almost complete understanding of the compound's unusual superconducting abilities. By contrast, physicists took more than 40 years to come up a theory to explain the first superconductors discovered in 1911, and the so-called high-temperature superconductors, discovered in 1986, continue to befuddle theorists 16 years later.

Magnesium diboride proved easier to solve because it acts like a traditional superconductor, with a few wrinkles.

Writing in the Aug. 15 issue of Nature, Dr. Cohen and other scientists at Berkeley reported that starting with the basic properties of the constituent atoms — magnesium and boron — they were able to calculate properties of magnesium diboride that matched what had been observed in experiments, including the maximum superconducting temperature — minus 389 degrees Fahrenheit. "Almost right on," said Dr. Steven G. Louie, a professor of physics at Berkeley and another of the researchers.

While magnesium diboride cannot superconduct unless it is much colder than the high-temperature superconductors, it is much cheaper and easier to work with. Some expect the material to find use in magnets for magnetic resonance imaging machines and certain electronic devices that now use low-temperature superconductors.

The compound's relatively high superconducting temperature comes from its unique electronic configuration. The boron atoms line up in honeycomb-shaped layers that resemble chicken wire — the same structure as graphite — but some of the bonds between the boron atoms are missing electrons.

In superconductivity, electrons collect into pairs at low enough temperatures, enabling them to travel without bouncing off atoms, eliminating electrical resistance. In magnesium diboride, in addition to the usual current-carrying electrons, the vacancies in the boron bonds also pair up. The bonds between the vacancies are especially strong and not as easily shaken apart by thermal vibrations, allowing the material to continue superconducting at warmer temperatures.

"I think we understand it quite well," Dr. Louie said. "Most of the issues have been resolved."

Other scientists had worked out the same basic ideas, but the Berkeley calculations are the most accurate.

Scientists have also made progress in preparing magnesium diboride for practical use. Two research groups this week publish papers on how to fabricate high-quality thin films — an essential step on the road to constructing electronic devices. Earlier techniques produced films with rough surfaces or could not be easily incorporated into processes for electronic devices where several layers of different materials need to be built upon each other.

The difficulty is that magnesium does not readily stick to a surface, preferring to float as vapor.

"If it doesn't stick, there isn't any magnesium," said Dr. Xiaoxing Xi, a professor of materials science at Pennsylvania State University. Without magnesium, there is no magnesium diboride.

Dr. Xi and other scientists at Penn State and the University of Michigan solved the sticking problem by putting high-pressure magnesium vapor in the chamber as they make the films on top of flat sapphire crystals that act as a template. The vapor keeps the magnesium in the film intact.

The films, less than one hundred-thousandth of an inch thick, are like almost perfect slices of single crystals. The results appear in the current issue of the journal Nature Materials.

"These are remarkably good films made by an extremely simple, cheap method," said Dr. John M. Rowell, a professor at Northwestern University, who wrote an accompanying commentary. "It's probably the easiest way to deposit relatively thick films."

Researchers at the University of Wisconsin led by Dr. Chang-Beom Eom, a professor of materials science and engineering, report similar films made with a different technique in the current issue of Applied Physics Letters. They lay down boron atoms first, then heat them in magnesium vapor at 1,500 degrees. "That reacts, and it forms a crystalline magnesium diboride," Dr. Eom said.

The hope is that the thin films can be turned into devices like Josephson junctions, in which a superconducting current tunnels across a gap for use for ultrafast switches or sensitive detectors of magnetic fields. Josephson junctions cannot be consistently and reliably made from high-temperature superconductors.

Over the past year, scientists have found ways to increase the amount of current carried by magnesium diboride by about a factor of 10 and have improved its performance in high magnetic fields, necessary for applications like magnetic resonance imaging machines.

A startup company, Hyper Tech Research Inc. in Troy, Ohio, is experimenting with techniques to make magnesium diboride wires up to 100 yards in length, and it hopes to extend that to 1,000 yards sometime next year. Michael Tomsic, president of Hyper Tech, said the company aimed to start selling wires in a couple of years.

While researchers have improved magnesium diboride's current-carrying capacity and resilience to magnetic fields, the maximum superconducting temperature has not budged from what Dr. Akimitsu originally reported: minus 389 degrees.

All of the usual tricks — substituting different atoms, putting it under pressure — have all lowered the temperature, not raised it. The Berkeley calculations may offer new avenues for experimentation.

"What we need to do now," said Dr. Louie of Berkeley, "is be more clever."