Woodstock West: Celebrating the New MgB2 Superconductors
Physicists pack Seattle Convention Center for marathon session on new superconductors
The excitement was palpable in the Grand Ballroom at the Westin Hotel in Seattle on Monday night, March 12, as physicists, attending the APS March Meeting from around the world, gathered for a mammoth technical session discussing the discovery and subsequent experimental results on the newly discovered superconducting compound magnesium diboride (MgB2), first discovered less than two months ago in a laboratory in Japan. Speakers flew in from Japan, Korea, Switzerland, Italy, Britain, China, France, the Netherlands and Germany, in addition to numerous speakers from the US. A total of 79 ultra-short (2-minute) papers were presented, with the session running past one the following morning. It was quickly dubbed "Woodstock West," in memory of the so-called "Woodstock of Physics" at the 1987 APS March Meeting where the discovery of high-temperature superconductivity was first announced.
Like many historical breakthroughs in science, the compound's discovery was partially serendipitous (although this was not the view of the discoverers-see Members in the Media). Jun Akimitsu's research group at Aoyama-Gakuin University in Tokyo were attempting to make a chemical analogue of CaB6, a semiconducting material that becomes ferromagnetic, like iron, when doped with a small amount of electrons. They tried to replace calcium with magnesium, which is directly above it in the periodic table. One of their starting materials was MgB2, a common compound known since 1953, which had been overlooked by physicists for decades in the search for new superconductors. "It's just that nobody bothered to cool it down and measure its superconducting properties," says David Cardwell of Cambridge University.
It was while routinely measuring the properties of MgB2 before using it as a dopant in high temperature superconductors that Akimitsu's group made the startling discovery that the compound had a transition temperature of about 39K. The previous highest transition temperature for a metallic superconductor - niobium tin - was 20K. Akimitsu's group and several others have already begun to explore whether it may be possible to raise the superconducting transition temperature of MgB2 further by lacing the compound with other elements.
"Discovery of superconductivity at 39K in the simple hexagonal diboride compound MgB2 proves that there are still remarkable scientific surprises," says J.D. Jorgensen of Argonne National Laboratory. From a physics standpoint, the chief interest in the compound is the possibility that the old BCS theory, which has proven useful for low temperature metallic materials but not for the higher temperature ceramic materials, might still be relevant at 40K, where the MgB2 materials become superconducting. "How much this discovery changes the path of materials physics depends on whether MgB2 is a solitary example of a new way of making high-temperature superconductors, or whether it represents only the tip of the iceberg," says Princeton University's Robert Cava. "For the high-Tc copper oxides, we haven't found the bottom of the iceberg yet, even after 15 years of looking. I, for one, hope this iceberg is just as deep."
Of course, the other key question is whether the compound might be amenable to technological applications. Although most scientists agree that it is too early to speculate about how the material might perform in devices, there are promising signs. Both magnesium and boron are common materials, inexpensive and easy to work with - in fact, MgB2 is a commodity item that can be bought off the shelf from chemical companies. Its transition temperature greatly exceeds those of the conventional metallic superconductors, and studies on polycrystalline materials indicate that naturally occurring grain boundaries do not significantly inhibit current flow, another significant advantage over the cuprate superconductors.
The most promising potential application is the commercial production of superconducting wires out of MgB2, which should be able to carry more current than the copper oxide superconductors, and could possibly be cooled by electric refrigerators rather than liquid helium because of the higher transition temperature. However, "A great deal of work remains to be done to develop wires of superconducting borides that are robust and cost-effective to manufacture," cautions Alex Malozemoff, chief technology officer of American Superconductor, which manufactures High-Temperature Superconducting (HTS) wires for electric power applications. "If these materials prove to be practical, it is likely that it will take five to ten years to get them out of the laboratory and into the marketplace." He points out that the HTS materials discovered in 1986 are just now in the early stages of commercialization.
Ultimately, though, the importance of the discovery and the special session is what they communicate to the general public about the noblest aspects of the scientific endeavor: the excitement of new discovery spurring a flurry of related research at laboratories around the world, culminating in a collegial gathering to interact and share results for the greater good. Most of those at the "Woodstock West" session echoed the sentiments of one observer in attendance, who enthused, "This is what physics is all about."
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