Reliving the Good Old Days
This year also marks the 50th anniversary of the BCS theory of superconductivity. Speakers at a special evening session at the March Meeting spoke about the history and impact of that theory.
The events leading up to the Woodstock of Physics conference began in 1986 when Georg Bednorz and Alex Muller, at IBM Zurich, made their discovery of a lanthanum-based cuprate perovskite that superconducts at 35K.
At the 2007 March Meeting, Bednorz recounted how he and Muller had worked for months on the project before making the discovery. They were working with copper oxides, rather than conventional metallic alloys, and had tried material after material with no success. It was frustrating at times, but they kept going, Bednorz said during the 2007 press conference. “We didn’t know whether it would be successful.” So he and Muller kept the work low key, working after hours, using colleagues’ equipment.
Finally, they hit upon a La-Ba-Cu-O compound. Initially they had seen only hints of superconductivity, and colleagues were skeptical that this unlikely ceramic compound would superconduct. By October 1986, however, they had found the optimum composition and had observed that the material exhibited the Meissner effect, considered definitive proof that the compound was superconducting, and they sent their paper off for publication. They won the Nobel Prize in 1987.
What made the discovery so exciting, said Bednorz, was that superconductivity had been known about and studied for decades, since Kamerlingh Onnes first discovered the phenomenon in 1911. “The excitement is that even in an established field, revolution is still possible,” he said.
The initial high Tc discovery was confirmed by a Japanese group and then by Paul Chu of the University of Houston. Chu discovered Y-Ba-Cu-O, the first compound to superconduct above liquid nitrogen temperature. While working towards the discovery, he went many nights with only three hours of sleep, and only saw his family at Christmas, Chu recalled at this year’s meeting.
By March 1987, dozens of research groups were working on similar high Tc compounds. About 50 speakers spoke at the marathon “Woodstock of physics” session held at the APS meeting in New York that year. Two thousand physicists packed the room and overflowed into the hallway until after 3 am, when the session finally ended.
The public was excited too, expecting this development would lead to amazing applications such as extremely efficient power transmission and superfast levitating trains. Paul Grant, who was at IBM Almaden in 1987, said in the 2007 press conference that even high school students got in on the high Tc excitement by producing one of these relatively simple compounds in their own school lab. Grant also remembered how physicists were treated like rock stars for that short period of time. For instance, bouncers at trendy New York nightclubs brought physicists to the front of the line. All they had to do was show an APS meeting badge.
Speakers at the 2007 March meeting also discussed progress since 1987. Since then, more than 100 high Tc materials have been discovered. Y-Ba-Cu-O is still best for many applications, Chu said. Under pressure, Y-Ba-Cu-O still holds the record for highest Tc, at 164K.
Many questions remain about high temperature superconductivity, and many of the expected applications have not appeared, speakers pointed out. At the time nothing seemed impossible; more great developments were expected to be just around the corner. But while engineers have made a number of minor improvements in high Tc materials, there have been no major breakthrough in the past 20 years. No one has made a room temperature superconductor, and it is not known whether such a material is possible.
There are some applications for the high Tc materials, but no company is making a profit on high temperature superconductivity, said Grant. Chu predicted that high Tc wires might actually first be used by developing countries that don’t already have a power infrastructure.
Moreover, there is still no accepted theory that explains high temperature superconductivity, making it an important unsolved problem, speakers said. “It rivals the unification of the forces,” Grant said.
Although there is no explanation for high temperature superconductivity, conventional low temperature superconductors are explained by the BCS theory, which celebrates its 50th anniversary this year. A special evening session at the March Meeting commemorated that anniversary.
In 1957, John Bardeen, Leon Cooper, and Robert Schrieffer, developed the first complete microscopic theory of superconductivity. In the theory, electrons form “cooper pairs” that move in a coordinated manner. At the March meeting, Doug Scalapino of the University of California, Santa Barbara, said the BCS theory was a major milestone. “It was a remarkably complete description.” He said. “I think they changed the way we think about condensed matter physics.”
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