PRL Top Ten: #2

NUMBER TWO
Superconductivity at 93K in a New Mixed-Phase Y-Ba-Cu-O Compound System

(M. K. Wu et al., Phys. Rev. Lett. 58 (1987), 908), 4171 Citations

This is the ninth in a series of articles by James Riordon. The first article appeared in the November 2002 issue.

The Chinese Lunar year of the Tiger was drawing to a close on January 27, 1987 as Maw-Kuen Wu and his graduate students, Jim Ashburn and T. J. Torng, toiled in their lab at the University of Alabama in Huntsville (UAH). They were testing the resistivity of samples from a batch of oxide compounds, hoping to find superconducting transitions as the materials were cooled in a liquid helium system. The researchers, along with Wu's colleague and mentor Ching-Wu "Paul" Chu of the University of Houston, had good reason to suspect that their samples might exhibit critical temperatures above the record 23.2 K superconductor discovered in 1973. But in the race to find high-T_C superconductors, the modest collaboration was at best a long shot in a field dominated by teams at IBM, Bell Labs, and Argonne National Laboratory, as well as groups in Russia, China, and Japan. In fact, the equipment that Wu, Torng, and Ashburn had at their disposal in the modest UAH lab was relatively ill-suited for measuring the 93 K critical temperature of the now famous blend of yttrium, barium, and copper oxide (YBCO) that they stumbled across on the eve of the year of the rabbit. "At the time," says Chu, "thermometers in low temperature labs worldwide were seldom calibrated to above 25 K."

In order to quickly confirm the startlingly high transition temperature of YBCO, Wu and his students replaced the liquid helium in their system with liquid nitrogen. At 77 K, the material was indeed superconducting. Just in case some fortuitous impurity had crept into the sample, they whipped up another sample of the YBCO compound. Once again, the sample resistivity fell to zero in the liquid nitrogen. "We were extremely excited right after we did our first measurement on the sample," recalls Wu, "When the result was confirmed after a second test, my colleagues at the department immediately shared our excitement and gave us all the support we needed. My department chairman immediately supported us with travel funds so that we could fly to Houston to carry out further measurements to confirm the results."

The editors of PRL pushed the paper through the review process at a breakneck pace, and it appeared in print barely a month later. The excitement soon spread to the physics community as a whole, and sparked a legendary fervor at the 1987 March APS meeting that led to a marathon high-T_C superconductor session. The New York Times dubbed the event "The Woodstock of Physics." The researchers were besieged by the press, public, and other scientists. One inquiry made a particular impression on Wu. "There was a very interesting call from a scientist in Russia," says Wu, "who proposed to use the high-T_C material to create a huge superconducting magnet energy storage device that crossed Alaska and Siberia so that we no longer needed to worry about the energy shortage and consequently, we could then have and enjoy world peace."

While it is startling that the modest Huntsville-Houston collaboration could break what the researchers called in their letter the "technological and psychological temperature barrier of 77 K" for high-T_C superconductivity, Chu explains that it was a logical progression from their previous work.

"The prevailing thinking in the field was that superconductivity above 30 K was impossible due to lattice instabilities," says Chu. "However, our [previous] high pressure studies demonstrated that instabilities in the material system known at the time did not have a large negative effect on the transition temperature. In other words, a T_C above 30 K must be possible. We decided that one of the best ways was to look at materials where new mechanisms might be in operation. The low electron density state compound of Ba-Pb-Bi-O which has a T_C of [about] 13 K, high at the time, offered the possibility. This was because it did not have any transition metal elements, which was considered necessary for high-T_C superconductors. We proposed that the high-T_C might be due to a new mechanism. In fact, in the last two conversations I had with Bernd Matthias, the pioneer in high-T_C field, right before his untimely death in 1980, we both agreed that the future of high-T_C would be in perovskite-like oxides. The positive pressure effect on these and related compounds led us to the YBCO."

Which is not to say that Chu was completely free of doubts. "In spite of all [our] precautionary steps, I still worried about the possibility of artifacts. I still remember [asking] my former students more than once before other labs reproduced the results? 'Can there be a phenomenon other than superconductivity that can give rise to the same effect? Please think and think hard'.  Deep in my heart, I was worrying that my superconductivity career would come to an abrupt end if it was an artifact."

Their discovery was, of course, soon verified by groups around the world. In the last sixteen years, compounds related to YBCO have been produced with ever increasing values of T_C, including a mixture of mercury, barium, and copper oxide that makes the transition to a superconductor at 134 K under atmospheric pressure, and 164 K under higher pressures.

Wu suspects, however, that substantial leaps in T_C will require some other, as yet unknown, class of materials. It is a sentiment that is apparently shared by many other physicists—the recent discovery of superconductivity in magnesium dibromide led to a packed session at the 2001 March APS meeting which has been called by some the "Woodstock West" of physics, in memory of the original Woodstock of Physics in 1987. But short of the discovery of room temperature superconductors, it is unlikely that any increase in T_C will generate the kind of excitement that followed the announcement of YCBO. "Back in early 80's," says Chu, "I was joking with Maw-Kuen that if one day we could find superconductor with T_C above 77 K, we should write a paper with only one sentence about the discovery." The published paper is a bit longer than that, but at two and a half pages, it's brief even for a PRL.

These days, Chu is continuing his work on basic and applied aspects of high-T_C superconductivity at the University of Houston, the Lawrence Berkeley Laboratory, and in Hong Kong, where he is president of the Hong Kong University of Science and Technology. Wu has also continued to study oxide superconductors, and is now director of the Institute of Physics at Taiwan's Academia Sinica.