When J. Georg Bednorz and Karl Alexander Müller discovered superconductivity in a new class of compounds at a significantly higher temperature, their findings set off a flurry of further research.
Superconductivity, the disappearance of all resistance to the flow of electricity in a material, has challenged experimentalists and theoreticians alike since its discovery in 1911. In that year, Heike Kamerlingh Onnes noted that mercury loses its resistance to the conduction of electricity completely when it is cooled to 4 Kelvins. Despite widespread attempts in the years after 1911, scientists found no substances that would superconduct at temperatures more than a few tenths of a degree above absolute zero. By the early 1970’s, the highest recorded temperature at which any material was found to superconduct was 23.3 Kelvins. Physicists did not come up with a plausible theory for superconductivity until the late 1950’s. That theory, developed by John Bardeen,
The reason for the search was simple: The development of superconducting devices held great promise of technological advancement and economic and social benefits. Superconducting compounds appeared so promising because they carried electrical currents indefinitely, whereas normal conductors of electricity, such as copper wire, offered some resistance to the flow of electricity and dissipated much of the power they carried in the form of heat. The promise of cheap, efficient, resistanceless power transmission was only one of the reasons physicists were so keen to find superconductors that operated at higher temperatures than the frigid regions of the bottom end of the temperature scale.
One of the more important reasons superconductors had not lived up to their potential was that large, very expensive liquid helium cooling systems were required to keep the superconducting material operating. When the superconductor warmed up to a point above a “transition temperature,” the material would revert to its normal state of conductivity. Another reason was that many superconductors could not remain superconducting and still carry the large electrical currents or withstand the high magnetic fields required for many applications. Raising the temperature at which materials would superconduct was the major focus of research for decades.
Over the years, physicists tried intermetallic compounds (combinations of several metals), thin films, and organic compounds in their search for superconductors that would operate at temperatures higher than 23.3 Kelvins. In the 1960’s, an oxide (a compound containing oxygen and other chemical elements) was shown to superconduct, but its transition temperature was so low (less than 1 Kelvin) that it was of no practical use. The discovery was important, however, in that a new class of compounds—the oxides—was shown to have the potential of superconductivity. The early 1970’s saw oxide superconductors achieve the respectable transition temperature of 14 Kelvins—still below the record high transition temperature but high enough to dispel the notion that high-temperature oxide superconductors could not exist.
Karl Alexander Müller (left) and J. Georg Bednorz.
Karl Alexander Müller, a fellow at IBM’s Zurich Research Laboratory, was convinced that metallic oxides were promising materials to test for high-temperature superconductivity, in part because the IBM laboratory had a long tradition of research in these oxides. In this context, “high temperature” was taken to be any temperature above 23.3 Kelvins. In early 1983, Müller enlisted J. Georg Bednorz to assist in the search. Bednorz fabricated a variety of samples of a lanthanum, nickel, and oxygen compound, altering the relative amounts of the constituent elements in the hope of creating a compound that would superconduct. Carefully measured amounts of powdery oxides containing the necessary chemical elements were combined with citric acid and ethylene glycol (automobile antifreeze) and then heated to 8,316 degrees Celsius. The black powdery residue was then compressed under high pressure into a pellet and allowed to cool slowly. Neither the chemicals nor the equipment needed to fabricate the samples was particularly costly or difficult to obtain.
Measurement after measurement on sample after sample demonstrated that Bednorz and Müller were not making any real progress toward their goal of achieving superconductivity at temperatures above 23.3 Kelvins. They could make measurements only in the evening because they had to share equipment with another group at the laboratory. Work on their project almost came to an end, but in 1985 they gained sole use of automatic measuring equipment. At the end of 1985, Bednorz read an article by three French physicists concerning the interesting properties of a lanthanum-barium-copper-oxide compound. The French physicists were not looking for superconductivity; they were concerned with other properties of the compound. Bednorz realized that including copper in the compound he and Müller were using theoretically should increase the chances that the compound would superconduct.
Bednorz quickly fabricated a sample of the material. Measurements made on the mixtures of the lanthanum-barium-copper-oxide produced in late January of 1986 showed the onset of superconductivity at around 10 Kelvins. Slight alterations in the relative amounts of the constituents of the compound yielded transition temperatures of 35 Kelvins. Bednorz and Müller repeated the measurements and confirmed that their superconductor had surpassed the twelve-year-old record by 12 Kelvins. Bednorz and Müller were excited because they had raised significantly the highest temperature at which superconductivity occurred and demonstrated that a new class of compounds could superconduct.
Knowing that any announcement of a new record for the transition temperature of a superconducting metallic oxide would be greeted with skepticism, Bednorz and Müller wanted to be absolutely sure of their results. There could be no doubt that their compound was superconducting if they could demonstrate the Meissner-Ochsenfeld effect
Bednorz and Müller were met initially with skepticism as they gave presentations at scientific meetings, despite the compelling evidence that their compound showed the Meissner-Ochsenfeld effect. This period of skepticism was very short-lived, however. In late November, 1986, a group of scientists in Japan reported that they were able to reproduce Bednorz and Müller’s results. Soon after, Paul C. W. Chu
One measure of the excitement that this discovery engendered in the scientific community was the large number of scientific papers that flooded scientific journals. In 1987, the year that Bednorz and Müller’s results became widely known in the scientific community, more than three thousand papers appeared in the scientific literature concerning superconductivity from laboratories all over the world.
The race was on to identify the superconductor’s structure so that even higher temperatures could be achieved through further altering of the compound. Chu raised the transition temperature to more than 50 Kelvins in December, 1986, by applying intense pressure to the compound. He also substituted the rare earth element yttrium for the lanthanum in the original compound. The transition temperature reached an amazing 93 Kelvins. Aside from the great jump in the record temperature, Chu’s discovery was significant in showing that liquid nitrogen, which was cheap and easy to use, could be used as a coolant. The dream of room-temperature superconductors now seemed much closer.
The high point in the short-term impact of Bednorz and Müller’s work came in March, 1987. Scientists were coming up with so many results so fast that scientific journals could not keep up. A special session of the American Physical Society’s annual solid-state physics meeting was arranged so that the scientists could share their results. An hour before the session was scheduled to begin, more than three thousand scientists were waiting to enter the hall, and television monitors were set up outside to broadcast the session to the overflow crowd. Dozens of scientists presented their results, and the presentations continued until after 3:00
With their discovery, Bednorz and Müller invigorated the scientific community’s search for compounds that superconduct at higher temperatures: They demonstrated that superconductivity could occur in oxides (which are usually electrical insulators at room temperature) at record high temperatures. Bednorz and Müller’s work also refuted the growing belief that perhaps superconductivity could not occur above 23 Kelvins. Chu substituted one chemical element in Bednorz and Müller’s compound for another and raised the transition temperature to the point where inexpensive and easy-to-use liquid nitrogen could be used for cooling. For their contributions to the field of superconductivity, Bednorz and Müller were awarded the 1987 Nobel Prize in Physics.
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Hazen, Robert. The Breakthrough: The Race for the Superconductor. New York: Summit Books, 1988. Provides an “insider’s view” of the scientific process. Recounts the discovery of the 93-Kelvin superconductor and then presents a firsthand, day-by-day account of the attempts by Hazen and his colleagues to identify the superconducting compound. Vividly portrays the sense of excitement in the scientific community after the discovery of high-temperature superconductors. -
Langone, John. Superconductivity: The New Alchemy. Chicago: Contemporary Books, 1989. Readable and nontechnical account of superconductivity’s history, applications, and future by a longtime science writer. Focuses on the technical innovations expected from high-temperature superconductivity. Includes glossary and index. -
Mayo, Jonathan L. Superconductivity: The Threshold of a New Technology. Blue Ridge Summit, Pa.: TAB Books, 1988. Uses straightforward, nontechnical language to explain how superconductivity works and how superconductors are constructed. Describes the applications of superconductor technology. Includes many illustrations and a good bibliography. -
Nowotny, Helga, and Ulrike Felt. After the Breakthrough: The Emergence of High-Temperature Superconductivity as a Research Field. New York: Cambridge University Press, 1997. Describes Bednorz and Müller’s work and examines the research that followed it, focusing on the factors that influenced the development of that research. Appendixes offer chronologies of events in the history of superconductivity. Includes bibliographic references and index. -
Simon, Randy, and Andrew Smith. Superconductors: Conquering Technology’s New Frontier. New York: Plenum Press, 1988. Intended for readers who have no prior background in physics, electronics, or other pertinent fields. Explores the nature of superconductivity, its history, and theoretical understanding of the phenomenon. Surveys a wide variety of practical uses for superconductivity and evaluates the impacts that breakthroughs in superconductivity may have on future technology. -
Tinkham, Michael. Introduction to Superconductivity. 2d ed. Mineola, N.Y.: Dover, 2004. Text designed for graduate students in physics and experimental physicists emphasizes physical arguments over theory. Begins with an informative historical overview.
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