Superconductivity
During the early 1900s, the Dutch physicist, Heike Kamerlingh Onnes carried out a series of studies on low temperature phenomena. The most striking result he obtained was the liquefaction of helium gas in 1908, the last gas to be liquefied.
After accomplishing the liquefaction of helium, Kamerlingh Onnes turned his attention to a study of the behavior of materials at the near-absolute zero temperatures of liquid helium. Perhaps his most startling discovery was that of superconductivity, the tendency of a substance to lose all resistance to the flow of electrical current. Kamerlingh Onnes observed that the metals he studied each became superconducting at some characteristic temperature, which he referred to as the metal's transition temperature. He also noted that the imposition of an external magnetic field could eliminate superconductivity in a metal, even below its transition temperature.
The property of superconductivity has some obvious practical applications. In most electrical devices, a large fraction of the energy that flows into the device is wasted in overcoming resistance within the device. If the device could be made with superconducting materials, its efficiency could be greatly improved, often by fifty percent or more.
However, research on superconductivity made relatively little progress in the seven decades after Kamerlingh Onnes' discovery. The highest observed transition temperature increased only from about 4 K in mercury, as observed in 1911 by Kamerlingh Onnes, to 23.3 K in compounds and alloys of niobium, as noted by Gavaler, Testardi, Wernick, and Royerin 1973. Scientists questioned whether superconductivity would ever become a practical reality for use in electrical devices.
Some effort was being devoted during this time to theories of superconductivity and empirical concepts that might lead to the prediction of superconducting materials. In the 1930s, for example, the German scientists Fritz and Heinz London tried to develop a theoretical explanation for superconductivity. Fritz, the elder brother, was born in Breslau, Germany (now Wroclaw, Poland) on March 7, 1900. He attended the universities of Bonn, Frankfurt, Göttingen, Paris, and Munich, earning his doctorate from the last of these. He emigrated to the United States in 1939 and became professor of theoretical chemistry at Duke University. He died in Durham, North Carolina, on March 30, 1954.
Fritz London was especially interested in problems of spectroscopy and the application of quantum mechanics to the chemical bond. His 1927 study of the hydrogen molecule (with W. Heitler) is considered a major contribution to the understanding of chemical valence. In 1932, Fritz became interested in the topic of superconductivity. His brother, Heinz, was then doing research on that topic for his Ph.D. at the University of Breslau. Heinz was born in Bonn on November 7, 1907 and attended the university there. After receiving his degree from Breslau in 1934, Heinz joined his brother Fritz at Oxford, where he remained for the rest of his life. He died there on August 3, 1970.
The two brothers worked together to develop a theory of electron flow in superconductors. They concluded that the electrical current is actually confined to a very thin outer layer in the superconducting material. The discovery of the Meissner-Ochsenfeld effect in 1933 provided them with valuable data that allowed them to refine their theory even further.
Another approach to the study of superconductivity was that developed by the German-American physicist, Bernd Teo Matthias. Born in Frankfurt on June 8, 1919, Matthias studied at the University of Rome before earning his doctorate at the Massachusetts Institute of Technology. Beginning in 1950, he attempted to discover some general rules about the known superconducting materials, hoping that he would find a way to predict other materials that would also be superconductive. He found that metals with certain numbers of valence electrons and materials with certain crystal structures were more likely to be superconductors. Based on these general rules, he invented an alloy of niobium and tin that has a transition temperature of 18 K.
The most significant breakthrough in superconductivity theory occurred with the announcement of the BCS theory in 1957. The acronym BCS stands for the names of the three scientists--John Bardeen, Leon Cooper, and J. Robert Schrieffer--who developed the theory. According to the BCS theory, an electron moving through a crystal lattice tends to distort the lattice in such a way as to create a wave of positive charge resulting from the dislocation of positive ions in the lattice. This positive charge attracts to it a second electron which, with the first electron, forms a Cooper pair that sweeps through the crystal. The accumulation of Cooper pairs, all flowing in the same direction, results in the resistantless flow characteristic of a superconductor.
The theoretical work of the Londons, Matthias, and the BCS team had relatively little impact on the discovery of new superconducting materials. Then, in 1986, a startling breakthrough occurred. Two physicists working at the IBM Research Laboratories in Zurich-- K. Alex Müller and Georg Bednorz--announced the discovery of a new type of material that becomes superconductive at 35 K. The material was a ceramic substance, an oxide of barium, lanthanum, and copper.
In little more than a year, other researchers around the world produced similar ceramics, containing various mixed oxides of yttrium, barium, copper, lanthanum, and/or strontium. Transition temperatures for these materials shot up from 35 K to 40 K to 52 K to nearly 100 K by February of 1987. At long last, there seemed to be some possibility of making superconducting electrical devices that can operate at near-room temperatures.
Perhaps the most dramatic moment of all during this period came in January 1987 when Ching-Wu "Paul" Chu, Mau-Kuen Wu, and a number of their collaborators reported a material that becomes superconductive between 90 and 100 K. The announcement set off a frenzy of experimentation among scientists around the world, all trying to duplicate the results of the Chu-Wu team. Although those results were uniformly successful, significant further progress beyond this point had not yet been made five years later.
One of the first of these applications was planned to be the Superconducting Super Collider (SSC). The SSC is designed to be the world's most powerful particle accelerator. The most important factor holding back its construction in the 1980s was the cost of its magnets. The size and cost of traditional electromagnets would have been far too great to make the SSC a practical machine. With the availability of new superconducting materials, however, the potential cost of building the SSC dropped considerably.
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