John Bardeen, Leon N. Cooper, and John Robert Schrieffer were the first physicists to explain how some metals, approaching absolute zero (-237.59 degrees Celsius), lose their electrical resistance. These so-called superconductors would become an important part of later developments in electronics and computer science.
In 1911, Dutch physicist Heike Kamerlingh Onnes, later a Nobel laureate in physics, observed in his study of the electrical resistance of mercury, as its temperature was lowered to the vicinity of absolute zero, that at -237.59 degrees Celsius, resistance decreased so markedly that it could not be measured with the instruments available. Onnes had discovered, and was soon to report on, superconductivity. For nearly half a century later, the phenomenon was tested extensively in the laboratory with various metals and was confirmed by far-ranging experiments, yet defied scientific explanation.
John Bardeen read David Shoenberg’s Superconductivity
By 1940, Bardeen had begun to formulate his own theory of superconductivity. His thinking, however, was diverted into other channels with the onset of World War II. His work at the Naval Ordnance Laboratory between 1941 and 1945 led him into the research on the transistor effect for which he was awarded his first Nobel Prize in
Bardeen’s interest in superconductivity was rekindled in 1950 when he learned of the discovery of the isotope effect—as it turned out, one of the missing pieces in the intricate puzzle of superconductivity. The new research on the isotope effect demonstrated that even though superconductivity is an electronic event, it is dependent in a significant way upon the vibrations of the crystal lattice within which the electrons travel. With the shift in his research interests, Bardeen moved in 1951 to the University of Illinois at Urbana-Champaign, where he became professor of physics and electrical engineering.
Because research on superconductivity is so complex that a researcher working alone cannot carry it out, Bardeen sought a recommendation from the Institute for Advanced Study at Princeton for a field theorist to collaborate with him. Leon N. Cooper, a recent recipient of a Ph.D. in physics from Columbia University, joined Bardeen at Illinois. In 1956, they were joined in their research by one of Bardeen’s graduate students, John Robert Schrieffer
Bardeen’s method—to segment complex, seemingly insoluble problems into smaller problems capable of solution—led to his dividing the collaboration into investigations of the constituent parts of the problem. He assigned to Schrieffer the task of working on the thermodynamic properties of superconduction and to Cooper the task of investigating its electrodynamic properties.
It was already known that twenty-six metallic elements, including lead and tin, are superconductors in their common forms, although each has a different transition temperature within a small range of degrees immediately above absolute zero. Another ten alloys share the same property. It was known that superconductors with relatively high transition temperatures are not good conductors in their usual state. The highest transition temperature, -214.44 degrees Celsius, was found in an alloy of aluminum, niobium, and germanium.
Bardeen and his colleagues knew that no convincing evidence had yet suggested that superconductivity takes place only at extremely low temperatures. Much of its practical potential is intimately connected to being able to achieve the phenomenon at higher temperatures, although temperatures close to absolute zero can be achieved inexpensively with liquid nitrogen.
The immediate task, however, was to understand and explain plausibly the dynamics of a phenomenon that had for a half century been known to exist and about which much was already known, although no convincing explanation for superconductivity had been forthcoming from the extensive experiments that physicists throughout the world were conducting.
As early as 1950, Bardeen had traced the relationship of the electron-phonon interaction to superconductivity. The first important clue to this interaction, which Bardeen had adjusted when the isotope effect was finally articulated, came in realizing that the superconducting transition temperature varies inversely as the square root of the isotopic mass. Bardeen conjectured that the energy gaps he had discovered in 1940 arose from dynamic interactions with phonons rather than from small lattice displacements. This theory presupposes that electrons have their own energy in a field of phonons.
The interaction of electrons brought about by the background crystal lattice occurs when a single electron causes the lattice, which has a degree of elasticity, to become slightly distorted as it moves. This was thought to occur because of the coulomb attraction of a negatively charged electron with the lattice, whose charge is positive. If another electron sensed this distortion, it seemed to be affected by it just enough to cause a weak interaction and a pairing.
In an article in the Physical Review of July 17, 1950, however, Bardeen observes that in variations in the temperature of mercury with isotopic mass, superconductivity results from the interaction of electrons with lattice vibrations, not from a period lattice distortion, as had previously been supposed. Bardeen also suggested ways of calculating the energy as the temperature approaches zero, an indispensable step that earlier measuring equipment was not sensitive enough to permit being carried out.
As early as 1956, Cooper had shown that when two electrons are in the presence of a high-density fluid of electrons, they interact and attract each other, even though their interaction may not be strong. In doing so, they bind, forming a pair, often designated as a Cooper pair, with opposite momentums. A year later, Bardeen and his colleagues discovered the means of constructing a wave function in which all the significant electrons formed into pairs. Then, by adjusting the wave function to minimize its free energy, they were able to use it as a means of developing a complete microscopic theory of superconductivity, the initial findings of which they published in February, followed by additional evidence during the next six months.
One can perhaps best visualize the electrons’ actions by likening them to people in a crowded railway station. The people are squeezed together in close proximity, all moving in specific directions. They will predictably bump into anything in their paths, including each other. If something causes all of them to shift to one side of the space while they continue moving, the motion will be totally chaotic and unpredictable. At every contact with another person or object, energy will decrease substantially or disappear entirely, which is what happens to electrons in normal metals when they collide with each other or with anything anomalous in the crystal lattice, such as impurities.
If current is run into the electric field, each collision will dissipate energy, causing conductivity that will steadily diminish the energy. If, however, the people (electrons) are weakly paired, as the Cooper pairs are, separated by a distance that is precisely the length of coherence, the resistance of all the people (electrons) that are not part of the pair is reduced by about a hundred times. Among electrons, it is this state that approaches superconductivity.
It was also discovered that besides lacking resistance, superconductors can prevent external magnetic fields from entering their own interiors, making them perfectly diamagnetic. The quantum wave function in superconductivity differs in quality from normal state wave functions, as the Bardeen-Cooper-Schrieffer (BCS) theory purports.
The search for perpetual motion antedates the discovery of superconductivity by hundreds of years. The two phenomena are closely related, however, in that each promises extremely efficient means of transmitting the energy that fuels developed societies and offers hope to underdeveloped ones. The articulation of the BCS theory solved a dilemma that had occupied scientists for more than half a century. It suggested ways to develop new, ultrasensitive, and highly accurate devices for measuring voltages, currents, and magnetic fields, as well as for improving heavy engineering equipment significantly and developing sophisticated, hypersensitive computer elements.
Because of the resistance it encounters in transmission, much of the energy that reaches its destination over conventional power lines is lost before it arrives where it will be put to use. Superconductive lines eliminate the resistance, thereby increasing exponentially the amount of energy that can be transmitted to a given destination.
The potential the theory unlocked for building superfast trains is impressive. Running in troughs, hovering above superconductive rails, such trains can achieve incredible speeds safely, efficiently, and economically, as is seen in some of the so-called bullet trains that serve Japan and parts of Europe. The electromagnets fundamental to the operation of such trains have been derived from the diamagnets that experiments in superconductivity revealed.
Although superconductivity is being achieved at higher temperatures, particularly when copper and oxygen atoms are present to form chains of atoms in the crystal, some problems relating to the process remain, such as brittleness, the dispersion of impurities at the surfaces of the crystals, and chemical instability under some situations, such as high humidity. Research in the field, however, is yielding promising results with such materials as compounds of bismuth.
Experiments are under way to find new superconductive materials whose transition temperature is high, possibly above room temperatures. Researchers are also considering the creation of environments in which the attractive interaction mechanism between electrons is stronger than the electron-lattice interaction that has been present in all superconductors to date.
It has been suggested that the interiors of neutron stars, and of at least one of the planets, could be superconductive. The BCS theory has, indeed, broad cosmic implications for those who are attempting to understand the beginnings of the universe. It is known that the time it takes for a supercurrent to decay exceeds 100,000 years. The building of great superconductors in both the United States and Europe will enable scientists to understand more fully and measure more accurately the persistence of superconducting loops. Some scientists have conjectured that the lifetime of some persistent currents is infinite.
Among the immediate advances the BCS theory made possible was research in such areas as elementary particle and electroweak theory. David Pines, for example, has applied the theory to the concept of the superfluidity of nuclear matter. The impact of the BCS theory is sufficiently broad and profound that one can only speculate on how many new worlds it has opened to scientists. Certainly, it marks a turning point in the way the scientific community has viewed many of the phenomena that have been central concerns to scientists for decades, even centuries.
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Bogoliubov, Nikolai N., V. V. Tolmachev, and D. V. Shirkov. A New Method in the Theory of Superconductivity. New York: Consultants Bureau, 1959. Valuable for its explanations of the various unsuccessful attempts to explain superconductivity. A highly technical book, it makes one appreciative of the quantity of research worldwide that preceded the articulation of the BCS theory. -
Hoddeson, Lillian. “John Bardeen and the Discoveries of the Transistor and the BCS Theory of Superconductivity.” In A Collection of John Bardeen’s Publications on Semiconductors and Superconductivity. Urbana: University of Illinois Press, 1988. Hoddeson’s introduction to this book honoring John Bardeen on his eightieth birthday is informed, accurate, and readable. Hoddeson has perhaps the clearest understanding of any available writer of the significance of the BCS theory. Her own background in physics is strong and her writing style is appealing. Some technical language, but accessible to readers willing to put forth some effort. -
London, Fritz. Superfluids. New York: John Wiley & Sons, 1950. This book was one of two major reasons that John Bardeen revived his research in superconductivity after a decade’s lapse. London’s new insights piqued Bardeen’s curiosity and were the precipitating force for the major redirection in his career. -
Matricon, Jean, and Georges Waysand. The Cold Wars: A History of Superconductivity. Translated by Charles Glashausser. New Brunswick, N.J.: Rutgers University Press, 2003. Comprehensive history of the theory and practical application of superconductivity; includes a chapter on Bardeen’s work that places it in its larger context. Bibliographic references and index. -
Schrieffer, John Robert. The Theory of Superconductivity. New York: Benjamin, 1964. One of the major researchers in the group that articulated the BCS theory, Schrieffer presents an accurate explanation, although it is extremely technical and may be inaccessible to those with insufficient backgrounds. Various summaries in the book are easier to understand than much of the supporting text. -
Shoenberg, David. Superconductivity. Cambridge, England: Cambridge University Press, 1938. Shoenberg’s explanation of superconductivity presents the problem in remarkable (and at times, diffuse) detail, although at the time of his writing, no solution to the problem had been forthcoming. This book was extremely influential to John Bardeen, who was led by it to an early (1940) statement of a theory of superconductivity, later to be supplanted by the BCS theory.
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