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Credit: AIP Emilio Segrè Visual Archives
Left to right: Bardeen, Cooper and Schrieffer, after accepting an honorary degree from the University of Illinois in 1978.
In 1957, John Bardeen, Leon Cooper, and Robert Schrieffer presented their complete theory of superconductivity, finally explaining a phenomenon that had been a mystery to physicists since its discovery in 1911.
In 1911, Heike Kamerlingh Onnes, in his quest to study materials at ever lower temperatures, happened to find that the electrical resistance of some metallic materials suddenly vanished at temperatures near absolute zero. He called the phenomenon superconductivity, and scientists soon found additional materials that exhibited this property.
But no one could completely explain how it worked. For the next few decades, many prominent physicists worked to develop a theory of the mechanism underlying superconductivity, but no one had much success, and some despaired of figuring it out. One such physicist, Felix Bloch, was quoted as proposing "Bloch's theorem: Superconductivity is impossible."
Richard Feynman also later recalled that he had "spent an awful lot of time in trying to understand it and doing everything by means of which I could approach it… I developed an emotional block against the problem of superconductivity, so that when I learned about the BCS paper I could not bring myself to read it for a long time."
While theorists were making little progress in the years following Onnes's discovery, experimentalists were discovering some interesting features of superconductors. In 1933, Walther Meissner found that superconductors would expel a magnetic field, an effect that makes it possible to levitate a magnet. The discovery of the Meissner effect, as it is called, added a new wrinkle that any theory of superconductivity would have to explain. John Bardeen made an attempt at the problem of superconductivity, but then went on to other work.
Some physicists did have partial success in explaining superconductivity. The brothers Fritz and Heinz London came up with a theory that explained some of its features, but didn't provide a mechanism at the microscopic level. In 1950, Herbert Frohlich proposed that superconductivity might have to do with interactions between the electrons and the vibrations of the crystal lattice, or phonons. Around that time, experimenters observed that the critical temperature, at which a material becomes superconducting, is related to the atomic mass of the superconductor. Frohlich's theory did explain this isotope effect, but couldn't account for other properties of superconductivity such as the Meissner effect.
At the time, Bardeen had been working on other research, but the discovery of the isotope effect renewed his interest in the problem of superconductivity. Bardeen and David Pines built on the explanation of the isotope effect. They took into account the electron–phonon interactions that Frohlich had considered, but they also determined how at low energies in a crystal lattice, electrons could overcome the Coulomb repulsion and attract each other.
Another piece of the puzzle was contributed by Leon Cooper, who suggested that interactions with the lattice would allow electrons with opposite spins to combine to form strongly correlated pairs. The electrons in these Cooper pairs, as they are called, do not have to be close together, but they can move in a coordinated manner. Cooper realized the motion of these pairs could explain how electrons could flow with no resistance in a superconductor. These pairs would form at low temperature; adding energy would break up the pairs, returning the material to a normal, non-superconducting, state.
The next insight came from Robert Schrieffer, a student of Bardeen at the University of Illinois. In New York early in 1957 to attend the APS annual meeting, Schrieffer had an idea while riding on the subway. He figured out how to mathematically describe the enormous collection of Cooper pairs in a superconductor with one single wave function. Upon returning to Illinois, he told Bardeen and Cooper about the breakthrough, and they realized that the problem of superconductivity was solved.
Bardeen, Cooper, and Schrieffer put all these insights together to form a complete theory, in which electrons, through interaction with lattice vibrations, form Cooper pairs, which move in a coordinated manner, rather than randomly as in a normal conductor, allowing electricity to flow with no resistance.
"Well, I think we've explained superconductivity," the usually quiet Bardeen announced one day. In April of that year, Bardeen, Cooper and Schrieffer published a short paper in Physical Review entitled "Microscopic Theory of Superconductivity." They submitted their full detailed report, appropriately titled "Theory of Superconductivity," to the Physical Review in July 1957, and it was published in December.
In 2007, at the 50th anniversary of the BCS theory, APS President Leo Kadanoff (right) presented the historic sites plaque to UIUC Chancellor Richard Herman.
The BCS theory was extremely successful, explaining in detail the mechanism of superconductivity and associated effects, and it agreed amazingly well with experimental data. "All of the hitherto puzzling features of superconductors fitted neatly together like the pieces of a jigsaw puzzle," Bardeen later recalled. BCS theory was quickly accepted as correct.
Bardeen, Cooper, and Schrieffer were awarded the Nobel Prize in 1972 for their theory of superconductivity. This was Bardeen's second Nobel Prize in physics–his first was shared with William Shockley and Walter Brattain for the transistor in 1956.
The theory works for conventional superconductors, but does not explain the high temperature superconductors first discovered 20 years ago, so puzzles still remain. However, BCS theory has had an impact far beyond superconductivity, as scientists have found states analogous to the BCS superconductor in astrophysics and nuclear physics.
In 2007, the University of Illinois hosted a celebration of the 50th anniversary of the BCS theory. At that occasion, APS President Leo Kadanoff presented a plaque that was accepted on behalf of the university by Chancellor Richard Herman. The citation on the plaque read "In this building, the home of the University of Illinois' Physics Department from 1909 to 1959, John Bardeen, Leon Cooper, and J. Robert Schrieffer created the 'BCS' theory of superconductivity, a great achievement of theoretical physics, in 1956-1957. For their work, they were awarded the 1972 Nobel Prize in Physics."