Snapshots from Physics History

July 1957: Bardeen, Cooper and Schrieffer
submit their paper, "Theory of Superconductivity"

Bardeen, Cooper and Schrieffer
Source: AIP Emilio Segrè Visual Archives
Bardeen, Cooper and Schrieffer (left to right)
Fifty years ago, 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 the 20th century.

In 1911, Heike Kamerlingh Onnes, in his quest to study materials at ever lower temperatures, discovered 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.

One physicist, Felix Bloch, proposed a new theorem: “Superconductivity is impossible.” Richard Feynman added that he had “spent an awful lot of time in trying to understand it…and [eventually] developed an emotional block against the problem of superconductivity…”

Although physicists made little progress figuring out superconductivity following Onnes’ discovery, experimentalists discovered 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 added a new wrinkle that any theory of superconductivity would have to explain. John Bardeen gave it try, but then went on to other work.

Explaining superconductivity
Some physicists experienced partial success explaining superconductivity. Brothers Fritz and Heinz London developed 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 explained 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’s at the University of Illinois. In 1957, Schrieffer figured out how to mathematically describe the enormous collection of Cooper pairs in a superconductor with one single wave function.

Superconductivity solved
Together, Bardeen, Cooper and Schrieffer formed a complete theory: 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.

They submitted their report, titled “Theory of Superconductivity,” to the Physical Review in July 1957, and it was published in December.

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. He received the other for the discovery of the transistor. The theory works for conventional superconductors, but does not explain the high temperature superconductors first discovered 20 years ago.

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