Superconductivity: A Superlatively Difficult Puzzle

Placing a single atomic layer of iron selenide (balls and sticks) on top of a material called STO (triangles) significantly raises the temperature at which it superconduct. Image by SLAC National Accelerator Laboratory.

In 1911 the Dutch physicist Heike Kamerlingh Onnes discovered that the resistivity of mercury suddenly drops to immeasurably small values below 4.2 Kelvin. He called this new state of matter superconductivity.

During the 48 years between this discovery and the first quantum theory of superconductivity, numerous scientists tried and failed to understand the phenomenon, among them the brightest minds in theoretical physics of the 20th century. Albert Einstein, Niels Bohr, Werner Heisenberg, Max Born, Lev D. Landau, and many others all wrote down highly interesting yet incorrect theories of superconductivity.

Einstein proposed in 1922 that superconductivity is a consequence of the motion of electrons in the valence shell of chemical bonds; he made a falsifiable prediction. It was Kamerlingh Onnes himself who disproved that theory.

Niels Bohr and Ralph Kronig had elegant theories involving electrons that crystallize and then coherently move as an organized electron-solid, a proposal that preceded the actual theory of electron crystals by Eugene Wigner.

Felix Bloch and Landau proposed that electrons in a superconductor enter a state with spontaneous currents. That theory was shown to be incorrect by Bloch himself, who jokingly formulated a theorem stating that every theory of superconductivity can be disproved. Landau’s theory did lead to the correct phenomenology of superconductivity, which he formulated with Vitaly Ginzburg. In addition, energy expansions of the type first used by Landau in his incorrect work are at the heart of every phenomenology of phase transitions, the Higgs mechanism in particle physics, and the theory of the inflationary expansion of the universe.

In 1957 the microscopic theory of superconductivity that holds today was finally formulated by John Bardeen, Leon N. Cooper, and J. Robert Schrieffer at the University of Illinois at Urbana-Champaign. BCS theory—named for Bardeen, Cooper, and Schrieffer—is among the most outstanding intellectual achievements in theoretical physics. It demonstrated that interactions between ions and electrons lead to bound states made up of two electrons, called Cooper pairs. The thermodynamic and electrodynamic properties that follow from the formation of these pairs are in excellent agreement with experiment. Cooper pairing is now believed to occur in all known superconductors, even if for many systems the mechanism for pairing, i.e., the pairing glue, continues to be hotly debated.

Superconductivity was obviously a tough problem. Given that the nature of magnetism was essentially understood as soon as quantum mechanics was formulated by Heisenberg and Schrödinger in 1927, it is amazing that it took another 30 years for the superconductivity puzzle to be resolved. Clearly, even the most beautiful theories emerge in ways that are much less direct than how they are taught in the classroom. A wrong theory can be interesting, inspiring, and may turn out to be useful in another setting. As long as we are willing to admit them, mistakes and failures are a natural and healthy part of the scientific discourse. //

Superconductivity occurs in many metals at low temperatures. It exists in organic conductors and copper oxide–based ceramics at temperatures up to 130 Kelvin. It has recently been observed at high pressure in a compound made up of hydrogen and sulfur at temperatures as high as 200 Kelvin, i.e., above “room temperature” in Antarctica. Superconductors are promising building blocks for future quantum computers and have made their way into hospitals, where they are responsible for the large magnetic fields needed in magnetic resonance imaging. As a macroscopic quantum phenomenon, superconductivity has fascinated and captivated physicists more than any other condensed state of matter.