When electrons couple up, further quantum trickery makes superconductivity unavoidable. Normally, electrons can’t overlap, but Cooper pairs follow a different quantum mechanical rule; they act like particles of light, any number of which can pile onto the head of a pin. Many Cooper pairs come together and merge into a single quantum mechanical state, a “superfluid,” that becomes oblivious to the atoms it passes between.
BCS theory also explained why mercury and most other metallic elements superconduct when cooled close to absolute zero but stop doing so above a few kelvins. Atomic ripples make for the feeblest of glues. Turn up the heat, and it jiggles atoms and washes out the lattice vibrations.
Then in 1986, IBM researchers Georg Bednorz and Alex Müller stumbled onto a stronger electron glue in cuprates: crystals consisting of sheets of copper and oxygen interspersed between layers of other elements. After they observed a cuprate superconducting at 30 kelvins, researchers soon found others that superconduct above 100, and then above 130 kelvins.
The breakthrough launched a widespread effort to understand the tougher glue responsible for this “high-temperature” superconductivity. Perhaps electrons bunched together to create patchy, rippling concentrations of charge. Or maybe they interacted through spin, an intrinsic property of the electron that orients it in a particular direction, like a quantum-size magnet.
The late Philip Anderson, an American Nobel laureate and all-around legend in condensed-matter physics, put forth a theory just months after high-temperature superconductivity was discovered. At the heart of the glue, he argued, lay a previously described quantum phenomenon called superexchange—a force arising from electrons’ ability to hop. When electrons can hop between multiple locations, their position at any one moment becomes uncertain, while their momentum becomes precisely defined. A sharper momentum can be a lower momentum, and therefore a lower-energy state, which particles naturally seek out.
The upshot is that electrons seek situations in which they can hop. An electron prefers to point down when its neighbor points up, for instance, since this distinction allows the two electrons to hop between the same atoms. In this way, superexchange establishes a regular up-down-up-down pattern of electron spins in some materials. It also nudges electrons to stay a certain distance apart. (Too far, and they can’t hop.) It’s this effective attraction that Anderson believed could form strong Cooper pairs.
Experimentalists long struggled to test theories like Anderson’s, since material properties that they could measure, like reflectivity or resistance, offered only crude summaries of the collective behavior of trillions of electrons, not pairs.
“None of the traditional techniques of condensed-matter physics were ever designed to solve a problem like this,” said Davis.
Super-Experiment
Davis, an Irish physicist with labs at Oxford, Cornell University, University College Cork, and the International Max Planck Research School for Chemistry and Physics of Quantum Materials in Dresden, has gradually developed tools to scrutinize cuprates on the atomic level. Earlier experiments gauged the strength of a material’s superconductivity by chilling it until it reached the critical temperature where superconductivity began—with warmer temperatures indicating stronger glue. But over the last decade, Davis’ group has refined a way to prod the glue around individual atoms.
They modified an established technique called scanning tunneling microscopy, which drags a needle across a surface, measuring the current of electrons leaping between the two. By swapping the needle’s normal metallic tip for a superconducting tip and sweeping it across a cuprate, they measured a current of electron pairs rather than individuals. This let them map the density of Cooper pairs surrounding each atom—a direct measure of superconductivity. They published the first image of swarms of Cooper pairs in Nature in 2016.
