When electrons couple, more quantum tricks make superconductivity inevitable. Normally, electrons cannot overlap, but Cooper pairs obey a different quantum mechanical rule; they act like particles of light, any number of which can accumulate on the head of a pin. Many Cooper pairs come together and merge into a single quantum mechanical state, a “superfluid” that is no longer aware of the atoms between which it is moving.
The BCS theory also explained why mercury and most other metallic elements become superconductive when cooled near absolute zero, but not above a few Kelvin. Atomic waves are the weakest glue. Crank up the heat, and it jiggles atoms, washing out lattice vibrations.
Then, in 1986, IBM researchers Georg Bednorz and Alex Müller stumbled upon a stronger electron glue in cuprates: crystals composed of layers of copper and oxygen interspersed between layers of other elements. After observing one cuprate superconducting at 30 Kelvin, the researchers soon found others superconducting above 100 and then above 130 Kelvin.
The breakthrough sparked 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 specific direction, like a quantum-sized magnet.
The late Philip Anderson, an American Nobel laureate and all-around legend in condensed matter physics, proposed a theory just months after discovering high-temperature superconductivity. At the heart of the glue, he argued, lies a previously described quantum phenomenon called superexchange — a force that arises from the ability of electrons to jump. When electrons are allowed to hop between multiple locations, their position becomes uncertain at any instant while their momentum is well defined. A sharper momentum may be a lower momentum and therefore a lower energy state that particles naturally seek.
The result is that electrons seek situations where they can hop. For example, an electron prefers to point down when its neighbor is pointing up, since this distinction allows the two electrons to jump back and forth between the same atoms. In this way, superexchange creates 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 jump.) It’s this effective attraction that Anderson believed could form strong Cooper pairs.
Experimentalists have long struggled to test theories like Anderson’s because material properties they could measure, such as reflectivity or resistance, gave only crude summaries of the collective behavior of trillions of electrons, not pairs.
“None of the traditional solid-state physics techniques were ever developed to solve a problem like this,” Davis said.
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 study cuprates at the atomic level. Previous experiments measured the strength of a material’s superconductivity by cooling it until it reached the critical temperature at which superconductivity begins – with warmer temperatures indicating stronger glue. But over the past decade, Davis’ group has refined a way to pierce the glue around individual atoms.
They modified an established technique called scanning tunneling microscopy, in which a needle is drawn across a surface to measure the current of electrons bouncing back and forth between the two. By exchanging the needle’s normal metal tip for a superconducting tip and sweeping it over a cuprate, they measured a current of electron pairs rather than individuals. This allowed them to map the density of Cooper pairs surrounding each atom – a direct measure of superconductivity. They released the first image of crushing Cooper couples in Nature in 2016.