High-Temperature Superconductivity Understood at Last - Quanta Magazine

Séamus Davis, who led the new experiment at the University of Oxford.

The new measurement matches a prediction based on the theory, which attributes cuprate superconductivity to a quantum phenomenon called superexchange.

Electrons deftly wended their way through the wire without generating heat when they collided with its atoms — the origin of resistance.

A new experiment led by the condensed matter physicist Séamus Davis at the University of Oxford all but settles the origin of high-temperature superconductivity, a puzzle Davis has worked on for 25 years.

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.

(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.

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.

That same year, an experiment by Chinese physicists provided a major piece of evidence supporting Anderson’s superexchange theory: They showed that the easier it is for electrons to hop between copper and oxygen atoms in a given cuprate, the higher the cuprate’s critical temperature (and thus the stronger its glue).

Davis and his colleagues sought to combine the two approaches in a single cuprate crystal to more conclusively reveal the nature of the glue.

They used a traditional scanning microscope with a metal tip to stick electrons onto some atoms and pluck them from others, mapping the hopping energies across the cuprate.

They then swapped in a cuprate tip to measure the density of Cooper pairs around each atom.

Where electrons struggled to hop, superconductivity was weak.

Davis’ finding that hopping energy is linked with superconductivity strength, published this month in the Proceedings of the National Academy of Sciences, strongly implies that superexchange is the super glue enabling high-temperature superconductivity.

Superexchange isn’t a new idea, so plenty of researchers have already thought about how to fortify it, perhaps by further squishing the copper and oxygen lattice or experimenting with other pairs of elements.