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A New Form of Electron Organization Offers High-Temperature Superconductivity

Conductive materials with no resistance are closer than ever.
​Superconductor links being developed at CERN. Image: ​CERN

A team of physicists at Columbia University may have discovered a possible new way of bringing superconductor technology to the masses—or at least the masses that can't count on regular access to temperatures near absolute zero. The potential is massive: electrical conductivity with zero energy loss, like a sled without friction just gliding forever unresisted.

Superconductors have been around for a good while now. In 1911, as the culmination of nearly a century of low-temperature deep-thinking, the Dutch physicist Kamerlingh Onnes noted the abrupt disappearance of electrical resistance in a mercury wire submerged in a bath of liquid helium cooled to just 1.5 degrees Kelvin, the coldest temperature ever achieved at the time.

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Like our frictionless sled above, electrons traveling through a superconductor just keep going. If you were to take a single loop of Onnes's supercooled mercury wire and apply some current, it would spin around and around in that loop forever. This has actually been demonstrated, with electrical currents persisting in coils of superconductive material for years unimpeded.

Physicists and electrical engineers have been obsessed with the phenomenon ever since Onnes's demonstration, for good reason, but achieving high-temperature superconductivity has been elusive, a puzzle that by now has taken over a century to just barely get started.

The Columbia group's superconductor is from a group of materials characterized by titanium-oxypnictide, a compound discovered only in the 1990s. This is where they observed a whole new variety of electronic order, what physicist Simon Billinge likens to discovering an untouched, undiscovered Egyptian tomb.

"As we try and solve the mysteries behind unconventional superconductivity," Billinge said in a statement provided by Brookhaven National Labs, "we need to discover different but related systems to give us a more complete picture of what is going on—just as a new tomb will turn up treasures not found before, giving a more complete picture of ancient Egyptian society."

Imagine a conductor material as some mountain creek. To float from top to bottom, an electron is going to careen and bounce from rock to rock, losing momentum on every collision, which would be released as thermal energy: heat. In a conductor, those rocks are ions, atoms with unbalanced charges. By the time the electrons reach the bottom, they will have dissipated a whole lot of energy as heat, which is basically waste.

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In a typical superconductor, electrons join forces, splitting off into partnerships called Cooper pairs. Normally, such pairings would be forbidden; particles with the same charge repel each other, after all. In a warm conductor, they do just this, as the thermal energy of the material—the ceaseless jostling and shuffling of an energetic system—is enough to break apart the relatively weak bond keeping would-be electron pairs together. So, the normal situation is that electrons scatter and act as individuals, acting as the fluid in our mountain creek example. Cooper pairs offer an alternative.

When so many electrons pair up, they stop acting like a normal fluid and condense. This condensation means that the whole current is kind of acting as a single entity. This entity enforces a lower limit on the excitation energy of the flowing electrons, which in turn overrides the usual scattering behavior of the particles. This new condensate treats the mountain stream as if it were just a vertical chute, falling through it as if the rocks didn't exist.

In high-temperature superconductors, the electrons form stripes (superstripes) or checkerboard patterns in the material, which are organizations that also enforce an excitation lower limit. Crucially, such arrangements break what's known as "translational symmetry" within the material. This just means that instead of looking the same in every direction, you'll find the landscape looking differently from different perspectives: a bunch of charge over here, a lot less over there. Before, the conductor was like an orderly lattice, and now it's full of odd peaks (shown above).

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The high-temperature superconductor demonstrated by the Columbia team breaks a different sort of order called rotational symmetry. The lattice of translational symmetry is again intact, but if you were to spin around it wouldn't look quite the same. If in front of you were an "S," behind you might be the mirror image of an "S" or an "S" otherwise rotated. This conductor landscape is again asymmetric, but in a different way.

This is called nematic ordering and, once again, it creates the sort of superstructure needed for superconductivity, but without the need to preserve Cooper pairs with supercold temperatures.

Symmetry can be explored in a material by firing a beam of particles through it at different temperatures and observing the resulting patterns and distortions. Billinge and his team determined that titanium-oxypnictide does indeed exhibit the elusive nemacity, opening an entirely new doorway into the hopeful realm of high-temperature superconductivity. Says Billage: "This new pharaoh's tomb indeed contained a treasure: nematicity."

A pre-publication open-access version of this paper is available at arVix.