A mysterious superconductor’s wave could reveal the physics behind the materials

High-temperature superconductors exhibit a phase of matter called a ‘pair-density wave’

High-temperature superconductor

High-temperature superconductors can exhibit unusual behavior, for example, levitating above a magnet. Now, scientists have found a feature called a pair-density wave in the materials.

Julian Litzel/Wikimedia commons (CC BY SA-3.0)

Physicists have finally captured a superconductor’s wave.

The first direct evidence of a phase of matter known as a pair-density wave helps reveal the physics that underlies mysterious high-temperature superconductors, which conduct electricity without resistance at surprisingly high temperatures. The wave was detected using a scanning tunneling microscope, researchers report April 1 in Nature.

Physicists had suspected that pair-density waves existed in these materials, and previous experiments had hinted at their existence. But without direct proof, scientists couldn’t advance their understanding of the materials. “Investigating and proving [that] this phase not only might exist, but actually does exist, is very important,” says theoretical physicist Eun-Ah Kim of Cornell University, who wasn’t involved in the work.

High-temperature superconductors wowed physicists when the materials came on the scene in the 1980s. Known as cuprates because they contain copper, the materials conduct electricity without resistance at temperatures much higher than most other superconductors, some around 100 kelvins (about –173° Celsius) or higher (SN: 12/8/17).

Although still chilly, such temperatures are much easier to attain than the nearly absolute zero temperatures required for many superconductors. The materials’ discovery led to high hopes that a room-temperature superconductor could soon be found, possibly leading to new technologies such as vastly more energy-efficient electric grids, magnetically levitated trains and powerful supercomputers.

But, decades later, a room-temperature superconductor has yet to appear on the scene. What’s more, scientists still don’t fully understand the physics that makes these materials so special. In particular, “we want to understand the microscopic mechanism of how superconductivity occurs in those materials,” says physicist Kazuhiro Fujita of Brookhaven National Laboratory in Upton, N.Y. Now, scientists are drawing a bit closer to a solution.

In superconductors, electrons buddy up into duos called Cooper pairs, a partnership that allows them to slip smoothly through the material without resistance (SN: 5/13/15). In these materials, scientists observe a gap in the energies of electrons, rather than a continuous spectrum.

Physicists predicted that, in high-temperature superconductors, the gap in the electrons’ energies would periodically vary across the surface of the material in a strange kind of wave. That effect might be linked to another unusual state that exists in the same materials at higher temperatures, called the pseudogap phase. That state inhabits a strange purgatory: It’s neither superconductor nor insulator, and it conducts electricity but not all that well.

Fujita and colleagues detected the wave by skimming across the surface of a superconducting compound — a bismuth-based copper oxide — with a scanning tunneling microscope. The microscope has an extremely thin tip that detects electrons that pass across the space between the superconductor and tip via a quantum process known as tunneling. In this case, the researchers also affixed a tiny piece of superconductor to the microscope’s tip, to search for electrons tunneling from one bit of the superconductor to another. The energy gap, the team reported, periodically varied across the surface of the material in a wave, as predicted.

“This is actually a direct measurement of the pair-density wave component,” says theoretical physicist Eduardo Fradkin of the University of Illinois at Urbana-Champaign. “It’s a really exciting experiment.”

That pseudogap phase may be important in the quest to increase the temperature range of high-temperature superconductors. The new result could help scientists understand that phase better by illuminating how these materials behave as they warm up.

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