Researchers observe ‘locked’ electron pairs in cuprate superconductor

Since their discovery a century ago, superconductors and their mysterious atomic properties have amazed researchers. These special materials allow electricity to flow through them without loss of energy. They even allow trains to levitate.

But superconductors typically only work at extremely low temperatures. When these materials are heated, they become ordinary conductors, which allow electricity to flow but with some energy loss; or insulators, which do not conduct electricity at all.

Researchers have been working hard to find superconducting materials that can perform their mission at higher temperatures, perhaps even at room temperature one day. Discovering or making such a material could revolutionize modern technology, from computers and cell phones to the power grid and transportation. What’s more, the unique quantum state of superconductors also makes them excellent building blocks for quantum computers.

Researchers have observed that a characteristic necessary for superconductors, called electron pairing, occurs at much higher temperatures than previously thought, and in a material where it is least expected: an antiferromagnetic insulator. Although the material does not have zero resistance, the discovery suggests that researchers may be able to find ways to make similar materials for superconductors that operate at higher temperatures.

The research team from SLAC National Accelerator Laboratory, Stanford University and other institutions published their results in Science.

“The electron pairs tell us they’re ready to become superconductors, but something is stopping them,” says Ke-Jun Xu, a graduate student in applied physics at Stanford and co-author of the paper. “If we can find a new way to synchronize the pairs, we could apply it to building higher-temperature superconductors.”

Electrons out of sync

Over the past 100 years, researchers have learned a lot about exactly how superconductors work. We know, for example, that for a material to be a superconductor, electrons must pair up, and those pairs must be coherent—that is, their motions must be synchronized. If the electrons are paired but incoherent, the material can end up being an insulator.

In superconductors, electrons behave like two reluctant people at a dance party. At first, neither person wants to dance with the other. But then the DJ plays a song that both people like, which allows them to relax. They notice that the other person enjoys the song and are drawn in from afar—they have teamed up but have not yet become cohesive.

The DJ then plays a new song, a song that both people love. Suddenly, the two people pair up and start dancing. Soon, everyone at the dance party follows their example: they all come together and start dancing to the same new tune. At this point, the party becomes coherent; it is in a superconducting state.

In the new study, the researchers observed electrons at an intermediate stage, where the electrons had their eyes fixed on them, but did not get up to dance.

Cuprates act strangely

Soon after superconductors were discovered, researchers discovered that it was vibrations in the underlying material that allowed electrons to pair up and dance. This type of electron pairing occurs in a class of materials known as conventional superconductors, which are well-known, said Zhi-Xun Shen, a Stanford professor and researcher at the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC, who oversaw the research. Conventional superconductors operate at temperatures typically near absolute zero, below 25 Kelvin, at ambient pressure.

Unconventional superconductors, such as the copper oxide, or cuprate, material used in the current study, operate at much higher temperatures, sometimes as high as 130 Kelvin. In cuprates, it is generally thought that something beyond lattice vibrations helps to pair the electrons. Although researchers are not sure what is behind this phenomenon, a leading candidate is the fluctuation of electron spins, which causes the electrons to pair up and dance around with higher angular momentum.

This phenomenon is known as wave channeling. The first signs of such a new state were observed in an experiment at SSRL about thirty years ago. Understanding what causes electron pairing in cuprates could help design superconductors that operate at higher temperatures.

In this project, the scientists chose a family of cuprates that had not been studied in depth because its maximum superconducting temperature was relatively low (25 Kelvin) compared to other cuprates. Worse still, most members of this family are good insulators.

To observe the atomic details of cuprate, the researchers shined ultraviolet light on samples of the material, which has the effect of ejecting electrons from the material. When the electrons are bound, they are slightly more resistant to ejection, resulting in an “energy gap.” This energy gap persists up to 150 Kelvin, suggesting that the electrons are paired at much higher temperatures than the zero-resistance state at about 25 Kelvin. The most unusual result of this study is that the pairing is strongest in the most insulating samples.

The cuprate under study may not be the material that can achieve room-temperature superconductivity, around 300 Kelvin, Shen said. “But maybe in another family of superconducting materials, we can use this knowledge to get some clues to get closer to room temperature,” he said.

“Our results open up a potentially rich new avenue,” Shen said. “We plan to study this coupling gap in the future to help design superconductors using new methods. On the one hand, we plan to use experimental approaches similar to SSRL to better understand this incoherent coupling state. On the other hand, we want to find ways to manipulate these materials to perhaps force these incoherent pairs into sync.”