One of the Secrets to Superconductors has been Decoded

The microscopic structure of high-temperature superconductors has long puzzled scientists seeking to harness their virtually limitless technological potential. Now at last researchers have deciphered the cryptic structure of one class of the superconductors, providing a basis for theories about how they manage to transport electricity with perfect efficiency when cooled, and how scientists might raise their operating temperature closer to room temperature.

Scientists believe room-temperature superconductivity would have an impact on a par with that of the laser, a 1960 invention that now plays an important role in an estimated $7.5 trillion in economic activity.

“In the same way that a laser is a hell of a lot more powerful than a light bulb, room-temperature superconductivity would completely change how you transport electricity and enable new ways of using electricity,” said Louis Taillefer, a professor of physics at the University of Sherbrooke in Quebec.

Though further tests are needed, Sachdev’s theory is garnering support from many experts, who say it succinctly captures key features of the materials.

The rate of progress in recent months has been “almost overwhelming for us,” Comin said. With better experimental tools at their disposal, he and other researchers described rushing to publish one interesting new result after another as fascinating papers by their competitors piled up on their desks.

Arxiv – Intra-unit-cell Nematic Density Wave: Unified Broken-Symmetry of the Cuprate Pseudogap State

The biggest question in the field is, what force binds the electrons together?” Taillefer said. “Because if you can understand the force, you can strengthen the force.”

Researchers confirmed that charge density waves are a ubiquitous phenomenon in cuprates and that they strenuously oppose superconductivity, prevailing as the temperature rises.

By applying Sachdev’s algorithm to a new round of data, Davis and his group mapped out the structure of the charge density waves, showing that the d-wave distribution of electrons was, indeed, their source.

“The paper establishes that the two patterns are the same,” Sachdev said. “It just works beautifully.”

The results fully confirmed the earlier report by Damascelli, Comin and their co-authors, which used X-ray data to reveal the same d-wave form of the charge density waves. Although Damascelli’s group reached the milestone first, Davis says his team went further. “Basically they published indirect evidence for the same state as we have visualized directly,” he said.

The waves’ structure is particularly suggestive, researchers say, because superconducting pairs of electrons also have a d-wave configuration. It’s as if both arrangements of electrons were cast from the same mold. “Until a few months ago my thought was, OK, you have charge density waves, who cares? What’s the relevance to the high-temperature superconductivity?” Damascelli said. “This tells me these phenomena feed off the same interaction.”

Many theorists believe both phenomena are caused by a quantum mechanical effect called antiferromagnetism, a tendency in some materials for neighboring electrons to spin in opposite directions. The effect sets up a chessboard pattern of electrons spinning upward and downward. Just as squares along diagonals on a chessboard have the same color, electrons positioned along 45-degree angles spin the same way.

Antiferromagnetism has long been considered one of the likeliest agents to be responsible for high-temperature superconductivity. Proponents of the idea argue that the force coupling electrons is essentially an attraction between oppositely spinning neighbors. This explains why the electron pairs always form along the cardinal directions in the crystal lattice but never along the diagonals — another d-wave arrangement. This well-known d-wave nature of superconductivity is slightly different from that of charge density waves. But in a theory that Sachdev and his collaborators have developed, “we find that the two d-waves are indeed linked to each other.”

Even if Sachdev’s theory (or some other) turns out to be correct, it remains to be seen whether materials scientists can find a way to significantly turn up the heat on superconductivity. It might simply be impossible. But in recent years, these scientists have proved remarkably successful at tweaking the knobs on nature’s raw materials. “They are usually miraculous at doing this,” Davis said.

Sachdev’s theory makes a suggestion. It indicates that the two types of pairs, electron-electron and electron-hole, are equally likely consequences of antiferromagnetism, so changing the strength of that interaction won’t help superconductivity dominate over charge density waves. But there are other differences between the pairs — for instance, electron-hole pairs move more sluggishly through the cuprate lattice — and altering a certain property of the material would kill off these slow movers. How to tweak this property is, Sachdev said, “obviously the question we are thinking about.”

Other researchers have their own ideas about how to increase the temperature range across which superconductivity dominates over charge density waves. Some refused to divulge their approaches. “The prize is so big,” said Hoffman, explaining the competitiveness of the field. “If somebody finds a room-temperature superconductor, that’s huge, in terms of personal fame, in terms of gifts to humanity, in terms of prestige and legacy.”

The biggest advantage of elevating superconductivity to room temperature would be accessibility. Just as the laser and computers unexpectedly yielded the Internet, many uses of superconductivity are probably still unknown. The point is to make the technology available and see what happens. “Right now there are a bunch of highly specialized guys in the lab fooling around with superconductivity,” Taillefer said. “That’s not what you want. You want the whole planet fooling around with superconductivity.”

SOURCES – Wired, Arxiv

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