Scientists watch high-temperature superconductivity emerge from magnetism

(Phys.org) – Scientists at SLAC National Accelerator Laboratory and Stanford University have shown for the first time how high-temperature superconductivity emerges from magnetism in a pnictide of iron, a class of materials with great potential for make devices that conduct electricity with 100 percent efficiency.

In experiments at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), the team “doped” the material – one of two known types of high-temperature superconductor – by adding or subtracting electrons to enhance its superconducting abilities. Next, they used a beam of ultraviolet light to measure changes in the electronic behavior of the material as it was cooled to a temperature where superconductivity becomes possible.

The researchers saw the two states battling for dominance: At first, the material’s electrons were all aligned with their spins pointing in specific directions, a characteristic of magnetism. But when the temperature dropped, a few electrons teamed up, like dancers at a party, to effortlessly conduct electricity; then a few others; until finally all the active electrons find partners and the material is entirely superconducting, a much more complex behavior.

The results, published on April 25 in Nature Communicationare an important step towards understanding how high temperature superconductors work – information scientists need to fulfill their dream of designing superconductors with more useful properties that operate at near room temperature for a variety practical applications.

Complexity emerges from simple ingredients

“For a while, magnetism and superconductivity coexist, it’s no surprise,” said Ming Yi, graduate student at Stanford Institute of Materials and Energy Sciences (SIMES) and lead author. of the report. “But we wanted to see how these two phases interact with each other. Now we finally have the high-resolution tools we need to see these changes at a microscopic level, and we find that the same electrons that were participating in the “magnetic order have shifted to participate in the superconducting order. These two orders are competing for the same electrons.”

By comparing their experimental data with simulation results, the researchers determined that the magnetism and superconductivity in the iron pnictide were intertwined at a very fine microscopic level, rather than occupying larger, separate pools in the material. The simulations were led by Lawrence Berkeley National Laboratory theorists Lex Kemper, Stanford graduate student Nachum Plonka, and SIMES director Thomas Devereaux.

“It’s a great example of ’emergence,’ where simple ingredients give rise to complex behavior,” said co-author Zhi-Xun Shen, SLAC and Stanford professor and SLAC Science Advisor. and technology. “Emergence is a major theme in modern research on the organizing principles of nature,” he said. “Our hope is that research on quantum systems like this, which are very simple model systems, will eventually give us some insight into these organizing principles.”

Explore a mysterious material

Discovered in 1986, high-temperature superconductors carry electricity without any loss at much hotter temperatures than conventional superconductors, which must be cooled to at least 30 kelvins (minus 243 degrees Celsius). Yet scientists have failed to operate high-temperature superconductors above minus 138 degrees Celsius.

Although these materials have the potential to save money and energy in a number of applications, from transporting electricity over long-distance power lines to operating maglev trains, the high cost and the logistics of keeping them cold and their hard-to-handle properties have held their back.

As in ordinary superconductors, electrons in high-temperature superconductors form pairs to conduct current. But the mechanism behind this pairing in high-temperature materials – the “glue” that holds electrons together – is still unknown, said Donghui Lu, senior scientist at SSRL and one of the study’s lead researchers.

Another mystery: in theory, superconductivity and magnetism are not supposed to coexist; the presence of one should drive out the other. But previous studies have shown that they can actually exist in the same material, and scientists are eager to learn the details of how and why this happens.

While this study doesn’t answer those burning questions, it does allow scientists to take a closer look at the details of what happens when superconductivity emerges.

The results may also shed light on the other known family of high-temperature superconductors, the copper-based cuprates, the scientists wrote, and comparing the results of the two may lead to “a possible understanding of the mechanism of superconductivity.” unconventional”.

In addition to SLAC and SIMES, which is a joint SLAC/Stanford institute, researchers from Stanford University, Lawrence Berkeley National Laboratory, Nanjing University and the University of California-Berkeley have contributed to this work. Some measurements were made at Berkeley Lab’s Advanced Light Source. The work at Stanford, SLAC and the Advanced Light Source was funded by the US Department of Energy Office of Science.