After 15 years of research, scientists reveal the fourth signature of superconducting transitions

Artistic interpretation of the superconducting transition. Credit: SLAC National Accelerator Laboratory

The results cap 15 years of detective work to understand how these materials transition to a superconducting state where they can conduct electricity without loss.

35 years ago, researchers were thrilled when an exciting new class of superconducting materials was discovered.

These copper oxides or cuprates, like other superconductors, conducted electricity without resistance or loss when cooled below a specific degree – but at temperatures significantly higher than scientists expected. . This increased the possibility of operating them at temperatures near room temperature for power lines and other perfectly efficient uses.

The research quickly confirmed that it highlighted two additional classic features of the transition to a superconducting state. The material expelled magnetic fields when superconductivity occurred, allowing a magnet placed on a piece of the material to hover above the surface. And during the transition, their heat capacity – the amount of heat needed to raise their temperature by a certain amount – showed a noticeable anomaly.

But despite decades of effort with a variety of experimental tools, the fourth signature, which can only be seen on a microscopic scale, has remained elusive: the way electrons pair up and condense into some sort of soup of electrons when the material changes from its normal state. to a superconducting state.

Now, a research team from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University has finally revealed this fourth signature with precise, high-resolution measurements made with Angular-Resolved Photoemission Spectroscopy, or ARPES, which uses light to eject electrons from material. Measuring the energy and momentum of these ejected electrons reveals the behavior of the electrons inside the material.

Four classic Superconductor SLAC signatures

How do you know if a material is a superconductor? Four classic signatures are shown here. From left to right: 1) It conducts electricity without resistance when cooled below a certain temperature. 2) It expels magnetic fields, so a magnet placed on it will levitate. 3) Its heat capacity – the amount of heat required to raise its temperature by a given amount – exhibits a distinctive anomaly when the material transitions to a superconducting state. 4) And at this same transition point, its electrons pair up and condense into a kind of electron soup that allows current to flow freely. Now experiments at SLAC and Stanford have captured this fourth signature in cuprates, which become superconductors at relatively high temperatures, and have shown that it occurs in two distinct stages and at very different temperatures. Knowing how it happens in great detail suggests a very practical new direction for research into these enigmatic materials. Credit: Greg Stewart, SLAC National Accelerator Laboratory

In a recent article published in Naturethe team confirmed that the cuprate material they studied, known as Bi2212, made the transition to a superconducting state in two distinct steps and at very different temperatures.

“Now we know what happens during the superconducting transition in great detail, and we can think about how to achieve this at higher temperatures,” said Sudi Chen, who led the study while was a graduate student at Stanford. “It’s a very practical direction.”

Stanford Professor Zhi-Xun Shen, a research fellow at the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC who supervised the research, said: “This is the culmination of 15 years scientific detective work to try to understand the electronic structure of these materials, and it provides the missing link for a holistic picture of unconventional superconductivity. We knew these materials should produce distinctive spectroscopic signatures when paired electrons coalesce into a quantum condensate; what is amazing is that it took so long to find it.

Unconventional transitions

In conventional superconductors, discovered in 1911, electrons overcome their mutual repulsion and form so-called Cooper pairs, which immediately condense into a sort of soup of electrons that allows electric current to flow unhindered.

But in unconventional cuprates, scientists have hypothesized that electrons pair up at one temperature but don’t condense until they’re cooled to a significantly lower temperature. only then does the material become superconducting.

While the details of this transition had been explored with other methods, until now it had not been confirmed with microscopic probes like photoemission spectroscopy which study how matter absorbs light and emits electrons. It is an important independent measure of the behavior of electrons in the material.

Shen began his scientific career at Stanford just as the discovery of the new cuprate superconductors was coming to light, and he has devoted more than three decades to unlocking their secrets and improving photoemission spectroscopy as a tool to do so.

For this study, cuprate samples made by collaborators in Japan were examined at two ARPES facilities – one in Shen’s lab at Stanford, equipped with an ultraviolet laser, and the other at the radiation light source. Stanford Synchrotron (SSRL) at SLAC with help from SLAC staff scientists and long-time collaborators Makoto Hashimoto and Donghui Lu.

Stanford SLAC synchrotron radiation light source

Stanford Professor Zhi-Xun Shen (center) and SLAC staff scientists Makoto Hashimoto (left) and Donghui Lu are seen in early 2020 on a Stanford synchrotron radiation beamline at SLAC. Experiments here and at Stanford, led by Stanford doctoral student Sudi Chen (not pictured), have revealed the long-sought fourth signature of the superconducting transition – the point where pairs of electrons begin to conduct electricity without loss – in a cuprate material. Credit: Jacqueline Ramseyer Orrell/SLAC National Accelerator Lab

Peel a physics onion

“Recent improvements in the overall performance of these instruments have been an important factor in achieving these high quality results,” Hashimoto said. “They allowed us to measure the energy of the ejected electrons with more precision, stability and consistency.”

Lu added: “It is very difficult to have a complete understanding of the physics of high-temperature superconductivity. Experimenters use different tools to probe different aspects of this difficult problem, which provides deeper insights.

Shen said studying these unconventional materials long-term has been like peeling layers of an onion to reveal the surprising and interesting physics within. Now, he said, confirming that the transition to superconductivity occurs in two distinct stages “gives us two knobs that we can adjust to cause materials to superconduct at higher temperatures.”

Sudi Chen is now a postdoctoral fellow at the University of California, Berkeley. Researchers from the National Institute of Advanced Industrial Science and Technology in Japan, the Lorentz Institute for Theoretical Physics at Leiden University in the Netherlands, and the DOE’s Lawrence Berkeley National Laboratory also contributed to this work, which was funded by the DOE Office of Science. SSRL is a user installation of the DOE Office of Science.

Reference: “Unconventional spectral signature of Tvs in a pure d-wave superconductor by Su-Di Chen, Makoto Hashimoto, Yu He, Dongjoon Song, Jun-Feng He, Ying-Fei Li, Shigeyuki Ishida, Hiroshi Eisaki, Jan Zaanen, Thomas P. Devereaux, Dung-Hai Lee, Dong-Hui Lu and Zhi-Xun Shen, January 26, 2022, Nature.
DOI: 10.1038/s41586-021-04251-2

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