ITHACA, NY – Superconductors – metals in which electricity flows without resistance – show promise as the defining material of the near future, according to physicist Brad Ramshaw, and are already being used in medical imaging machines, research on drug discovery and quantum computers built by Google and IBM.
However, the extremely low temperatures that conventional superconductors need to operate – a few degrees above absolute zero – make them too expensive for large-scale use.
In their quest to find more useful superconductors, Ramshaw, Dick & Dale Reis Johnson Assistant Professor of Physics at the College of Arts and Sciences (A&S), and his colleagues have discovered that magnetism is key to understanding the behavior of electrons in temperatures » superconductors. With this discovery, they solved a 30-year-old mystery surrounding this class of superconductors, which operate at much higher temperatures, over 100 degrees above absolute zero. Their paper, “Fermi Surface Transformation at the Pseudogap Critical Point of a Cuprate Superconductor,” published in Nature Physics on March 10.
“We’d like to understand what makes these high-temperature superconductors work and incorporate that property into another material that’s easier to adopt in technologies,” Ramshaw said.
A central mystery of high-temperature superconductors is what happens to their electrons, Ramshaw said.
“All metals have electrons, and when a metal becomes a superconductor, the electrons pair up with each other,” he said. “We measure something called the ‘Fermi surface,’ which you can think of as a map of where all the electrons are in a metal.”
To study how electrons pair up in high-temperature superconductors, researchers constantly change the number of electrons through a process called chemical doping. In high-temperature superconductors, at a certain “critical point,” electrons seem to disappear from the Fermi surface map, Ramshaw said.
The researchers focused on this critical point to understand what makes the electrons disappear and where they go. They used the world’s most powerful steady-state magnet, the 45 Tesla Hybrid Magnet from the National High Magnetic Field Laboratory in Tallahassee, Florida, to measure the Fermi surface of a high-temperature carbon oxide superconductor. copper as a function of electron concentration. just around the critical point.
They discovered that the Fermi surface changes completely when the researchers pass the critical point.
“It’s like looking at a real map and all of a sudden most of the continents are gone,” Ramshaw said. “That’s what we found happens to the Fermi surface of high-temperature superconductors at the critical point – most of the electrons in a particular region, a particular part of the map, disappear.”
It was important for the researchers to note not just which electrons were disappearing, but which ones in particular, Ramshaw said.
They built different simulation models based on several theories and tested whether they could explain the data, said Yawen Fang, a doctoral student in physics and lead author of the paper.
“At the end of the day, we have a winning model, which is the one associated with magnetism,” Fang said. “We move confidently from the well-understood side of the material, comparing our technique, to the mysterious side beyond the critical point.”
Now that they know which electrons are disappearing, the researchers have an idea why – it has to do with magnetism.
“There have always been hints that magnetism and superconductivity are linked in high-temperature superconductors, and our work shows that this magnetism appears to appear right at the critical point and gobble up most of the electrons,” Ramshaw said. “This critical point also marks the concentration of electrons where superconductivity occurs at the highest temperatures, and higher temperature superconductors are the goal here.”
Knowing that the critical point is associated with magnetism provides insight into why these particular superconductors have such high transition temperatures, Ramshaw said, and perhaps even where to look to find new ones with even higher transition temperatures.
“It’s a 30-year-old debate that predates our study, and we’ve found a simple answer,” said Gaël Grissonnanche, postdoctoral fellow at Cornell’s Kavli Institute for Nanoscale Science and co-first author.
This research was funded in part by the National Science Foundation, the Azrieli Global Scholars Program at the Canadian Institute for Advanced Research, and the Kavli Institute for Nanoscale Science at Cornell.
For more information, see this Cornell Chronicle story.
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Transformation of the Fermi surface at the critical point of the pseudogap of a cuprate superconductor
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