Superconductors – metals in which electricity flows without resistance – hold promise as the defining material of the near future, according to physicist Brad Ramshaw at Cornell University. They are already used in medical imaging machines, drug discovery research and quantum computers being built by Google and IBM.
However, the super-low temperatures – a few degrees above absolute zero – that conventional superconductors need to function make them too expensive for widespread use.
As part of their quest to find more useful superconductors, Ramshaw, assistant professor of physics in the College of Arts and Sciences, and colleagues have now discovered that magnetism is key to understanding the behavior of electrons in so-called ‘high-temperature’ superconductors.
With this finding, the researchers have solved a 30-year-old mystery surrounding this class of superconductors, which can function at much higher temperatures than conventional superconductors – more than 100K above absolute zero. They report their findings in a paper in Nature Physics.
“We’d like to understand what makes these high-temperature superconductors work and engineer that property into some other material that is easier to adopt in technologies,” Ramshaw said.
A central mystery to high-temperature superconductors is what happens with their electrons. “All metals have electrons, and when a metal becomes a superconductor, the electrons pair up with each other,” Ramshaw explained. “We measure something called the ‘Fermi surface’, which you can think of as a map showing where all the electrons are in a metal.”
To study how electrons pair up in high-temperature superconductors, researchers continuously change the number of electrons through a process known as chemical doping. But in high-temperature superconductors, at a certain ‘critical point’, electrons seem to vanish from the Fermi surface map, Ramshaw said.
The researchers zeroed in on this critical point to figure out what makes the electrons vanish, and where they go. Using the strongest steady-state magnet in the world – the 45-tesla hybrid magnet at the National High Magnetic Field Laboratory in Tallahassee, Florida – they measured the Fermi surface of a copper-oxide high temperature superconductor as a function of electron concentration, right around the critical point.
This revealed that the Fermi surface changes completely as the researchers dial past the critical point.
“It’s as if you were looking at a real map and all of a sudden most of the continents just disappeared,” 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, vanish.”
It was important for the researchers to note not just that electrons were vanishing, but which ones in particular. So they built different simulation models based on several theories and tested whether these models could explain the data, said Yawen Fang, a doctoral student in physics and lead author of the paper.
“In the end, we have a winning model, which is the one associated with magnetism,” Fang said. “We are stepping confidently from the well-understood side of the material, benchmarking our technique, into the mysterious side past the critical point.”
As well as knowing which electrons vanish, the researchers also now have an idea why – and it has to do with magnetism.
“There have always been hints that magnetism and superconductivity are related in high-temperature superconductors, and our work shows that this magnetism seems to appear right at the critical point and gobble up most of the electrons,” Ramshaw said. “This critical point also marks the electron concentration where the superconductivity happens at the highest temperatures, and higher-temperature superconductors are the goal here.”
Knowing that the critical point is associated with magnetism offers insight into why these particular superconductors have such high transition temperatures, Ramshaw said, and maybe even where to look to find new ones with even higher transition temperatures.
“It is a 30-year-old debate that precedes our study, and we came up with a straightforward answer,” said Gaël Grissonnanche, a postdoctoral fellow with the Kavli Institute at Cornell for Nanoscale Science and co-first author of the paper.
This story is adapted from material from Cornell University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.
The Fermi surface on the left shows the arrangement of electrons in a copper-oxide high temperature superconductor before the ‘critical point’. After the critical point, most of the electrons vanish, as shown in the Fermi surface on the right. Image: Cornell University.