Study finds nickelate superconductors are inherently magnetic
“This study looked at the fundamental properties of nickelates compared to cuprates, and what that can tell us about unconventional superconductors in general,” said Jennifer Fowlie, postdoctoral researcher at the Stanford Institute for Materials and Energy Sciences (SIMES) of the SLAC who conducted the experiments.
Some researchers believe that magnetism and superconductivity compete in this type of system, she said. others think you can’t have superconductivity unless magnetism is nearby.
“While our results don’t settle this question, they do highlight areas where more work probably needs to be done,” Fowlie said. “And they mark the first time that magnetism has been examined in both the superconducting state and the normal nickelate state.”
Harold Hwang, Professor at SLAC and Stanford and Director of SIMES, said: “This is another important piece of the puzzle that the research community is putting together as we work to frame the properties and the phenomena at the heart of these exciting materials.
Enter the muon
Few things are easy in this area of research, and the study of nickelates has been more difficult than most.
While theorists predicted more than 20 years ago that their chemical similarity to cuprates made it likely that they could harbor superconductivity, nickelates are so difficult to manufacture that it took years of testing before the SLAC and Stanford team does not succeed.
Even then, they could only make thin films of the material – not the thicker chunks needed to explore its properties with common techniques. A number of research groups around the world have been working on easier ways to synthesize nickelates in all their forms, Hwang said.
The research team therefore turned to a more exotic method, called spin-relaxation of low-energy muons, which can measure the magnetic properties of thin films and which is only available at the Institut Paul Scherrer (PSI) in Switzerland.
Muons are fundamental charged particles similar to electrons, but 207 times more massive. They remain only 2.2 millionths of a second before breaking down. Positively charged muons, which are often preferred for experiments like these, decay into a positron, a neutrino, and an antineutrino. Like their electronic cousins, they spin like tops and change the direction of their rotation in response to magnetic fields. But they can only “sense” these fields in their immediate surroundings – up to about a nanometer, or a billionth of a meter, away.
At PSI, scientists use a beam of muons to incorporate small particles into the material they want to study. When muons decay, the positrons they produce fly away in the direction of the muon’s rotation. By tracing the positrons back to their origins, researchers can see which direction the muons were pointing when they disappeared and thus determine the material’s overall magnetic properties.
Find a workaround
The SLAC team asked to experiment with the PSI system in 2020, but then the pandemic made it impossible to travel in or out of Switzerland. Fortunately, Fowlie was a postdoc at the University of Geneva at the time and was already planning to come to SLAC to work in Hwang’s group. She therefore launched the first series of experiments in Switzerland with a team led by Andreas Suter, a senior scientist at PSI and an expert in extracting information on superconductivity and magnetism from muon decay data.
After arriving at SLAC in May 2021, Fowlie immediately began making various types of nickelate compounds that the team wanted to test in their second set of experiments. When travel restrictions ended, the team was finally able to return to Switzerland to complete the study.
PSI’s unique experimental setup allows scientists to embed muons at precise depths in nickelate materials. From there, they were able to determine what was going on in each ultra-thin layer of various nickelate compounds with slightly different chemical compositions. They discovered that only the layers containing nickel atoms were magnetic.
Interest in nickelates is very high around the world, Hwang said. Half a dozen research groups have published their own methods of synthesizing nickelates and are working to improve the quality of the samples they study, and a large number of theorists are trying to come up with ideas to guide research in productive directions.
“We’re trying to do what we can with the resources we have as a research community,” he said, “but there’s still a lot more we can learn and do.”
Major funding for this work came from the DOE Office of Science and the Gordon and Betty Moore Foundation’s Emergent Phenomena in Quantum Systems Initiative. Fowlie’s work is also supported by a postdoctoral fellowship from the Swiss National Science Foundation.
Quote: Jennifer Fowlie et al., Natural PhysicsAugust 1, 2022 (10.1038/s41567-022-01684-y)
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