Study shows that nickelate superconductors are intrinsically magnetic

Electrons find each other repulsive. Nothing personal – it’s just that their negative charges repel each other. So to get them to mate and travel together, as they do in superconducting materials, requires a little nudge.

In old-school superconductors, discovered in 1911, which conduct electricity without resistance, but only at extremely cold temperatures, the impetus comes from vibrations in the material’s atomic lattice.

But for newer, “unconventional” superconductors — which are particularly exciting for their potential to work for things like lossless power transmission at near room temperature — no one knows exactly what the push is, although researchers think they could be strips of electric charge, waves of flipping electron spins creating magnetic excitations, or a combination of things.

Hoping to learn more by looking at the problem from a slightly different angle, researchers at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory synthesized another unconventional family of superconductors — the nickel oxides, or nickelates. Since then they have spent three years studying the properties of the nickelates and comparing them to one of the most famous unconventional superconductors, the copper oxides or cuprates.

And in an article published in natural physics Today, the team reported a significant difference: Unlike cuprates, nickelates have their magnetic fields always on.

Magnetism: friend or foe?

Nickelates, the scientists said, are intrinsically magnetic, as if each nickel atom were clutching a tiny magnet. This is true whether the nickelate is in its non-superconducting or normal state, or in a superconducting state where electrons have paired up and formed a sort of quantum soup that can host intertwined phases of quantum matter. Cuprates, on the other hand, are not magnetic in their superconducting state.

“This study examined the fundamental properties of the nickelates compared to the cuprates and what that can tell us about unconventional superconductors in general,” said Jennifer Fowlie, a postdoctoral fellow at SLAC’s Stanford Institute for Materials and Energy Sciences (SIMES) who led the study experiments.

Some researchers believe magnetism and superconductivity compete with each other in this type of system, she said; others think there can be no superconductivity unless magnetism is nearby.

“While our results don’t answer that question, they do indicate where more work likely needs to be done,” Fowlie said. “And they mark the first time magnetism has been studied in both the superconducting and normal states of nickelates.”

Harold Hwang, professor at SLAC and Stanford and director of SIMES, said, “This is another important piece of the puzzle that is binding the research community together as we work to uncover the properties and phenomena at the heart of these exciting materials.”

Enter the muon

Few things are simple 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 resemblance to the cuprates made it likely that they could host superconductivity, nickelates are so difficult to make that it took the SLAC and Stanford team years of trying to succeed.

Even then, they could only make thin films of the material — not the thicker chunks needed to study its properties using traditional techniques. A number of research groups around the world have been working on simpler ways to synthesize nickelates in some form, Hwang said.

Therefore, the research team turned to a more exotic method called low-energy muon spin rotation/relaxation, which can measure the magnetic properties of thin films and is only available at the Paul Scherrer Institute (PSI) in Switzerland.

Muons are fundamentally charged particles similar to electrons but 207 times heavier. They only last for 2.2 millionths of a second before decaying. Positively charged muons, which are often preferred for such experiments, decay into a positron, a neutrino, and an antineutrino. Like their electron cousins, they spin like tops, changing the direction of their spin in response to magnetic fields. But they can only “feel” these fields in their immediate vicinity – up to a distance of about a nanometer, or a billionth of a meter.

At PSI, scientists use a muon beam to embed the small particles in the material to be examined. As the muons decay, the positrons they produce fly in the direction the muon is spinning. By tracing the positrons back to their origin, the researchers can see which way the muons were pointing when they disappeared, and thus determine the overall magnetic properties of the material.

find workaround

The SLAC team applied to experiment with the PSI system in 2020, but then the pandemic made entry and exit to Switzerland impossible. 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. So she launched the first round of experiments in Switzerland with a team led by Andreas Suter, senior scientist at PSI and an expert in extracting information about superconductivity and magnetism from muon decay data.

Upon arriving at SLAC in May 2021, Fowlie immediately began preparing different types of nickelate compounds for the team to test in their second round of experiments. When travel restrictions were lifted, the team was finally able to return to Switzerland to complete the study.

The unique experimental setup at PSI enables the scientists to embed muons precisely deep into the nickelate materials. From this they were able to determine what was going on in each wafer-thin layer of different nickelate compounds with slightly different chemical compositions. They discovered that only the layers containing nickel atoms were magnetic.

Interest in the nickelates is very high around the world, Hwang said. Half a dozen research groups have published their own methods for synthesizing nickelates and are working to improve the quality of the samples they study, and a large number of theorists are trying to generate insights to steer the research in productive directions.

“We’re trying to do what we can with the resources that we have as a research community,” he said, “but there’s a lot more we can learn and do.”