Study finds nickelate superconductors are intrinsically magnetic

Electrons are repulsive to each other. Nothing personal, it’s just that their negative charges repel each other. So getting them to pair up and travel together, as they do with superconducting materials, takes a bit of a push.

In old-school superconductors, which were discovered in 1911 and conduct electrical current without resistance, but only at extremely fast speeds. low temperaturesthe push comes from vibrations in the material’s atomic lattice.

But in the newer “unconventional” superconductors, which are especially exciting because of their potential to operate near room temperature for things like lossless transmission of power, no one knows for sure what the push is, though the researchers think could involve stripes. of electrical charge, flip-flopping waves electron spins that create magnetic excitations, or some 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 family of unconventional superconductors: nickel oxides, or nickelates. Since then, they have spent three years researching the properties of nickel plating and comparing it to one of the most famous. unconventional superconductorscopper oxides or cuprates.

And in an article published in physics of nature today, the team reported a significant difference: unlike cuprates, the magnetic fields in nickelates are always active.

Magnetism: friend or foe?

Nickel platings, the scientists said, are inherently magnetic, as if each nickel atom were holding onto a tiny magnet. This is true whether the nickel plated it is either in its non-superconducting, or normal, state, or in a superconducting state where electrons have paired up and formed a kind of quantum soup that can host entangled phases of quantum matter. Cuprates, on the other hand, are not magnetic in their superconducting state.

“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, a postdoctoral researcher at the Stanford Institute for Materials and Energy Sciences (SIMES). from SLAC, who led the experiments

Some researchers think that magnetism and superconductivity compete with each other in this type of system, he said; others think that you can’t have superconductivity unless magnetism is nearby.

“While our results don’t answer that question, they do highlight where more work probably should be done,” Fowlie said. “And they mark the first time magnetism has been examined 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 the research community is being put together as we work to frame the properties and phenomena at the heart of these exciting materials.”

Enter the muon

Few things come easy in this field of research, and studying nickel plating 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 make that it took years of trying before the SLAC and Stanford team succeeded.

Even then, they were only able to make thin films of the material, not the thicker chunks needed to explore its properties with common techniques. Various research groups around the world have been working on easier ways to synthesize nickelates in any form, Hwang said.

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

Muons are fundamental charged particles that are similar to electrons, but 207 times more massive. They stick around for just 2.2 millionths of a second before decaying. 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, changing the direction of their spin in response to magnetic fields. But they can “sense” those fields only in their immediate surroundings, up to about a nanometer, or a billionth of a meter, away.

In PSI, scientists use a beam of muons to embed tiny particles in the material they want to study. When muons decay, the positrons they produce fly out in the direction the muon is spinning. By tracing the positrons back to their origins, the researchers can see which way the muons were pointing when they disappeared and thus determine the general magnetic properties of the material.

Find a solution

The SLAC team requested to do experiments 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 on Hwang’s group. So he started the first round of experiments in Switzerland with a team led by Andreas Suter, a senior scientist at PSI and an expert in extracting information about 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 round of experiments. When travel restrictions ended, the team was finally able to return to Switzerland to finish the study.

The unique experimental setup at PSI allows scientists to embed muons at precise depths in the nickel plating materials. From this, they were able to determine what was going on in each super-thin layer of various nickelate compounds with slightly different chemical compositions. They found that only the layers containing nickel atoms were magnetic.

Interest in nickel plating is very high around the world, Hwang said. Half a dozen research groups have published their own ways of synthesizing nickelates and are working to improve the quality of the samples they study, and a host 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 so much more we can learn and do.”