Study finds nickel superconductors are intrinsically magnetic

Study finds nickel superconductors are intrinsically magnetic

The muon, the center, rotates like a vertex within the atomic lattice of a thin layer of superconducting nickel. These elementary particles can sense the magnetic field created by spinning electrons up to a distance of a billionth of a metre. By embedding muons in four nickel compounds at the Paul Scherer Institute in Switzerland, researchers at SLAC and Stanford discovered that the nickel they tested hosts magnetic excitation whether or not it is in its superconducting states — further evidence in the long quest to understand just how unconventional superconductors can be. Voltage without loss. Credit: Jennifer Foley/SLAC National Accelerator Laboratory

Electrons find some of them repulsive. Nothing personal – just that negative charges repel each other. So getting them to pair up and travel together, as they do in superconducting materials, requires a bit of a push.

In old-school superconductors, discovered in 1911 that conduct an electric current without resistance, but only in its extremes. cold temperaturesthe alert comes from vibrations in the atomic lattice of matter.

But in newer, “unconventional” superconductors — which are particularly exciting because of their ability to operate at near room temperature for things like lossless energy transfer — no one knows for sure what the alert is, although the researchers think It may include lines of electric charge, stirring waves electron spins That creates magnetic excitation, 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 have synthesized another unconventional family of superconductors — nickel or nickel oxides. Since then, they have spent three years investigating the properties of nickel and comparing it to one of the most famous unconventional superconductorscopper or copper oxides.

In a research published in Nature Physics Today, the team reports a major difference: Unlike copper, the magnetic fields in nickel are always on.

Magnetism: friend or foe?

Scientists said that nickel is intrinsically magnetic, as if each atom of nickel were holding a small magnet. This is true whether nickel It is in a non-superconducting, normal, or superconducting state where electrons pair up and form a kind of quantum soup that can host entangled phases of quantum matter. On the other hand, cuprates are not magnetic in their superconducting state.

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

She said that some researchers believe that magnetism and superconductivity compete with each other in this type of system; Others believe that you can’t get superconductivity unless magnetism is close to you.

“While our results do not resolve this question, they do highlight where more work is likely to be done,” Foley said. “This is the first time that magnetism has been investigated in both the normal state and the superconductivity of nickel.”

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

Enter the muon

Few things are easy in this area of ​​research, and studying nickels has been more difficult than most.

While theorists predicted more than 20 years ago that their chemical similarity to cuprate made it likely that they would be able to host superconductivity, nickel is so difficult to make that it took years of trying before the SLAC and Stanford team succeeded.

Even then, they could only make thin films of the material – not the thick pieces needed to explore its properties with common techniques. Huang said a number of research groups around the world are working on easier ways to manufacture nickel in any form.

So the research team turned to a more exotic method, called low-power myung Rotation/relaxation, it 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 that are similar to electrons, but are 207 times larger. They remain about 2.2 millionths of a second before they decay. Positively charged muons, which are often preferred in experiments like this, decay into a positron, a neutrino, and an antineutrino. Like their electron cousins, they spin like vertices and change the direction of their spin in response to magnetic fields. But they can “feel” these fields only in their immediate surroundings – about one nanometer, or one billionth of a metre.

At PSI, scientists use a beam of muons to embed small particles into the material they want to study. When muons decay, the positrons they produce fly in the direction of the muon’s rotation. By tracing the positrons back to their origins, researchers can work out which direction the muons were pointing when they disappeared, and thus determine the material’s overall magnetic properties.

Find an alternative solution

The SLAC team applied for trials of the PSI system in 2020, but then the pandemic has made it impossible to travel in or out of Switzerland. Fortunately, Foley was a postdoctoral researcher at the University of Geneva at the time and was already planning to come to SLAC to work in Hwang’s group. So the first round of experiments began in Switzerland with a team led by Andreas Suter, a senior PSI scientist and expert in extracting information about superconductivity and magnetism from muon decay data.

After arriving at SLAC in May 2021, Fowlie immediately began making different types of nickel compounds that the team wanted to test in the second round of testing. When Travel Restrictions Finished, the team was finally able to return to Switzerland to finish school.

PSI’s unique experimental setup allows scientists to embed muons at minute depths into nickel materials. With this in mind, they were able to determine what happens in each ultrathin layer of different nickel compounds with slightly different chemical compositions. They discovered that only the layers containing nickel atoms are the magnetic ones.

Huang said that interest in nickels is very high around the world. Half a dozen research groups have published their own methods for making nickel and improving the quality of the samples they study, and a large number of theorists are trying to come up with insights to guide the research in fruitful directions.

“We try to do what we can with the resources we have as a research community,” he said, “but there is still a lot we can learn and do.”

A new leap in the understanding of nickel oxide superconductors

more information:
Jennifer Foley, Intrinsic magnetism in superconducting infinite layer nickel, Nature Physics (2022). DOI: 10.1038 / s41567-022-01684-y.

the quote: Study finds that nickel superconductors are intrinsically magnetic (2022, August 1) Retrieved on August 2, 2022 from

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