First study of nickelate's magnetism finds a strong kinship with cuprate superconductors

Date:
July 8, 2021
Source:
DOE/SLAC National Accelerator Laboratory
Summary:
Are new nickelate superconductors close kin to the original high-
temperature superconductors, the cuprates? The first study of their
magnetic properties says the answer is yes. Scientists have found
important similarities but also subtle differences between the two.



FULL STORY ==========================================================================
Ever since the 1986 discovery that copper oxide materials, or cuprates,
could carry electrical current with no loss at unexpectedly high
temperatures, scientists have been looking for other unconventional superconductors that could operate even closer to room temperature. This
would allow for a host of everyday applications that could transform
society by making energy transmission more efficient, for instance.


========================================================================== Nickel oxides, or nickelates, seemed like a promising candidate. They're
based on nickel, which sits next to copper on the periodic table, and the
two elements have some common characteristics. It was not unreasonable
to think that superconductivity would be one of them.

But it took years of trying before scientists at the Department of
Energy's SLAC National Accelerator Laboratory and Stanford University
finally created the first nickelate that showed clear signs of superconductivity.

Now SLAC, Stanford and Diamond Light Source researchers have made the
first measurements of magnetic excitations that spread through the
new material like ripples in a pond. The results reveal both important similarities and subtle differences between nickelates and cuprates. The scientists published their results in Science today.

"This is exciting, because it gives us a new angle for exploring how unconventional superconductors work, which is still an open question
after 30- plus years of research," said Haiyu Lu, a Stanford graduate
student who did the bulk of the research with Stanford postdoctoral
researcher Matteo Rossi and SLAC staff scientist Wei- Sheng Lee.



========================================================================== "Among other things," he said, "we want to understand the nature of the relationship between cuprates and nickelates: Are they just neighbors,
waving hello and going about their separate ways, or more like cousins
who share family traits and ways of doing things?" The results of this
study, he said, add to a growing body of evidence that their relationship
is a close one.

Spins in a checkerboard Cuprates and nickelates have similar structures,
with their atoms arranged in a rigid lattice. Both come in thin, two-dimensional sheets that are layered with other elements, such as
rare-earth ions. These thin sheets become superconducting when they're
cooled below a certain temperature and the density of their free-flowing electrons is adjusted in a process known as doping.

The first superconducting nickelate was discovered in 2019 at SLAC and Stanford. Last year, the same SLAC/Stanford team that performed this
latest experiment published the first detailed study of the nickelate's electronic behavior. That study established that in undoped nickelate, electrons flow freely in nickel oxide layers, but electrons from the intervening layers also contribute electrons to the flow. This creates a
3D metallic state that's quite different from what is seen in cuprates,
which are insulators when undoped.



========================================================================== Magnetism is also important in superconductivity. It's created by the
spins of a material's electrons. When they're all oriented in the same direction, either up or down, the material is magnetic in the sense that
it could stick to the door of your fridge.

Cuprates, on the other hand, are antiferromagnetic: Their electron
spins form a checkerboard pattern, so each down spin is surrounded by
up spins and vice versa. The alternating spins cancel each other out,
so the material as a whole is not magnetic in the ordinary sense.

Would nickelate have those same characteristics? To find out,
researchers took samples of it to the Diamond Light Source synchrotron
in the UK for examination with resonant inelastic X-ray scattering,
or RIXS. In this technique, scientists scatter X-ray light off a sample
of material. This injection of energy creates magnetic excitations --
ripples that travel through the material and randomly flip the spins
of some of its electrons. RIXS allows scientists to measure very weak excitations that couldn't be observed otherwise.

Creating new recipes "What we find is quite interesting," Lee said. "The
data show that nickelate has the same type of antiferromagnetic
interaction that cuprates have. It also has a similar magnetic energy,
which reflects the strength of the interactions between neighboring
spins that keep this magnetic order in place. This implies that the same
type of physics is important in both." But there are also differences,
Rossi noted. Magnetic excitations don't spread as far in nickelates, and
die out more quickly. Doping also affects the two materials differently;
the positively charged "holes" it creates are concentrated around nickel
atoms in nickelates and around oxygen atoms in cuprates, and this affects
how their electrons behave.

As this work continues, Rossi said, the team will test how doping the
nickelate in various ways and swapping different rare earth elements
into the layers between the nickel oxide sheets affect the material's superconductivity - - paving the way, they hope, to discovery of better superconductors.

========================================================================== Story Source: Materials provided by
DOE/SLAC_National_Accelerator_Laboratory. Original written by Glennda
Chui. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. H. Lu, M. Rossi, A. Nag, M. Osada, D. F. Li, K. Lee, B. Y. Wang, M.

Garcia-Fernandez, S. Agrestini, Z. X. Shen, E. M. Been, B. Moritz,
T. P.

Devereaux, J. Zaanen, H. Y. Hwang, Ke-Jin Zhou, W. S. Lee. Magnetic
excitations in infinite-layer nickelates. Science, 2021 DOI:
10.1126/ science.abd7726 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210708170340.htm

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