Peeling back the layers: investigating the capping effect on nickelate superconductors

NSLS-II beamline scientists (top row, left to right) Jonathan Pelliciari, Claudio Mazzoli, (bottom row, left to right) Andi Barbour, Valentina Bisogni, Shiyu Fan, Vivek Bhartiya, Taehun Kim. Credit: Kevin Coughlin/Brookhaven National Laboratory
So-called “infinite layer” nickelate materials, characterized by unique crystalline and electronic structures, show great potential as high-temperature superconductors. Studying these materials remains difficult for researchers. They were synthesized as thin films and simply “capped” with a protective layer that could change the properties of the nickelate layered system.
To address this challenge, a team led by researchers from the National Synchrotron Light Source II (NSLS-II), an Office of Science user facility at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, has assembled two facilities. used complementary X-ray techniques. A variety of beamlines are available to gain new insights into these materials. Their results were published in Physical Review Letters.
New discoveries in a long history
Superconductivity was first discovered in mercury over 100 years ago. Superconducting materials can conduct current without resistance, so there is no power loss. When these materials become superconducting, a persistent current releases a magnetic field that allows them to float even on magnetic materials.
Initially, superconducting properties appeared to only emerge at extremely low temperatures of -415°F. However, in the mid-1980s, researchers discovered that copper-based oxide materials, or “copper oxides,” could exhibit these properties at -297.7°F. This spearheaded the study of “high temperature” superconductivity and the search for other cuprate-like high temperature superconductors.
If researchers can find a way to manipulate materials to become superconducting at more practical temperatures, they could one day eliminate energy loss in the power grid and create maglev trains, more efficient MRI machines, and high-speed maglev trains. It may help pave the way for other new technologies such as -Energy storage capacity for electric vehicles.
Recently, nickel-based materials have attracted attention as a new class of high-temperature superconductors similar to copper oxides. Neodymium nickelate becomes particularly interesting when strontium is added to its structure. This compound is known as an “infinite layer nickelate.” Nickel atoms are arranged in a two-dimensional square lattice that repeats infinitely in two dimensions, giving it its nickname “infinite.”
Superconductivity in nickelates has so far been observed only in very thin films. This raises the question whether the superconducting properties depend on the interaction at the interface between the nickelate material and its substrate or capping layer. Early studies yielded conflicting results regarding the properties of these materials.


The crystal structure of NdNiO2 is shown both capped (left) and uncapped (right). Credit: American Physical Society/Brookhaven National Laboratory
“This system is sensitive to water and oxygen,” explains Jonathan (Johnny) Periciari, a scientist at NSLS-II’s Soft Inelastic X-ray Scattering (SIX) beamline. This is due to the lack of a thick surface layer.
“Given how sensitive these systems are, even small changes or defects can affect material properties. We are wondering how much of a role this capping layer plays and how much I wanted to know if such a signal could be spurious.
To answer this question, the team used two beamlines at NSLS-II to investigate high-quality nickelate thin film samples with and without a strontium titanate capping layer, which is magnetically We checked whether it affects the properties and electronic properties. Magnetic properties are important because they relate to the material’s intrinsic electronic structure and are directly related to superconductivity.
Complete the picture with complementary techniques
Resonant elastic X-ray scattering (REXS), performed at NSLS-II’s coherent soft X-ray scattering (CSX) beamline, provides researchers with a detailed view of the structural properties of materials. This part of the experiment revealed the atomic and electronic structure of the infinite-layer nickelate thin film. They then measured how X-rays lose energy as they scatter from the film using resonant inelastic X-ray scattering (RIXS) performed at the SIX beamline.
By analyzing the density, motion, and interaction of electrons and spins, researchers have gained valuable insight into processes related to electronic and magnetic properties within materials.
Combining these perspectives gave us a complete picture of how the material behaves, especially the changes introduced by capping. The group found that magnetic fluctuations, or “spin excitations,” in the material exist regardless of whether a capping layer is applied, indicating that magnetism is an inherent property of these nickelates. Ta.
In the capped sample, these magnetic properties are slightly stronger due to interfacial effects. This may be due to slight structural adjustments at the interface where the capped layer contacts nickelate, crystal defects, or lattice disturbances. The data also confirmed that spin excitations in these materials are stable in the superconducting phase, similar to what is seen in cuprates.
“RIXS is very sensitive to magnetism,” said Shiyu Fan, a postdoctoral researcher at SIX and lead author of the study. “Perhaps the most important finding of this study is the change in spin waves with and without the capping layer, which points to the magnetic and superconducting properties unique to infinite layer nickelate materials.”
“The similarities between the copper oxide surface of superconducting cuprates and the nickel oxide surface of nickelates have led scientists to study superconductivity in nickelates for 25 years,” said CSX lead beamline scientist Claudio Mazzoli. I have done so,” he said.
“Now that it has finally been discovered, we need to understand the differences and similarities between these two cases and the physics behind them in order to control this fascinating phenomenon for technological applications.”
Further information: S. Fan et al. Capping effects on spin and charge excitations in parent and superconducting Nd1−xSrxNiO2, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.206501. For arXiv: DOI: 10.48550/arxiv.2409.18258
Provided by Brookhaven National Laboratory
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