Complementary X-Ray Techniques at NSLS-II Provide New Insights

A team led by researchers at the National Synchrotron Light Source II (NSLS-II), a US Department of Energy (DOE) Office of Science user facility at Brookhaven National Laboratory, used complementary X-ray techniques at two separate beamlines to acquire new insights into these materials. The results were reported in Physical Review Letters.

Nickelate materials, known as “infinite-layer”, have tremendous potential as high-temperature superconductors due to their unusual crystal and electronic structures. However, researchers continue to face challenges in studying these materials, as they have only been produced as thin films and then “capped” with a protective layer, which may affect the properties of the nickelate layered system.

New Discoveries in a Long History

Superconductivity in mercury was discovered more than a century ago. Superconducting materials allow current to flow through them without resistance, resulting in no power loss. As these materials become superconducting, the persistent electric current enables them to emit a magnetic field and levitate over magnetic objects.

Initially, superconducting characteristics appeared only at extremely low temperatures – -415 degrees Fahrenheit.

In the mid-1980s, researchers discovered that copper-based oxide compounds, or “cuprates,” can exhibit similar qualities at -297.7 degrees Fahrenheit. This sparked research into “high-temperature” superconductivity and the search for cuprate-like high-temperature superconductors.

If researchers can engineer materials to superconduct at higher, more realistic temperatures, they may eventually help eliminate energy losses in the power grid and open the door for other innovative technologies like high-capacity energy storage for electric vehicles, more efficient MRI machines, and maglev trains.

Nickel-based materials have lately gained attention as a new class of high-temperature superconductors similar to cuprates. When strontium is introduced into the structure of neodymium nickelate, it becomes very intriguing. The compound is characterized as an “infinite layer nickelate” because nickel atoms are arranged in a two-dimensional square lattice that repeats indefinitely in two dimensions, hence the name “infinite.”

Until now, superconductivity in nickelates has only been discovered in very thin sheets. This raises the question of whether the superconducting properties are determined by interactions at the nickelate material's interfaces with its substrate or capping layer. Early research yielded different results regarding the characteristics of these materials.

This system is sensitive to water and oxygen, so past studies used a very thin protective capping layer and attributed electronic orders to the lack of a thick surface layer. Given how sensitive these systems are, small changes or defects can also affect the material’s properties. We wanted to see how much of a role this capping layer was playing and what signals may be spurious.

Jonathan (Johnny) Pelliciari, Beamline Scientist, NSLS-II

To answer this question, the team used two NSLS-II beamlines to analyze high-quality nickelate thin film samples with and without a strontium titanate capping layer to see if the layer affected magnetic and electrical properties. Magnetic characteristics are important because they affect the material’s intrinsic electronic structure, which is directly related to its superconductivity.

Complementary Techniques Complete the Picture

NSLS-II's Coherent Soft X-ray Scattering (CSX) beamline provides researchers with a thorough insight into a material’s structural properties through resonant elastic X-ray scattering (REXS). This section of the experiment showed the atomic and electronic structures of infinite-layer nickelate thin films.

At the SIX beamline, resonant inelastic X-ray scattering (RIXS) was used to measure how X-rays lose energy when they scatter off the films. By measuring electron and spin density, mobility, and interactions, researchers gained useful insight into processes connected to the material’s electrical and magnetic properties.

Combining various perspectives revealed a complete picture of how the material behaved, particularly any alterations caused by capping. The group discovered that the material’s magnetic fluctuations, or “spin excitations,” occur whether or not the capping layer is applied, indicating that magnetism is an inherent property of these nickelates.

These magnetic properties in capped samples are only slightly greater due to interfacial effects, which could be caused by minor structural changes at the interface where the capped layer meets the nickelate, crystal defects, or lattice disorder. The research also indicated that spin excitations in these materials remain stable in the superconducting phase, as seen in cuprates.

RIXS is very sensitive to magnetism. Perhaps the most important finding of this research is the evolution of the spin wave in the presence or absence of the capping layer, which points to the magnetic and superconducting properties being intrinsic to the infinite layer nickelate material.

Shiyu Fan, Study Lead Author and Postdoctoral Researcher, SIX

“The similarity between copper oxide planes in superconducting cuprates and nickel oxide planes in nickelates have had scientists searching for superconductivity in nickelates for 25 years,” added Claudio Mazzoli, lead beamline scientist at CSX.

Mazzoli added, “Now that it has finally been found, we need to understand the differences and commonalities in these two cases and the physics behind them to gain control of this fascinating phenomenon for technological applications.”

The DOE Office of Science funded the research and facilities used.

Journal Reference:

Fan, S., et. al. (2024) Capping Effects on Spin and Charge Excitations in Parent and Superconducting Nd_1-xSr_xNiO₂. Physical Review Letters. doi.org/10.1103/PhysRevLett.133.206501