A team of researchers has identified atomic distortions that may be linked with high-temperature superconductivity in a promising class of nickel-based materials, offering new insight into how next-generation superconductors might be designed.

Understanding how and why superconductivity emerges at higher temperatures in some materials remains a major challenge in physics and materials science. A new study published April 1 in Nature shows how tiny changes in atomic structure can strongly influence whether a material becomes superconducting.

The study was led by Berit Goodge, Ph.D. ’22, Minerva Group leader at the Max Planck Institute for Chemical Physics of Solids and incoming assistant professor in the School of Applied and Engineering Physics (AEP) at the Cornell Duffield College of Engineering, and Lopa Bhatt, M.S. ’24, doctoral student in AEP. They worked with a team of scientists from institutions across the U.S. and Europe to investigate why the nickel-oxide (nickelate) compound bilayer lanthanum nickel oxide becomes superconducting when under high pressure in the bulk crystal form and when under strain in the thin film form.

The central question was, how do subtle changes in atomic structure, especially in the nickel–oxygen bonds, influence the electronic properties that enable superconductivity.

The team identified atomic distortions using a combination of advanced electron microscopy techniques, including electron ptychography pioneered by David Muller, Ph.D. ’96, the Samuel B. Eckert Professor of Engineering Physics, and his group in AEP, and advanced thin-film synthesis developed by collaborators at Stanford and Harvard, including AEP alumna Julia Mundy, M.S. ’13, Ph.D. ’14.

The nickelate compound has recently drawn attention for exhibiting superconductivity at relatively high temperatures when subjected to extreme conditions. In the study, the researchers investigated nickelate thin films grown on different substrates that impose varying amounts of strain on the material. By studying a systematic series of films with different strain, the researchers were able to identify essential structural characteristics and their effect on electronic changes.

Many of the questions about bilayer nickelates and their structure relate to the precise configuration of nickel and oxygen atoms, which are challenging to measure accurately by many experimental techniques. Leveraging recent advances in multislice electron ptychography allowed the researchers to resolve and quantitatively measure atomic positions with extremely high precision.

“Directly measuring the small atomic distortions in bulk crystals under hydrostatic pressure is challenging and beyond current experimental capabilities,” Bhatt said. “Instead, static stabilization of strained lanthanum nickelate thin films provides a platform well-suited to investigation using local atomic-resolution electron microscopy methods to quantitatively measure subtle distortions while avoiding artifacts from defects that limit other characterization techniques.”

Under compressive strain, the oxygen atoms rearranged in a higher symmetry configuration than in the tensile strained films. These observations echo those in superconducting bulk crystals subject to very high pressures: The nickel-oxygen bonds in compressively strained superconducting films adopt the same pattern that has been reported in superconducting bulk crystals under very high pressures. This suggests that increasing symmetry in the nickel-oxygen structure may be an essential ingredient to stabilize high-temperature superconductivity in these nickelates.

The researchers also found an increase in the vertical bond length in their superconducting thin films, while the same bond length decreases in the bulk superconducting crystals. This observation helps narrow down which structural manipulations are necessary for realizing superconductivity.

To understand why these structural changes are important for the material’s electronic properties, the researchers built a model using the data obtained by multislice electron ptychography to disentangle different types of coupled structural distortions such as tilting, stretching and bending in the nickel-oxygen bonds. They found that changes in bond lengths drive changes in the system’s energy levels, while changes in symmetry – especially rotations of the nickel-oxygen octahedra – strongly reduce mixing between certain electron orbitals, creating a cleaner electronic structure that may help electrons pair up and flow without resistance.

“Combining the expertise of multiple groups spanning synthesis, characterization and theory, we could build a systematic and comprehensive picture of how subtle atomic distortions in these nickelates may relate to superconductivity and how they can be controlled,” Goodge said. “This helps us understand how careful engineering of similar materials in the future could continue to enhance their superconducting properties and offers some inspiration of where to look for other promising compounds.”

Diane Tessaglia-Hymes is communications coordinator for Cornell Duffield College of Engineering.