Study finds strain and oxygen tuning produce similar orbital and magnetic changes in nickelate films

The researchers used X-ray absorption spectroscopy and resonant inelastic X-ray scattering on (La,Pr)3Ni2O7-delta thin films, varying epitaxial strain and oxygen content independently, according to the EurekAlert release. Compressive strain comes from the substrate on which the film is grown; oxygenation changes the film's composition. Both adjustments shifted the measured electronic and magnetic properties in broadly the same direction.

The question was what happens to electrons and magnetism when a non-superconducting nickelate is moved toward the superconducting state, the release said. The two independent controls let the researchers compare lattice compression with oxygen content inside one material platform.

Orbital channel and magnetism

As compressive strain rose or oxygen content was optimized, states tied to oxygen 2pz and nickel 3dz2 orbitals became progressively more delocalized, the study said. Spectral weight shifted toward oxygen-derived hole states, while features from initially localized nickel 3dz2 electrons grew broader and less pronounced.

The measurements point to a Ni 3dz2-O 2pz-Ni 3dz2 pathway linking adjacent nickel-oxygen layers. The study associates progressive delocalization of this interlayer channel with the evolution toward superconductivity, rather than establishing it as the sole cause. The same signature appeared under either tuning method, offering a concrete experimental benchmark for theories of nickelate superconductivity.

Long-range spin-density-wave order, pronounced in the non-superconducting state, weakened as the films approached superconductivity. Both its measured intensity and spatial correlation length fell, the study said. Magnetism did not disappear: short-range magnetic excitations survived after long-range coherence was strongly suppressed. They became more strongly damped, but their characteristic energy bandwidth stayed nearly unchanged.

That combination places constraints on any explanation, according to the study. A theory has to fit more interlayer orbital delocalization, less long-range magnetic coherence, and short-range excitations at a nearly unchanged energy scale.

Ambient-pressure context

Bulk Ruddlesden-Popper nickelates were first made superconducting under high pressure, which required squeezing samples in a diamond anvil cell and limited many experimental probes, Newswise reported. Thin-film growth made ambient-pressure superconductivity possible by applying compressive strain through the substrate and controlling oxygen content, the EurekAlert release said.

A 2023 study found that bilayer La3Ni2O7 under high pressure reached a transition temperature up to 80 K, a record for nickel-based superconductors, according to a review from the Institute of Physics, Chinese Academy of Sciences. In 2024, strain engineering via substrates enabled superconductivity in La3Ni2O7 thin films at ambient pressure, the review said.

Films superconduct more weakly than samples under external pressure because they are difficult to grow with high crystalline perfection, Newswise reported. That leaves room to improve crystallinity, the same account said.

What remains open

High-quality single crystals and films remain hard to prepare. How defects and lattice disorder affect electronic properties is still incompletely understood, and debate over the mechanism and electron pairing continues, the review said. The field needs advanced spectroscopic techniques, new experimental methods and theoretical modelling to pin down the pairing, it said.