The discovery of high-temperature superconductivity in Ruddlesden–Popper nickelates has opened a new direction in the search for unconventional superconductors. Unlike conventional metals, these materials host several active electronic orbitals and strong magnetic interactions, making it challenging to determine which microscopic changes are essential for superconductivity.
Superconductivity in bulk Ruddlesden–Popper nickelates was initially realized under high pressure. More recently, advances in thin-film growth has made superconductivity possible at ambient pressure by applying compressive strain through the epitaxial substrate and controlling the oxygen content. This provides researchers with a more accessible platform for investigating how superconductivity emerges.
A central question, however, has remained unresolved: what happens to the electrons and magnetism when the non-superconducting nickelate is driven toward the superconducting state?
In a new study published in National Science Review , researchers addressed this question using X-ray absorption spectroscopy and resonant inelastic X-ray scattering. They investigated (La,Pr) 3 Ni 2 O 7− δ thin films while independently varying two key parameters—epitaxial strain and oxygen content.
Remarkably, both tuning methods produced a similar microscopic evolution.
The first major change involved the electronic orbitals. As compressive strain increased or the oxygen content was optimized, electronic states associated with oxygen 2 p z and nickel 3 d z2 orbitals became progressively more delocalized. Spectral weight shifts toward oxygen-derived hole states, while features associated with initially localized nickel 3 d z2 electrons became broader and less pronounced.
These observations point to the growing importance of an electronic pathway extending between adjacent nickel-oxygen layers through the Ni 3 d z2 –O 2 p z –Ni 3 d z2 orbitals. The results suggest that the progressive delocalization of this interlayer electronic channel is an important part of the evolution toward superconductivity.
At the same time, the magnetic state underwent a striking transformation. Long-range spin-density-wave order, which is pronounced in the non-superconducting state, became progressively weaker as the system approaches superconductivity. Both its intensity and spatial correlation length decrease, providing direct evidence that long-range magnetic order competes with the superconducting state.
Yet magnetism did not simply disappear.
The researchers found that short-range magnetic excitations survived even after long-range magnetic coherence was strongly suppressed. These excitations become more strongly damped, but their characteristic energy bandwidth remain nearly unchanged. The persistence of these short-range magnetic correlations places important constraints on theories seeking to explain superconductivity in nickelates.
Together, the observations reveal a coherent microscopic pathway from the non-superconducting parent state toward superconductivity. Compressive strain and oxygenation act as two independent control parameters but drive the material in the same direction: electronic states associated with the interlayer orbitals become more delocalized, long-range magnetic order loses coherence, and robust short-range magnetic correlations remain.
The study therefore identifies an orbital-selective route toward superconductivity in Ruddlesden–Popper nickelates. The emergence and delocalization of the interlayer electronic channel, together with persistent short-range magnetism after the suppression of long-range spin-density-wave order, provide new experimental benchmarks for theories of nickelate superconductivity.
Beyond clarifying the microscopic evolution of this particular material, the findings also provide a potential roadmap for designing nickelate superconductors through controlled strain and oxygen engineering.
National Science Review