X-rays Reveal the Interlayer Glue Behind Superconductivity in Bilayer Nickelates

Few discoveries in modern condensed matter physics have energized the community quite like the rise of the nickelate superconductors. Since the first reports of superconductivity in layered nickel oxides, researchers have wondered whether these materials might rival the famous cuprates, whose superconducting transitions at temperatures well above those of conventional superconductors reshaped physics after 1986. Now a team led by researchers at Peking University, Zhejiang University and Nanjing University, working with synchrotron facilities in Taiwan and Berlin, has delivered one of the most direct answers yet to a question that has haunted the field: what, exactly, turns a bilayer nickelate into a superconductor? Their study, published in Nature Materials, points decisively to a quantum mechanical handshake between adjacent nickel-oxygen planes.

The material at the heart of the study is a bilayer nickelate, a compound in which pairs of nickel-oxide planes are stacked with a spacing of just a few angstroms. In these structures, each nickel atom sits at the center of an oxygen octahedron, and the electronic states near the Fermi level are dominated by nickel 3d orbitals. Two of these orbitals matter most. The dx2-y2 orbital extends within each plane and governs the in-plane conduction, much as it does in cuprate superconductors. The dz2 orbital, by contrast, points along the crystallographic c axis, out of the plane, and can overlap with the pz orbitals of apical oxygen atoms sitting above and below the nickel. This overlap creates a potential pathway for electrons to hop coherently from one layer to its partner in the bilayer.

Theoretical work had long suggested that this interlayer channel could be crucial. Calculations published since 2023 proposed that pressure-driven changes in the bilayer structure enhance dz2 orbital character at the Fermi level, enabling interlayer pairing mechanisms that range from s-wave states with sign changes between sheets of the Fermi surface to interlayer valence bond scenarios. But theory alone could not settle the matter, because the experiments needed to test these ideas, soft X-ray absorption spectroscopy and resonant inelastic X-ray scattering, are notoriously difficult to perform on superconducting nickelate samples. The materials are chemically sensitive, their surfaces degrade in air, and the superconducting fraction within bulk crystals is often inhomogeneous, making it hard to attribute spectroscopic signatures unambiguously to the superconducting phase.

The Chinese-led collaboration overcame this obstacle with a clever materials engineering strategy. They grew thin films of the bilayer nickelate (La,Pr)3Ni2O7 by pulsed laser deposition and protected them with a capping layer that preserves the delicate oxygen stoichiometry of the buried superconducting layers. Crucially, the team prepared a series of films spanning three distinct electronic ground states: insulating, metallic and superconducting. This controlled family of samples gave the researchers something no single superconducting crystal could provide, namely the ability to track how the electronic structure evolves as the material crosses from one quantum state to another, isolating the features that appear only when superconductivity does.

With the capped films in hand, the researchers deployed the full arsenal of modern synchrotron spectroscopy. X-ray absorption spectroscopy at the oxygen K edge and the nickel L3 edge probed the unoccupied electronic states, revealing the orbital character of the charge carriers. Resonant inelastic X-ray scattering, or RIXS, at the nickel L3 edge then measured the energy and momentum resolved spectrum of orbital excitations and spin excitations, the collective magnetic fluctuations that many theorists believe mediate pairing in unconventional superconductors. Resonant elastic X-ray scattering complemented these measurements by detecting static magnetic order, specifically the spin-density-wave order that competes with superconductivity in these compounds.

The results paint a coherent and strikingly clear multiorbital picture. Across all samples, the in-plane electronic structure is built from itinerant dx2-y2 states, which form the conductive backbone of the material regardless of whether the sample insulates, conducts or superconducts. What separates the superconductors from the rest is the out-of-plane channel. Only in the superconducting films did the spectra show evidence of coherent dz2-pz-dz2 hybridization, meaning that the dz2 orbitals on nickel atoms in adjacent layers, linked through the apical oxygen pz orbitals, merge into well-defined bonding and antibonding bands that straddle the bilayer. In the insulating and metallic samples, this interlayer coherence is absent or incoherent, and the two layers effectively fail to form a unified electronic entity.

The magnetic data reinforce the same conclusion. In the non-superconducting samples, static spin-density-wave order is prominent, indicating that electrons are locked into a periodic magnetic texture that gaps out parts of the Fermi surface and suppresses coherent metallic behavior. As the samples move into the superconducting regime, this static spin order is suppressed, and the spin excitations measured by RIXS become strongly damped, their spectral weight broadening and softening in a manner reminiscent of the paramagnons observed in cuprate superconductors. The coexistence of coherent interlayer hybridization, suppressed static magnetism and heavily damped but persistent spin fluctuations defines the narrow window in which superconductivity emerges, suggesting that magnetic fluctuations surviving the collapse of long-range order may still contribute to the pairing interaction.

Perhaps the most practically important finding concerns the tuning knobs that control the interlayer channel. The researchers found that both oxygen stoichiometry and epitaxial strain, the mechanical distortion imposed on the film by the underlying substrate, directly modulate the strength of the dz2-pz-dz2 hybridization. Oxygen vacancies disrupt the apical oxygen network that mediates the interlayer hopping, while strain alters the geometry of the octahedra and the spacing between layers. Because both parameters tune the same orbital channel, superconductivity in bilayer nickelates occupies only a narrow regime of interlayer coherence, explaining why samples grown or treated slightly differently can show wildly different transport behavior. This insight offers a concrete design rule for future experiments: optimize the apical oxygen environment and the interlayer spacing, and the superconducting fraction and transition temperature should follow.

The broader significance of this work lies in how it positions the nickelates relative to the cuprates. In cuprate superconductors, superconductivity is widely understood through a single-band picture in which doped holes in the dx2-y2 orbital pair through antiferromagnetic fluctuations, with the famous Zhang-Rice singlet capturing the essential physics. The bilayer nickelates refuse to fit into that framework. Their superconductivity requires simultaneous coherence in two orbital channels, an in-plane itinerant channel and an out-of-plane hybridized channel, making them genuinely multiorbital superconductors. This complexity is not a nuisance but an opportunity, because it adds new adjustable degrees of freedom, orbital occupancy, interlayer hopping and Hund’s rule coupling, that have no direct analogue in the cuprates and could in principle be exploited to push transition temperatures higher.

The path forward is now well marked. Thin film growth, already proven capable of ambient-pressure superconductivity in these materials, provides the platform for systematic strain engineering, while oxygen recycling protocols allow fine control of the apical oxygen network. Spectroscopists can now interrogate genuine superconducting samples rather than inferring properties from non-superconducting relatives, and the identification of coherent interlayer hybridization as the superconducting enabler gives theorists a sharp target for microscopic models of the pairing mechanism. If the nickelates continue to follow the trajectory that this study illuminates, the dream of a new family of high-temperature superconductors, one built not on a single orbital but on the quantum coupling of paired atomic planes, looks increasingly within reach.

Subject of Research: The role of interlayer orbital hybridization in superconductivity of bilayer nickelate thin films

Article Title: Interlayer hybridization promotes superconductivity in bilayer nickelates

Article References: Zhang, S., Zhang, M., Luo, Q., Tao, Z., Huang, H.-Y., Weschke, E., Li, K., Channagowdra, G., Li, J., Fu, J., Huang, D.-J., Xie, Y., Lu, Y., & Peng, Y. (2026). Interlayer hybridization promotes superconductivity in bilayer nickelates. Nature Materials. https://doi.org/10.1038/s41563-026-02750-z

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02750-z

Keywords: bilayer nickelates, superconductivity, interlayer hybridization, dz2 orbital, resonant inelastic X-ray scattering, X-ray absorption spectroscopy, spin-density wave, thin films, oxygen stoichiometry, epitaxial strain, multiorbital physics, cuprates