The problem
In bilayer La3Ni2O7, what interaction makes electrons form the pairs that carry current without resistance, and what pattern does the paired state take? Electrons can move within and between the nickel–oxygen layers through different orbitals. A pressure-motivated two-orbital Hubbard–Hund model finds an s± instability dominated by interlayer dz² singlets; s± means opposite signs between relevant gap components. Photoemission on particular substituted films finds a nodeless gap, without directly measuring that relative sign.[1] [3]
Scope
Pair formation in superconducting Ruddlesden–Popper bilayer nickelates derived from La3Ni2O7, distinguishing pressurized bulk crystals from substituted, strained films. Identify the pairing interaction and order parameter without assuming all nickelate families or sample regimes share an identical mechanism.
What remains open
Scientists have measured superconducting energy gaps, including a gap without nodes in particular films. These results narrow the possibilities. They do not by themselves identify the interaction that binds the pairs or prove the same mechanism in every sample.[2] [3]
Cluster calculations depend on chosen hoppings, interactions and accessible temperatures. Bulk point-contact fits and film spectroscopy must constrain the same proposed pairing kernel within their respective regimes. Phase-sensitive evidence and quantitative orbital-resolved predictions are still needed to select among mechanisms.[1] [2] [3] [4]
Why it matters
The two-layer structure lets researchers change the coupling between sheets and watch superconductivity respond. That makes these materials a useful test of how a crystal’s electronic structure can favor pairing.[4]
The pairing interaction would explain how changing the coupling between the two layers alters the tendency of electrons to form pairs. It would connect measured hybridization and spin excitations to the forces responsible for superconductivity, instead of treating them only as accompanying features. Keeping bulk and film conditions explicit is essential to deciding which physical mechanism the evidence actually tests.[1] [4]
Research directions
2 directions to explore
Relative gap phase
Determine the order-parameter phase structure in defined superconducting bilayer samples.
Orbital-resolved predictions
Predict responses to interlayer hybridization and material tuning within each bulk or film regime.
Definitions
3 terms
orbitals
Quantum states describing the spatial character of an electron around and between atoms. The bilayer models include nickel states directed within the layers and states that couple strongly between the layers.[1]
hybridization
Mixing between electronic states. In these bilayer nickelates, coherent mixing along the nickel–oxygen–nickel path connects the two layers; the film study associates changes in this mixing with superconductivity.[4]
Sources
Partial results
A nodeless superconducting gap was observed in substituted bilayer films. Scope: Ambient-pressure (La,Pr,Sm)3Ni2O7 films measured by ARPES.
This constrains gap structure but is not a phase-sensitive determination or unique pairing-kernel identification.
Source date: 2026-05-21Published May 2026 spectra report a finite gap across the Brillouin zone and an electron-boson coupling feature.Read the source ↗Coherent interlayer hybridization is linked to superconductivity. Scope: Capped substituted bilayer films with oxygen/strain control.
Published abstract accessed; causal interaction weights and bulk/film universality are not established by this association alone.
Source date: 2026-09-23September 2026 X-ray absorption and resonant inelastic scattering identify the evolving interlayer channel.Read the source ↗- Interlayer pairing in bilayer nickelates
Notes on Interlayer pairing in bilayer nickelates
Two-orbital Hubbard–Hund cluster calculations support interlayer singlets and an s± instability in a pressure-motivated model. This establishes a candidate interaction, not unique experimental identification.
- Revealing superconducting gap in La3Ni2O7−δ by Andreev reflection spectroscopy under high pressure
Notes on Revealing superconducting gap in La3Ni2O7−δ by Andreev reflection spectroscopy under high pressure
Primary high-pressure point-contact spectra support two fitted gap scales. Both s+s and s+d fits are discussed; the article calls for further evidence for definitive symmetry.
- Nodeless superconducting gap and electron-boson coupling in (La,Pr,Sm)3Ni2O7 films
Notes on Nodeless superconducting gap and electron-boson coupling in (La,Pr,Sm)3Ni2O7 films
Primary ARPES abstract reports a nodeless gap and an electron-boson coupling feature in ambient-pressure substituted bilayer films. It constrains the problem without measuring a unique pairing interaction.
- Interlayer hybridization promotes superconductivity in bilayer nickelates
Notes on Interlayer hybridization promotes superconductivity in bilayer nickelates
X-ray absorption and resonant inelastic scattering link coherent interlayer hybridization to superconductivity in capped bilayer films, providing recent constraints on pairing theories.