Knowledge

27 linked papers

This study employed the six-terminal method to perform high-precision transport measurements on Ruddlesden–Popper nickelate bilayer and trilayer single crystals, self-consistently extracting the in-plane and out-of-plane resistivities. The results reveal strong intrinsic electronic anisotropy, with out-of-plane resistivity exhibiting a non-monotonic temperature dependence that signals a universal interlayer coherent-to-incoherent crossover. Under pressure, the maximum superconducting transition temperature is inversely proportional to the resistivity anisotropy at ambient pressure, indicating that stronger interlayer electronic coherence favors superconductivity. Moreover, out-of-plane resistivity serves as a sensitive probe of magnetic and density-wave orders, whereas the in-plane resistivity shows a weaker response. These findings highlight interlayer coherence as a key tuning parameter that both tracks magnetic correlations and is closely linked to superconductivity, providing stringent constraints for microscopic theories of nickelate high-temperature superconductivity.

Using resonant X-ray scattering and spectroscopy, we investigate the relationship between spin-density wave (SDW) order and superconductivity in bilayer nickelate La₂PrNi₂O₇ thin films. Superconductivity is found to emerge only in regions without SDW and with complete oxygen stoichiometry, whereas oxygen deficiency promotes SDW order, indicating phase separation between the two. Further Ni-L₃ and O-K edge spectroscopy reveal that the superconducting phase exhibits a metallic ground state dominated by Ni d⁸ and oxygen ligand-hole character; oxygen deficiency induces electron localization and the appearance of low-energy excitations. Combined with theoretical analysis, we propose that ligand holes primarily reside on the interlayer apical oxygen sites, forming stable interlayer five-spin polaron states as the ground state of superconducting bilayer nickelates. This study demonstrates that oxygen stoichiometry is a key parameter controlling interlayer coupling and electronic structure, and that SDW order is not the intrinsic parent state of superconductivity.

4 linked papers

By stabilizing bilayer nickelate (La,Pr)₃Ni₂O₇ superconducting thin films with a protective capping layer and employing X-ray absorption and resonant inelastic X-ray scattering spectroscopy, this study directly probes the evolution of electronic structures across insulating, superconducting, and metallic states. Experimental and theoretical analyses reveal that the in-plane d_{x²-y²} states constitute an itinerant electron backbone, whereas superconductivity emerges only when the out-of-plane d_{z²}-p_z-d_{z²} interlayer hybridization becomes coherent, accompanied by suppression of static spin order and the appearance of strongly damped spin excitations. Oxygen stoichiometry and epitaxial strain jointly regulate this interlayer channel, confining the superconducting phase to a narrow window of interlayer coherence and correlation strength. These findings elucidate the microscopic prerequisites for superconductivity in bilayer nickelates and provide a multi-orbital framework to describe its emergent mechanism.

4 linked papers

11 linked papers

This review systematically summarizes theoretical progress on the interlayer pairing mechanism driven by strong correlation effects in bilayer nickelate superconductors. Starting from key experimental observations, the paper extracts core physical ingredients, including the hybridized electronic structure of Ni-3d({x^2-y^2}) and 3d({z^2}) orbitals, orbital-dependent electronic correlations, Hund’s coupling, and strong interlayer magnetic coupling, and introduces fundamental theoretical frameworks such as the bilayer two-orbital Hubbard model and its (t)-(J) variants. Emphasis is placed on the strong-correlation pairing mechanism rooted in an interlayer valence bond picture in the atomic limit of half-filled d({z^2}) orbitals, with particular stress on the hybridization mechanism: local singlet pairing of d({z^2}) electrons provides condensation energy, while hybridization with itinerant d(_{x^2-y^2}) orbitals promotes superconducting phase coherence. The review further analyzes the pairing symmetry, the dependence of critical temperature on various internal and external parameters, and non-trivial normal-state behaviors including Fermi liquid, non-Fermi liquid, weak insulating, and pseudogap regimes, and discusses the effects of pressure tuning, oxygen stoichiometry, and Kondo scattering induced by oxygen vacancies. The central conclusion points to an unconventional superconductivity picture driven by the synergy between local pairing and itinerant behavior, and briefly mentions weak-coupling theories based on spin fluctuations arising from Fermi surface nesting.

Motivated by the unresolved question of whether d-wave or s±-wave pairing dominates in the bilayer nickelate La3Ni2O7 and by the unclear connection between its spin order, magnetic excitations, and pairing mechanism, the authors use finite-temperature determinant quantum Monte Carlo simulations of a single-orbital bilayer Hubbard model to study pairing competition under conditions of relatively strong interlayer coupling and comparable intra- and interlayer antiferromagnetic correlation strengths. The results show that with increasing hole doping, spin stripes form spontaneously, accompanied by a crossover of the dominant pairing symmetry from d-wave to s±-wave; this crossover tracks changes in the relative strengths of the two types of spin correlation, with interlayer correlations exceeding intralayer correlations in magnitude. The odd channel of the dynamical spin structure factor further exhibits momentum-dependent reorganization of magnetic excitations, consistent with a fluctuating spin-stripe picture. On this basis, the authors argue that the spin-fluctuation structure is a key link connecting spin stripes and pairing competition and offers a minimal-model perspective for understanding La3Ni2O7; this is a prediction from model calculations rather than an established experimental mechanism.

Based on a bilayer two-orbital Hubbard model, this study employs the ground-state constrained path integral quantum Monte Carlo method to investigate the regulatory mechanisms of vertical electric fields (interlayer potential differences) and electron correlations on the competition between interlayer $s^{\pm}$ and intralayer $d$-wave pairing in bilayer nickelates. The results indicate that increasing the interlayer potential difference typically enhances the $d$-wave pairing vertex contribution and suppresses $s^{\pm}$ pairing, thereby shifting the pairing preference toward the $d$-wave channel; conversely, increasing the crystal field splitting enhances the relative competitiveness of $s^{\pm}$ pairing, while increasing the Hund coupling favors $s^{\pm}$ pairing under weak bias but enhances $d$-wave pairing under large bias. Microscopic analysis reveals that the electric field primarily induces interlayer charge redistribution within each orbital sector, accompanied by enhancements in the antiferromagnetic spin structure factor and local interorbital parallel spin correlations induced by Hund coupling. These findings elucidate the potential connections among charge redistribution, magnetic correlations, and pairing competition, providing a theoretical basis for understanding how layer polarization and multi-orbital interactions influence the relative pairing tendencies in bilayer nickelates.