Daily Overview: Today’s highlights focus on the microscopic pairing theory of the hybrid Ruddlesden–Popper nickelate La₅Ni₃O₁₁, while multifaceted studies of infinite-layer nickelate superconductors also reveal a series of key advances ranging from orbital-selective electron correlations to strange-metal critical behavior. Theoretically, a symmetry-based study combining DFT+DMFT indicates that superconductivity in La₅Ni₃O₁₁ arises from two-gap pairing of the Ni-e_g orbitals within the bilayer subsystem, where coupling of the interlayer d_{z²} orbital dominates the pairing, and the variation in transition temperature can be attributed to the decreasing ratio of interlayer to intralayer hopping parameters. Meanwhile, the global phase diagram of the bilayer two-orbital Hubbard model unifies seemingly contradictory phenomena in La₃Ni₂O₇—including orbital-dependent effective mass enhancement, downward shift of the bonding band, and a sharp drop in Drude weight—highlighting the critical role of interlayer antiferromagnetic superexchange in sustaining band splitting. In infinite-layer systems, high-quality ARPES measurements resolve well-defined quasiparticle peaks in La₀.₈Ca₀.₂NiO₂ for the first time, revealing momentum-dependent linear scattering in the imaginary part of the self-energy and marginal Fermi liquid behavior without any detectable pseudogap, suggesting that a pseudogap is not a prerequisite for high-temperature superconductivity. Another experiment directly observes zero resistance and a diamagnetic response in stoichiometric PrNiO₂ thin films, confirming intrinsic superconductivity of the parent compound; this superconducting phase is confined to an extremely narrow window of less than about 3% extra hole doping, well separated from the known ~20% doping superconducting dome, thereby forming a unique double-superconducting-region structure and exhibiting stronger upper critical field anisotropy. Furthermore, employing the Seebeck coefficient as a probe of heat capacity, a low-temperature logarithmic divergence is discovered at the critical doping point x* in infinite-layer nickelates, indicating that x* is a quantum critical point where the pseudogap phase terminates; the abrupt change in doped carrier density recapitulates the hallmark of the cuprate pseudogap, while the emergent T-linear resistivity around x* provides compelling evidence for the quantum critical origin of strange-metal behavior. arXiv submission processing window: 2026-07-21 00:00 to 2026-07-21 00:00 UTC.

1. Symmetry-Based Microscopic Theory of the Unconventional Pairing Mechanism in La$_5$Ni$_3$O$_{11}$

Summary: This study systematically investigates the low-temperature physics and unconventional pairing mechanism of the hybrid nickelate La₅Ni₃O₁₁ using a symmetry-based phenomenological approach combined with charge self-consistent density functional theory and dynamical mean-field theory. The monolayer subsystem is found to be in a Mott insulating state, and superconductivity primarily originates from the Ni-e_g orbitals in the bilayer subsystem. The system exhibits a two-gap superconducting feature, with the dominant pairing arising from interlayer coupling between d_{z²} orbitals and the secondary pairing from intralayer coupling within d_{x²-y²} orbitals. Compared with high-pressure La₃Ni₂O₇, the reduction of the superconducting transition temperature T_c in La₅Ni₃O₁₁ can be attributed to the weakening of interlayer pairing contributions, which is directly reflected in the decreased magnitude of the hopping parameter ratio |t_{\perp}^z/t_{\parallel}^{x}|. This unified picture provides a microscopic theoretical framework for understanding the superconducting pairing mechanism within bilayer NiO₂ planes and the role of the γ pocket.

2. Orbital-selective electron correlations in high-$T_{\rm c}$ bilayer nickelates: from a global phase diagram to implications for spectroscopy

Summary: By constructing the global phase diagram of a bilayer two-orbital Hubbard model, we investigate the orbital-selective electron correlations in the bilayer nickelate La3Ni2O7. At half-filling the system undergoes a Mott transition, and at the physical electron filling it exhibits strong orbital selectivity, manifested as interlayer spin singlet formation among z²-orbital electrons. This effect leads to significant band-structure renormalization with dramatic bandwidth narrowing, while the splitting between the z² bonding and antibonding bands remains at about 1 eV. These correlation features naturally explain the orbital-dependent effective mass enhancement and the sinking of the z² bonding band below the Fermi level observed in ARPES experiments, as well as the puzzling contrast in optical conductivity where the Drude weight drops sharply yet the interband peak position barely shifts. Theoretical analysis reveals that interlayer antiferromagnetic superexchange plays a key role in maintaining the band splitting, and intralayer correlations drive the z² band away from the Fermi surface. This work provides a unified microscopic picture for understanding the normal-state properties and the mechanism of high-temperature superconductivity in multilayer nickelates.

3. Marginal-Fermi-Liquid-like Behavior without Pseudogap in Infinite-Layer Nickelates

Summary: Using angle-resolved photoemission spectroscopy (ARPES) with an optimized surface treatment, we probe the low-energy electronic structure of the infinite-layer nickelate superconductor La0.8Ca0.2NiO2 and its parent LaNiO2 thin films, resolving clear quasiparticle peaks for the first time in this system. In La0.8Ca0.2NiO2, the imaginary part of the electron self-energy exhibits a roughly linear dependence on energy with a slope that progressively increases from the (π/2, π/2) to the (π, 0) direction, revealing momentum-dependent marginal-Fermi-liquid-like scattering. Both films display low-energy spectral weight that is gradually suppressed from the diagonal toward the antinodal region, with stronger suppression in the parent compound; however, finite Fermi-level spectral weight persists over the entire Fermi surface, and no leading-edge shift or back-bending is observed, indicating the absence of the pseudogap typical of cuprates. These findings demonstrate that momentum-selective correlations and marginal-Fermi-liquid-like scattering can emerge without a detectable pseudogap, providing a crucial benchmark for identifying the essential electronic ingredients of the high-Tc normal state and suggesting that the pseudogap is not a requirement, while charge-transfer energy and the orbital character of doped carriers may play decisive roles.

4. A Disconnected Superconducting Regime at the Parent Limit of Infinite-Layer Nickelates

Summary: This study finds that zero resistance and a diamagnetic response are observed in stoichiometric PrNiO₂ thin films without cation substitution and in the absence of a capping layer, thereby confirming the intrinsic superconductivity of the parent infinite-layer nickelate. Heterostructure engineering by inserting a non-superconducting buffer layer rules out contributions from substrate interface effects. Isovalent trivalent La substitution likewise preserves superconductivity, demonstrating that the phenomenon is not unique to PrNiO₂. In contrast, trace divalent Sr or Ca substitution (within about 3%) rapidly suppresses superconductivity so that it no longer appears. Combined with angle-resolved photoemission spectroscopy measurements, this reveals that the superconducting phase is confined to an extremely narrow hole-doping range within 3% beyond the parent PrNiO₂. A non-superconducting region exists between this phase and the previously established superconducting dome at approximately 20% divalent doping, giving rise to two separated superconducting regions on the phase diagram. Furthermore, the parent superconducting phase exhibits a significantly stronger upper critical field anisotropy. These results indicate that infinite-layer nickelates are not simply cuprate analogs but possess unique superconducting physics and segregated superconducting domains.

5. Quantum critical origin of strange-metals at the end of a pseudogap phase in infinite-layer nickelates

Summary: Infinite-layer nickelate superconductors provide a new platform to study the quantum critical origin of strange metals, yet their thin-film form forbids conventional calorimetry. Using the Seebeck coefficient as a proxy for low-temperature specific heat, we find at the critical doping x* that the high-temperature Seebeck response quantitatively matches the band structure measured by ARPES, indicating well-defined quasiparticles, while below 60 K, S/T develops a logarithmic divergence that persists to the lowest temperatures (after suppressing superconductivity with a magnetic field), marking x* as a quantum critical point that terminates the underdoped phase. Moreover, the Ni-d_{x^2-y^2} carrier density drops abruptly from 1+x above x* to x below x*, reproducing the hallmark signature of the pseudogap phase in cuprates. These results indicate that the underdoped region of infinite-layer nickelates is a pseudogap-like state, at whose end emergent strange metal behavior with T-linear resistivity provides strong evidence for a quantum critical origin of strange metals.