Daily Overview: Today’s highlights focus on an in-depth understanding of the electronic structure of mixed Ruddlesden-Popper nickelates. In [1], functional renormalization group studies reveal that lattice symmetry is a core factor in regulating the competition between spin density wave and superconductivity in bilayer nickelate La₃Ni₂O₇, suggesting that suppressing orthorhombic distortion via uniaxial strain may achieve bulk superconductivity at ambient pressure. [2] systematically compares the strain effects in trilayer La₄Ni₃O₁₀ thin films, finding that while compressive strain can suppress density wave order, it cannot completely eliminate in-plane octahedral rotations, leading to the absence of superconductivity—highlighting a key difference between trilayer and bilayer systems. [3] reports the observation of a superconducting onset temperature of 50–70 K in oxygen-deficient nickelate thin films grown on highly reduced SrTiO₃ substrates, accompanied by a paramagnetic Meissner effect and superconducting diode effect, attributed to the synergistic interaction between the film and substrate. [4] utilizes RIXS to observe plasmon excitations for the first time in low-valence nickelate Pr₄Ni₃O₈, where strong damping and low propagation velocity reveal unique charge screening behavior, providing quantitative constraints for analogies between nickelate and cuprate superconductors. [5] systematically evaluates the delafossite structure as a competing phase of infinite-layer oxides through high-throughput first-principles calculations, identifying the La-Ni combination as the optimal thermodynamic choice and offering guidance for stabilizing the infinite-layer phase via hole doping. These works advance the understanding of nickelate superconductivity mechanisms and materials design from multiple perspectives, including symmetry, strain engineering, interface effects, electronic excitations, and phase stability. arXiv submission processing window: 2026-06-23 00:00 to 2026-06-23 00:00 UTC.

1. Structural symmetry effects on the competition of density waves and superconductivity in bilayer nickelates

Summary: Using the functional renormalization group method, this study investigates the competition between spin-density wave order and superconductivity in the bilayer nickelate La₃Ni₂O₇ under both ambient and high-pressure crystal structures. By comparing weakly coupled multi-orbital models of the two structures, it is found that as the Hund coupling increases, the dominant instability transitions from superconductivity to a spin-density wave with a characteristic wave vector Q₁≈(π/2,π/2), consistent with experiments. Surprisingly, the non-interacting susceptibilities and fRG leading instabilities are nearly identical for the ambient and high-pressure structures, indicating that the emergence of superconductivity under pressure cannot be solely attributed to changes in low-energy electronic structure. Further analysis reveals that suppressing orthorhombic distortion is key: when the system approaches the tetragonal limit, symmetry-related spin-density wave fluctuations become nearly degenerate, thereby hindering long-range magnetic order and enhancing pairing interactions. These results highlight lattice symmetry as a crucial parameter in tuning the competing ordered states in bilayer nickelates and suggest that reducing orthorhombic distortion through uniaxial strain may enable bulk superconductivity at ambient pressure.

2. Persistent structural distortions and absent superconductivity in trilayer nickelate thin films

Summary: This study systematically investigates the strain effect in trilayer nickelate La₄Ni₃O₁₀ thin films through atomically precise synthesis, electrical transport measurements, picometer-resolution electron microscopy, and synchrotron X-ray diffraction. While compressive epitaxial strain effectively suppresses the parent density-wave order and enhances crystal symmetry (e.g., eliminating out-of-plane octahedral rotations), no superconductivity is observed even under the maximum compressive strain of -2.8%. Critical structural characterization reveals that compressive strain fails to completely eliminate the characteristic in-plane octahedral rotations in the thin films, which exhibit interlayer inequivalence between the inner and outer layers of each trilayer unit and persist robustly. Synchrotron X-ray diffraction shows that the amplitude of in-plane rotations decreases monotonically with compressive strain but does not vanish entirely. In contrast, in the bilayer system La₃Ni₂O₇, compressive strain fully suppresses all octahedral rotations, thereby inducing superconductivity. These results uncover a key difference between trilayer and bilayer systems, indicating that ambient-pressure superconductivity in trilayer nickelates cannot be achieved solely through epitaxial strain engineering, and alternative tuning methods need to be explored.

3. Experimental evidence of Tc enhancement above 50 K and diode and paramagnetic-Meissner effects, in Nickelate films on highly reduced $SrTiO_3$

Summary: Oxygen-deficient nickelate thin films were fabricated on highly reduced and conductive SrTiO₃ substrates, and through Meissner effect and transport measurements, a superconducting onset temperature of 50–70 K was observed, with zero resistance achieved at 20–25 K, indicating the presence of superconductivity in island-like regions within the film. A giant paramagnetic Meissner effect peak appeared at approximately 48 K, further supporting the occurrence of a superconducting transition near this temperature. Additionally, a non-reciprocal, hysteresis-free superconducting diode effect was observed, with its polarity fully polarizable and reversible. The thin films comprise a mixture of various Ruddlesden–Popper phases, including the infinite-layer phase. These enhanced superconducting properties are attributed to the synergistic effect between the oxygen-deficient films and the highly reduced SrTiO₃ substrates.

4. Observation of correlated plasmons in low-valence nickelates

Summary: Using resonant inelastic X-ray scattering (RIXS), plasmon excitations are observed for the first time in the metallic low-valence nickelate Pr4Ni3O8. Compared to cuprates, the plasmons in Pr4Ni3O8 exhibit stronger damping and lower propagation velocity, attributable to reduced electronic hopping strength and enhanced screening of long-range Coulomb interactions. Regarding temperature dependence, the plasmons in Pr4Ni3O8 soften with increasing temperature, whereas the plasmon energy in cuprates remains nearly unchanged while damping increases. These results reveal distinctive charge screening behavior in nickelates and impose quantitative constraints on the analogy between the two families.

5. Delafossites as an unexpected competing phase to infinite-layer oxides

Summary: Through high-throughput first-principles simulations, this study systematically compares the thermodynamic stability of delafossite (D1), ordered rock salt variant (D2), and infinite-layer (IL) oxides at ABO₂ stoichiometry, constructing phase diagrams encompassing 2,346 elemental combinations. The results demonstrate that for nickelates, palladates, and platinate, the delafossite structure exhibits stability comparable to or even superior to the infinite-layer phase, with competition between these two phases and the perovskite phase. Electronic structure analysis reveals that delafossite compounds feature an inverted cation order, with the Fermi surface dominated by d_{z^2} orbital contributions, distinctly different from the d_{x^2-y^2} characteristics of the infinite-layer phase. Among all candidate systems, the La-Ni combination is the thermodynamically optimal choice for stabilizing the infinite-layer structure. Furthermore, hole doping via Ca, Sr, and Ba systematically enhances the relative stability of the infinite-layer phase across the three transition metal families. These findings elucidate the fundamental challenges in synthesizing substrate-free bulk infinite-layer oxides and provide guidance for the experimental exploration of novel superconducting compounds.