Daily Overview: Today’s highlighted works focus on the effects of lattice structure, doping, and pressure on superconducting behavior in nickelate superconductors. [1] Heavily Sr-doped La₂SrNi₂O₇₋δ single crystals were synthesized for the first time by a high-pressure high-temperature flux method. At ambient pressure, they are found to possess a tetragonal I4/mmm structure and a 180° Ni−O−Ni bond angle; however, Sr doping drives the Ni-3d_z² band far away from half-filling, weakening the interlayer antiferromagnetic coupling and pairing, and no superconductivity is observed, providing important clues for understanding the conditions for superconductivity in bilayer nickelates. [2] The microscopic origin of the robust superconductivity of infinite-layer La₀.₈Sr₀.₂NiO₂ under pressure was theoretically revealed: pressure increases kinetic energy, reduces effective correlation, enhances interlayer hybridization, and induces self-doping, allowing the system to maintain d-wave pairing over a wide pressure range, while superconductivity is suppressed only in the high-pressure overdoped region. [3] Trilayer Sm₄Ni₃O₁₀₋δ single crystals were successfully synthesized; in their orthorhombic Pbca structure, the Ni−O−Ni bond angle is about 152.4°. They exhibit a density-wave order at about 180 K at ambient pressure and remain non-superconducting at 80 GPa, suggesting that the severe bond-angle deviation may hinder superconductivity. The three works clarify the key factors controlling nickelate superconductivity from the perspectives of doping, pressure, and layer number/bond angle, respectively. arXiv submission processing window: 2026-09-02 00:00 to 2026-09-02 00:00 UTC.

1. Heavily Sr-Doped La$_{2}$SrNi$_{2}$O$_{7-δ}$ as a Tetragonal Ruddlesden-Popper Phase at Ambient Pressure

Summary: This study reports the first successful synthesis of heavily Sr-doped La₂SrNi₂O₇₋δ single crystals by a high-pressure, high-temperature flux method. X-ray diffraction and scanning transmission electron microscopy confirm that the material adopts a tetragonal I4/mmm structure at ambient pressure, with a Ni−O−Ni bond angle of 180° along the c-axis, indicating that heavy doping effectively suppresses the distortion of NiO₆ octahedra. Resistivity measurements reveal metallic behavior with a low-temperature upturn, and no density-wave features are observed; however, neither applying pressure nor varying the oxygen content induces superconductivity. Density functional theory calculations show that holes introduced by Sr doping predominantly enter the Ni-3d_z² orbital, creating a large γ pocket on the Fermi surface and markedly reducing the occupation of this orbital, so that the Ni-3d_z² band deviates strongly from half-filling. The authors accordingly argue that this departure from half-filling weakens interlayer antiferromagnetic interactions and pairing, which is the key reason superconductivity does not emerge in this system. This work provides important clues for understanding the conditions required to realize superconductivity in bilayer nickelates.

2. Microscopic Origin of Pressure-Enhanced and Robust Superconductivity in Infinite-Layer La$_{0.8}$Sr$_{0.2}$NiO$_2$

Summary: Combining first-principles calculations, a pressure-dependent two-orbital model, self-consistent FLEX calculations, and the linearized Eliashberg equation, this study reveals the microscopic mechanism of pressure-enhanced and robust superconductivity in infinite-layer La₀.₈Sr₀.₂NiO₂. The results show that increasing pressure enlarges the kinetic energy scale, reduces the effective correlation strength U/t, enhances interlayer hybridization, and transfers holes from the La/Sr charge reservoir to the correlated Ni region; at low pressure, the enlarged kinetic energy scale and the approach to optimal intermediate coupling promote pairing, whereas at high pressure, pressure-induced self-doping drives the system into the overdoped regime and suppresses superconductivity, thereby forming a broad superconducting dome. Although compression significantly three-dimensionalizes the Fermi surface, the spin susceptibility relevant to pairing depends weakly on q_z and still peaks mainly near (π,π), so the Ni d_{x²−y²}-dominated d-wave pairing state remains stable over the calculated pressure range. This constrained low-energy pairing framework explains the unusual robustness of superconductivity in this material under megabar-level compression.

3. Expanding the trilayer Ruddlesden-Popper nickelate family: Synthesis and characterization of Sm$_4$Ni$_3$O$_{10-δ}$ single crystals

Summary: Here we report the first successful synthesis of single crystals of the samarium-based trilayer Ruddlesden–Popper nickelate Sm₄Ni₃O₁₀₋δ using a high-pressure, high-temperature (3.25 GPa, 1400 °C) flux method. Single-crystal X-ray diffraction and powder diffraction refinement show that the structure adopts the orthorhombic Pbca space group, with an Ni–O–Ni bond angle along the c axis of about 152.4°, significantly deviating from 180°. Magnetization and transport measurements at ambient pressure consistently reveal a density-wave transition at about 180 K; applying pressure up to 80 GPa partially suppresses the insulating behavior and density-wave order, but no superconductivity is observed. Density functional theory calculations indicate that the 3d_z² and 3d_x²−y² orbitals are separated from the other t₂g orbitals and are the main contributors to the Fermi surface. These results suggest that the severe deviation of the Ni–O–Ni bond angle may hinder the emergence of superconductivity, and the newly synthesized Sm₄Ni₃O₁₀₋δ provides a unique platform for studying the fundamental physics of Ruddlesden–Popper nickelates.