Paper

This study uses the bilayer t-J-J⊥ ladder as a theoretical model (inspired by bilayer cuprate superconductors and La3Ni2O7), where t is the in-plane electron hopping, J is the in-plane spin exchange, and J⊥ is the interlayer spin exchange; using the density matrix renormalization group method, it tunes J⊥ over a wide doping range δ=1/8–1/2 and, with spin-singlet pairing correlations as the main criterion, constructs the quantum phase diagram of the bilayer two-leg ladder. The results show that sufficiently large J⊥ can drive interlayer superconductivity in two parent states, the Luther-Emery liquid and the Luttinger liquid: when two Luther-Emery liquids are coupled, in-plane superconductivity can transform into interlayer superconductivity either through an intermediate charge-density-wave (CDW) phase over a finite doping interval or directly, and this CDW phase likely develops from the CDW state of the two-leg ladder at δ=1/4; when two Luttinger liquids are coupled, large J⊥ drives the system into interlayer superconductivity, whereas at intermediate J⊥ the in-plane Luttinger-liquid features remain stable and interlayer superconductivity exhibits enhanced quasi-long-range order with power-law exponent Kzz_SC~1. Results for the three-leg ladder further indicate that strong J⊥ can stabilize interlayer superconductivity over a wider doping range, providing a numerical basis for understanding interlayer pairing in bilayer cuprates and nickelates.

The recent discovery of high-temperature superconductivity in bilayer nickelate La3Ni2O7−δ single crystals under high pressure has sparked significant interest in the correlated electronic physics and unconventional pairing mechanisms within Ruddlesden-Popper phase nickelates, where the oxygen content plays a crucial role in both structure and superconductivity. Here we report the evolution of the structure and density wave order in La3Ni2O7−δ single crystals at ambient pressure under various oxygen annealing conditions. Structural analysis reveals that with the increase of annealing oxygen pressure, La3Ni2O7−δ undergoes a structural phase transition from an orthorhombic to a tetragonal phase. Magnetic torque measurements indicate that the density wave order, present in the orthorhombic phase, vanishes upon transition to the tetragonal structure, accompanied by an abrupt change in carrier concentration. A phase diagram illustrating the dependence of both the crystal structure and density wave order on the annealing oxygen pressure is mapped. Our findings suggest a promising pathway for studying the interplay between structure, density wave order, and superconductivity in bulk nickel-based materials.

This paper systematically measures the superfluid density of the infinite-layer nickel-based superconductor Nd1-xSrxNiO2 within the doping superconducting dome using the mutual inductance method. The results show that the superfluid stiffness is weak and exhibits an approximate square-root relationship with the superconducting transition temperature Tc. Additionally, a strong interaction between the Nd 4f magnetic moments and the superfluid is observed, leading to a significant suppression of the superfluid density at low temperatures, with an effect far beyond simple paramagnetic explanations, suggesting a coupling between magnetic order and the superconducting phase. These findings indicate that superconducting phase fluctuations play an important role in limiting Tc and reveal an unexpectedly strong coupling between rare-earth magnetic ions and the superfluid.

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.

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.

Bilayer Ruddlesden-Popper nickelates exhibit superconductivity near 80 K under high pressure, and recent work has stabilized RA₃Ni₂O₇ (RA = rare earth or alkaline earth element) superconducting thin films at ambient pressure via epitaxial strain, enabling transport, spectroscopic, microscopic, and device measurements. This review summarizes experimental progress on ambient-pressure superconducting bilayer nickelate thin films, covering synthesis routes, oxygen stoichiometry, substrate-induced strain, normal-state transport, superconducting properties, doping phase diagrams, and momentum-resolved electronic structure. Key unresolved issues include the reproducibility of phase-pure ultrathin films, the microscopic origin of the two-step superconducting transition, the roles of oxygen defects and substrate doping, the position of the Ni 3dz₂-derived γ band, and the pairing symmetry. The review concludes that future work must establish more quantitative links between crystal structure, orbital reconstruction, and superconductivity to deepen the understanding of this unconventional high-temperature superconducting system.

This study systematically analyzes the electronic correlation behavior of La₃Ni₂O₇ within the superconducting pressure range using first-principles simulations combined with constrained random phase approximation and ab initio molecular dynamics. The results show that, accompanying the structural phase transition, the effective on-site repulsion of Ni e_g orbitals is significantly enhanced, attributed to the dynamic balance between orbital localization and competing screening channels, particularly the spacer-layer La bands. This enhancement region aligns remarkably well with the experimentally observed right-triangular superconducting dome, reaching a peak correlation strength at 18 GPa that corresponds to the highest superconducting critical temperature. Finite-temperature simulations further clarify the boundaries of the structural phase diagram, while calculations on Ac₃Ni₂O₇ confirm the critical role of A-site cations in the pressure-driven evolution of electronic correlations. These findings directly reveal how structural changes drive unconventional superconductivity by modulating the strength of electronic correlations.

This study systematically investigated the evolution of PrNiOₓ thin films at various intermediate stages of topological reduction (x = 2–3) using soft X-ray absorption spectroscopy. By comparing Ni L-edge experimental spectra with single-cluster and double-cluster ligand-field model calculations, it was found that none of the samples exhibited a pure d⁹ electronic configuration. Quantitative analysis based on the charge sum rule revealed that even in the most reduced films, the average number of Ni 3d holes remained 1.35, while superconducting samples displayed higher hole counts, challenging previous assumptions regarding the hole doping limit. Concurrent changes in the O K-edge absorption spectra during reduction indicated the presence of O 2p holes even in the most reduced films. Collectively, these results suggest that a complex hole doping mechanism arises from the interplay between self-doping effects and oxygen non-stoichiometry.

Research indicates that the two-step superconducting transition observed in La₂PrNi₂O₇₋δ thin films originates from their granular superconducting nature, where two superconducting phases with distinct critical temperatures coexist and couple through a Josephson junction network. For films grown via pulsed laser deposition and subsequently ozone-annealed, transport measurements reveal a pronounced secondary low-temperature transition even when the residual resistance is minimal near 30 K, resulting in a zero-resistance temperature of only about 10 K. The hysteresis in magnetoresistance and the sensitive response to weak magnetic fields align with the effective field model of granular superconductors, ruling out the possibility of a spin-glass phase. Structural characterization identifies oxygen inhomogeneity and local structural disorder, such as monolayer phase intercalation, as the primary causes of the observed phase separation. These findings elucidate the microscopic mechanisms underlying the complex superconducting behavior in bilayer nickelate films and underscore that improving oxygen uniformity is crucial for achieving bulk superconductivity with higher zero-resistance temperatures, thereby providing a foundation for subsequent spectroscopic studies.

We report systematic magnetotransport measurements on high-crystallinity overdoped infinite-layer nickelate La₁₋ₓSrₓNiO₂ thin films (x = 0.20–0.24), revealing two prominent normal-state features in pulsed magnetic fields up to 62 T: the magnetoresistance violates Kohler’s rule and exhibits H-linear behavior at high H/T limits, while the normal-state resistivity consistently follows a T² dependence below 30 K. These results demonstrate the coexistence of H-linear magnetoresistance and T² resistivity in this model unconventional superconductor, providing new insights into the transport properties of the normal ground state that hosts superconductivity in infinite-layer nickelates.