Paper

Using Ni (L_3)-edge resonant inelastic X-ray scattering (RIXS), this work systematically investigates the evolution of electronic and spin excitations with carrier doping in coherently compressively strained La(_{3-x})Sr(_x)Ni(_2)O(_7)/SrLaAlO(_4) thin films, covering the superconducting ((x = 0, 0.09, 0.21)) and overdoped non-superconducting ((x = 0.38)) regimes. In the superconducting films, dispersive spin excitations persist along the ([H,H]) and ([H,0]) directions, with the dispersion remaining almost doping-independent and exhibiting minimal damping, while the spectral weight only moderately decreases, indicating robust bistripe spin correlations. However, in the non-superconducting film at (x = 0.38), the magnetic response becomes strongly broadened and weakened, accompanied by significantly enhanced damping and a spectral weight reduction of approximately 50%, signaling the collapse of coherent bistripe spin excitations. The simultaneous disappearance of magnetic coherence with superconductivity directly establishes the link between doping-controlled magnetism and superconductivity in layered nickelate thin films.

Using a minimal bilayer two-orbital model and the random phase approximation, we investigate the superconducting pairing symmetry and its doping evolution in pressurized La₃Ni₂O₇. In the undoped system, the most favorable pairing state is an s± wave whose gap function changes sign between different Fermi pockets; the analysis indicates that this unconventional pairing arises from repulsive interactions mediated by the magnetic odd mode of the bilayer nickelate. Hole doping enlarges the γ Fermi pocket, and the intrapocket repulsion driven by the magnetic even mode gradually strengthens and dominates the pairing interaction, eventually driving the pairing symmetry from s± wave to dxy wave in the heavily hole-doped regime. In contrast, under electron doping the s±-wave pairing persists and remains stable even in the deeply electron-doped region where a Lifshitz transition occurs, indicating that the γ Fermi pocket is not a necessary condition for superconductivity in bilayer nickelates; owing to favorable nesting between the α and β pockets that enhances spin fluctuations, the s± wave becomes even more robust in the absence of the γ pocket. This study reveals the doping control of pairing symmetry and provides a new route for testing the superconducting pairing mechanism in pressurized La₃Ni₂O₇.

Through muon spin rotation/relaxation and resistivity measurements combined with oxygen isotope substitution, the pressure and isotope effects on density wave transitions in the trilayer Ruddlesden-Popper nickelate La₄Ni₃O₁₀ were systematically investigated. Under ambient pressure, two incommensurate spin density wave (SDW) transitions were observed at 132 K and 80–90 K; the magnetic structure reveals that the outer two Ni layers exhibit an antiferromagnetically coupled SDW order, while the inner layer has a smaller magnetic moment, and a c-axis component of the magnetic moment emerges below T*. The abrupt onset of the internal field at T_SDW indicates that the SDW transition resembles a first-order phase change and is closely intertwined with the charge density wave (CDW) occurring at the same temperature. Under applied pressure, T_SDW, T*, and T_CDW are uniformly suppressed at a rate of approximately -13 K/GPa, differing from the behavior in bilayer La₃Ni₂O₇ where pressure increases the separation between SDW and CDW. Substitution of ¹⁶O with ¹⁸O raises T_CDW; in the region where CDW and SDW are intertwined, T_SDW also exhibits a significant isotope effect similar in magnitude to the shift in T_CDW, whereas no isotope effect is observed for the SDW at T* where it evolves independently. These results reveal the strongly intertwined nature of SDW and CDW in La₄Ni₃O₁₀ and suggest that pressure-induced suppression of the CDW order may be a key mechanism for high-pressure superconductivity in Ruddlesden-Popper nickelates.

Using first-principles density functional theory calculations, we systematically investigate the electron-doping effects of tetravalent element substitution in double-layer Ruddlesden-Popper nickelate La₃Ni₂O₇ thin films. Unlike cuprates, cerium (Ce) doping is found to be inefficient in introducing electron carriers into low-energy bands, whereas zirconium (Zr), hafnium (Hf), and thorium (Th) serve as effective electron dopants. These elemental substitutions significantly enhance the interlayer hopping integral t⊥ between Ni-dz² orbitals, potentially strengthening the interlayer superexchange coupling J⊥ and thereby potentially increasing the superconducting transition temperature Tc. Using the constrained random phase approximation to evaluate interaction parameters, we find that electron doping increases the occupancy of low-energy orbitals (including Ni-dx²-y² and dz² along with their hybridized oxygen orbitals) and alters the electron filling ratio between in-plane and interlayer orbitals. Structural analysis reveals that differences in dopant ionic radii cause variations in Ni–O bond lengths, with Zr and Hf inducing lattice contraction and Th exhibiting the strongest doping effect. These results indicate that Zr, Hf, and Th are promising candidates for achieving electron doping in La₃Ni₂O₇, offering a new avenue to clarify the ongoing debate over the electron pairing mechanism in this system.

By combining density functional theory and dynamical mean-field theory, the evolution of the electronic structure, magnetism, and correlation effects in the infinite-layer nickelate LaNiO₂ under electron and hole doping is investigated. The results reveal that, due to the presence of rare-earth 5d states, the self-doping effect of the Ni-d_{x²-y²} band exhibits significant asymmetry: hole doping strongly suppresses self-doping, whereas electron doping, while enlarging the rare-earth 5d electron pocket, does not effectively hole-dope the Ni-d_{x²-y²} band. This difference directly impacts the magnetic response—hole doping rapidly suppresses antiferromagnetic order, while electron doping maintains the antiferromagnetic state as the ground state. Despite these disparities, the electronic correlations in both doping regimes are dominated by the Ni-d_{x²-y²} orbital, suggesting that a single-band description may be applicable in both electron- and hole-doped regions.

Predicting superconducting transition temperature (Tc) remains challenging. This study proposes an interpretable, structure- and chemistry-aware Gaussian process model, GP-Tc, that encodes local bonding environments via graphlet histograms and constructs an effective kernel using Earth mover’s distance, enabling Tc prediction with uncertainty. Analysis shows that Tc across different superconducting families can be predicted using only the distribution of electron affinity differences between adjacent atoms, interatomic distances, and a small number of elemental features, revealing that electron affinity difference is a key chemical parameter linking local bonding to macroscopic superconductivity in a mechanism-agnostic manner. The model reproduces the experimental Tc range of the infinite-layer nickelate Nd0.8Sr0.2NiO2 and predicts superconductivity in stoichiometric PtPb3Bi, which is experimentally confirmed with Tc≈3 K. In addition, GP-Tc is openly accessible through a web interface and identifies high-priority candidate materials such as SrNiO2 and K(PRh)2.

Using ozone-assisted atomic layer epitaxy, the researchers grew La2PrNi2O7 thin films on NdAlO3 substrates, introducing an extreme compressive strain of up to −2.14% and achieving high-temperature superconductivity with an onset critical temperature of 60 K, a zero-resistance temperature of 33 K, and a diamagnetic transition at 20 K, while magnetotransport measurements confirmed a quasi-two-dimensional superconducting state. A comparison of the phase diagrams between the strained films and high-pressure bulk materials reveals that, although both suppress spin-density waves to drive superconductivity, the lattice responses diverge: the film’s c-axis parameter window markedly departs, whereas the in-plane parameters coincide with those of the bulk. Crucially, Hall measurements uncover a dichotomy in electronic character, as the optimal superconducting film exhibits a negative Hall coefficient indicative of electron-like behavior, in stark contrast to the positive, hole-like Hall coefficient of high-pressure bulk samples and non-superconducting tensile-strained films. These observations indicate that both strain and pressure strategies effectively tune the underlying correlation modulation beyond the constraints of a specific Fermi surface topology, thereby enabling superconductivity. This work establishes a macroscopic platform for studying multi-orbital physics in nickelates and provides a new dimension for exploring the mechanism of high-temperature superconductivity.

High-temperature superconductivity was discovered in the pressurized nickelate La3Ni2O7 which has a unique bilayer structure and mixed valence state of nickel. The properties at ambient pressure contain crucial information of the fundamental interactions and bosons mediating superconducting pairing. Here, using X-ray absorption spectroscopy and resonant inelastic X-ray scattering, we identified that Ni 3 $${d}_{{x}^{2}-{y}^{2}}$$, Ni 3 $${d}_{{z}^{2}}$$, and ligand oxygen 2p orbitals dominate the low-energy physics with a small charge-transfer energy. Well-defined optical-like magnetic excitations soften into quasi-static spin-density-wave ordering, evidencing the strong electronic correlation and rich magnetic properties. Based on an effective Heisenberg spin model, we extract a much stronger inter-layer effective magnetic superexchange than the intra-layer ones and propose two viable magnetic structures. Our findings emphasize that the Ni 3 $${d}_{{z}^{2}}$$orbital bonding within the bilayer induces novel electronic and magnetic excitations, setting the stage for further exploration of La3Ni2O7 superconductor.

Trilayer Ruddlesden-Popper phase La4Ni3O10 has been observed with Tc of ∼30 K at high pressure in a recent experiment, which further expanded the family of nickelate superconductors. In this study, we explored the effects of electronic correlations in La4Ni3O10 using density functional theory plus dynamical mean-field theory at ambient and high pressures. Our derived spectral functions and Fermi surface of the ambient pressure phase are nicely consistent with the experimental results by angle-resolved photoemission spectroscopy, which emphasized the importance of electronic correlations in La4Ni3O10. We also found the electronic correlations in pressurized La4Ni3O10 are both orbital-dependent and layer-dependent due to the presence of Hund’s rule coupling. There is a competition between the Hund’s rule coupling and the crystal-field splitting, and therefore, the Ni–O layers with weaker crystal-field splitting energy would have stronger electronic correlations.

The discovery of superconductivity with a critical temperature of about 80 K in La3Ni2O7 single crystals under pressure has received enormous attention. La3Ni2O7 is not superconducting under ambient pressure but exhibits a transition at T ∗ ≃ 115 K. Understanding the electronic correlations and charge dynamics is an important step towards the origin of superconductivity and other instabilities. Here, our optical study shows that La3Ni2O7 features strong electronic correlations which significantly reduce the electron’s kinetic energy and place this system in the proximity of the Mott phase. The low-frequency optical conductivity reveals two Drude components arising from multiple bands at the Fermi level. The transition at T ∗ removes the Drude component exhibiting non-Fermi liquid behavior, whereas the one with Fermi-liquid behavior is barely affected. These observations in combination with theoretical results suggest that the Fermi surface dominated by the Ni- $${d}_{3{z}^{2}-{r}^{2}}$$orbital is removed due to the transition at T ∗. Our experimental results provide pivotal information for understanding the transition at T ∗ and superconductivity in La3Ni2O7.