Paper

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.

We employ a density-functional theory plus dynamical mean-field theory framework to investigate the correlated electronic structure of the alternating single-layer–trilayer (1313) polymorph of La3⁢Ni2⁢O7, which becomes superconducting under pressure. At ambient pressure, the single layer is in a Mott-insulating regime and the low-energy physics is dominated by the trilayer block. Under pressure, the gap in the single-layer block closes due to orbital-selective physics, enabling charge transfer into the trilayer block. This change in effective doping of the trilayer block could be linked to the higher 𝑇𝑐 obtained in La3⁢Ni2⁢O7−1313 (∼80 K) when compared to the nominal trilayer La4⁢Ni3⁢O10 compound (∼30 K). We conclude that correlation-driven layer differentiation is crucial in the La3⁢Ni2⁢O7−1313 polymorph and that its low-energy physics aligns closely with the trilayer La4⁢Ni3⁢O10 compound (in spite of the apparent differences in nominal filling) rather than with the conventional bilayer La3⁢Ni2⁢O7.

Using polarized ultrafast pump-probe spectroscopy, the researchers comparatively investigated the normal-state electronic dynamics of bilayer La₃Ni₂O₇ and trilayer La₄Ni₃O₁₀ single crystals under ambient pressure. Both materials exhibit a density-wave transition accompanied by the opening of a quasiparticle relaxation bottleneck, yet their electronic responses display markedly different symmetries: trilayer La₄Ni₃O₁₀ remains optically isotropic across the entire temperature range, whereas bilayer La₃Ni₂O₇ shows clear twofold (C₂) rotational symmetry breaking—i.e., electronic nematicity—at low temperatures. This nematicity manifests in the anisotropy of slow quasiparticle relaxation dynamics and effective gap scale, and below 115 K it competes with a secondary isotropic order, leading to a non-monotonic temperature dependence of the nematic signal. This work reveals the presence of electronic nematic fluctuations in bilayer nickelates, which are absent in the trilayer system, suggesting a close relationship between electronic nematicity and high-pressure superconducting pairing in La₃Ni₂O₇, thereby providing key insights into the microscopic mechanism of this class of nickel-based superconductors.

This work uses ambient-pressure broadband infrared-to-visible reflectivity spectra (15 meV–3.2 eV) combined with high-temperature Raman measurements to investigate the electronic response of La3Ni2O7 before and after its transition at approximately 544 K from a NiO6 octahedrally tilted phase to an untilted structure. The results show that spectral weight undergoes pronounced redistribution over an exceptionally broad energy range, transferring from 1–3 eV excitations to low-energy excitations below 1 eV; in particular, two low-energy interband excitations gradually approach and merge upon entering the untilted phase, indicating a substantial reconstruction of the finite-energy electronic structure. Drude–Lorentz analysis further attributes these changes mainly to finite-energy excitations rather than the Drude response, and points to reconstruction of bilayer Ni 3d_{z^2}-derived electronic states, whose interlayer coupling is considered central to the superconducting mechanism of La3Ni2O7. On this basis, the authors argue that the transition to the untilted phase can directly reorganize electronic degrees of freedom relevant to high-temperature superconductivity and provide an ambient-pressure reference for the electronic structure of the superconducting state.

Building on the discovery of high-pressure superconductivity in trilayer nickelate La₄Ni₃O₁₀, this study employed density functional theory combined with dynamical mean-field theory (DFT+DMFT) to design and calculate the cobalt-based analogue La₄Co₂NiO₈Cl₂. By substituting the inner-layer Co in the high-pressure phase La₄Co₃O₁₀ with Ni and incorporating Cl to achieve electron doping, this compound acquires a crystal structure and strongly correlated electronic characteristics similar to those of superconducting La₄Ni₃O₁₀: the outer-layer Co orbitals exhibit strong effective mass enhancement and non-Fermi liquid behavior, while the inner-layer Ni behaves as a weakly correlated Fermi liquid; a flat band near the Fermi level originating from the outer-layer Co orbitals emerges around the M point; and there is pronounced orbital selectivity as well as local spin fluctuations mixing high-spin and low-spin states. These features are in close agreement with the key electronic states of La₄Ni₃O₁₀, indicating that La₄Co₂NiO₈Cl₂ is a promising candidate for realizing high-temperature superconductivity in cobalt-based layered compounds, providing a theoretical basis for subsequent experimental exploration.