Paper

Combining a correlated multi-orbital quasiparticle model derived from DFT+CDMFT with the T-matrix method, this study treats oxygen vacancy scattering within a semiclassical Boltzmann transport framework and reveals the microscopic origin of the Hall coefficient sign reversal in bilayer nickelate thin films. Multiband compensation alone is insufficient to explain the phenomenon; in-plane oxygen vacancies strongly suppress the transport channel dominated by the d_{x^2-y^2} orbital through orbital-selective scattering, driving the Hall coefficient across zero to become positive, whereas apical oxygen vacancies tend to make the Hall coefficient more negative. This pocket-resolved and orbital-selective scattering mechanism demonstrates that oxygen vacancies act not only as electron doping sources but also as active scattering centers whose spatial distribution directly controls the normal-state transport behavior, providing a theoretical framework for a unified understanding of the diverse Hall responses observed experimentally as a function of oxygen stoichiometry.

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.

This study proposes a novel route to electron doping in nickelate superconductors through heterostructuring. First-principles calculations reveal that upon inserting wide-bandgap insulating layers of LaXO₃ (X = Al, Ga, Sc) into La₂NiO₄, the additional (LaO)⁺ layers act as electron donors, releasing carriers into the Ni-3d orbitals and thereby achieving disorder-free electron doping of Ruddlesden–Popper nickelates. This doping naturally places La₂NiO₄:La₂AlO₄ in the optimal regime for d_{x^2−y^2}-wave superconductivity, with many-body methods—including dynamical vertex approximation, fluctuation exchange, and dynamical cluster approximation—predicting a superconducting critical temperature exceeding 50 K and reaching as high as 127 K. The strategy circumvents the disorder typically introduced by conventional chemical doping, and is equally applicable to other nickelates such as La₃Ni₂O₇, offering a viable scheme for comprehensively mapping the electron-doping phase diagram of nickelates and extending the approach to other transition metal oxide systems.

This study employs the variational Monte Carlo method to perform nonperturbative calculations on a bilayer two-orbital Hubbard model, revealing the hierarchical structure of superconducting pairing in the bilayer layered nickelate La₃Ni₂O₇. It is found that the primary pairing interaction originates from the bonding-anti-bonding splitting of the Ni 3d({z^2}) orbital, while orbital hybridization redistributes superconducting correlations into the 3d({x^2-y^2}) channel, despite its intrinsically weak pairing interaction. This distinction between the origin of pairing and the source of superconducting correlations explains why both orbital channels exhibit comparable long-range superconducting correlations, and the resulting s± state is robust against changes in Fermi surface topology, such as the disappearance of the α Fermi pocket. The results reconcile previously divergent theoretical perspectives on the pairing mechanism, indicating that the strength of superconducting correlations is primarily determined by the orbital character of the low-energy density of states, whereas the pairing interaction stems from orbital level splitting in the bilayer structure, highlighting the crucial role of orbital hybridization in stabilizing superconductivity in multilayer layered superconductors.

High oxygen pressure floating zone growth and crystal structure of the metallic nickelates R 4 Ni 3 O 10 ( R = La , Pr )

This study systematically investigates single-crystal and powder samples of Ruddlesden-Popper nickelates La_{n+1}Ni_nO_{3n+1} (n=1,2,3,∞) using a combination of powder and single-crystal X-ray diffraction, heat capacity, and differential scanning calorimetry measurements across a broad temperature range of 2–1000 K, revealing a previously overlooked high-temperature phase transition. For the n=2 and n=3 compounds, pronounced lattice-parameter anomalies are observed around 560 K: in the bilayer 2222 phase, the out-of-plane lattice constant exhibits a sudden increase while the in-plane parameter contracts, indicating an abrupt release of octahedral tilting, whereas the monolayer–trilayer 1313 polytype displays an isotropic volume collapse; in the trilayer n=3 phase, the monoclinic angle β shows a clear kink near this temperature, and heat-capacity and DSC data further confirm the thermodynamic character of the transition. This transition is entirely distinct from the known high-temperature tetragonal transition and the low-temperature density-wave transition, and the n=∞ perovskite LaNiO₃ shows no analogous behavior. The study establishes that this high-temperature phase transition is a universal feature of the nickelate RP series and emphasizes that, in the search for superconductivity, the potential influence of this high-temperature structural instability on low-temperature physical properties must be carefully considered.

This study employs time-resolved optical spectroscopy to investigate the ultrafast dynamics of high-energy electronic excitations in bilayer nickelate La₃Ni₂O₇ from 10 K to room temperature at ambient pressure. Two high-energy electronic excitations originating from distinct interband transitions are identified at approximately 1.8 eV and 2.4 eV, revealing different density wave (DW) gaps of about 54 meV and 67 meV, respectively. The relaxation dynamics of these two excited states are well described by the Rothwarf-Taylor model. Additionally, four coherent Raman-active phonon modes are observed, exhibiting varying coupling strengths to the different electronic excitations. The phonon softening upon heating from about 100 K to room temperature can be explained by a semi-quantitative model incorporating thermal expansion and anharmonic phonon-phonon coupling, while the deviation of measured phonon frequencies from the model fit at low temperatures suggests an additional contribution from electron-phonon coupling. This work directly demonstrates the complex gap structure and phonon dynamics in this material, providing key insights into its density wave mechanism and many-body effects.

This study reports the discovery of high-magnetic-field-stabilized reentrant superconductivity in (Sm,Eu,Ca,Sr)NiO₂ infinite-layer nickelate thin films. Through resistive and radio-frequency inductive measurements, in addition to the low-field superconducting state, another superconducting state characterized by a sharp resistivity drop was observed in the high-field region, with a transition temperature of approximately 9.6–11.7 K in low-Tc samples, while in high-Tc samples (up to 31.7 K) the low-field and high-field superconducting phases merge, and the upper critical field far exceeds the Pauli paramagnetic limit. The phase diagram can be accurately described by a Werthamer-Helfand-Hohenberg (WHH) model modified with an internal exchange field, indicating that this phenomenon originates from the Jaccarino-Peter compensation mechanism, where the internal exchange field generated by the Eu²⁺ magnetic moments counteracts the applied magnetic field. This finding realizes for the first time field-induced reentrant superconductivity in materials with relatively high superconducting transition temperatures, offering a new pathway toward developing superconducting magnets and devices capable of operating under magnetic fields of several tens of teslas.

Single crystals of Ruddlesden–Popper nickelates La4Ni3O10 were grown by means of the floating-zone technique at oxygen pressure of 20 bar. Our results reveal the effects of the annealing process under pressure on the crystal structure. We present the requirements for crystal growth and show how a reported ferromagnetic impurity phase can be avoided. The different growth and post-annealing processes result in two distinct phases 𝑃 21∕𝑎 and Bmab in which the metal-to-metal transitions occur at 152 K and 136 K, respectively.

Researchers have developed a technique to integrate free-standing infinite-layer Nd₀.₈₅Sr₀.₁₅NiO₂ thin films into diamond anvil cells, thereby overcoming the difficulties of measuring such films under high-pressure conditions. By applying pressures up to approximately 90 GPa to the films, they observed that the superconducting transition temperature (T_c) increased monotonically and linearly from about 17 K at ambient pressure to roughly 74.2 K, with an enhancement rate of approximately 0.65 K/GPa and no signs of saturation. This linear, non-saturating pressure dependence of T_c markedly differs from the pressure-induced overdoping that leads to T_c suppression in most copper oxide superconductors and bilayer nickelates, suggesting that the pairing strength in infinite-layer nickelates can be elevated to unexpectedly high levels. Furthermore, measurements of the upper critical field and coherence length confirm the pressure-induced enhancement of the superconducting state. This study provides a new pathway for continuously enhancing superconductivity through lattice compression, and the developed free-film high-pressure technique holds promise for broad application to other two-dimensional materials.