Paper

Using polarization-resolved electronic Raman scattering, this study systematically maps the momentum-selective spin-density-wave (SDW) gap in the trilayer nickelate La4Ni3O10. The experiments reveal that SDW-induced spectral weight redistribution occurs simultaneously on the α pocket at the Brillouin zone center and on part of the β pocket near the zone boundary, with a corresponding gap energy of approximately 55 meV, whereas no comparable spectral weight suppression is observed in the diagonal region of the β pocket, indicating that this region remains nearly gapless. This momentum-space gap topology contrasts sharply with that of the bilayer nickelate La3Ni2O7, where only the β pocket exhibits an anisotropic SDW gap. These results establish distinct momentum-space gap topologies between bilayer and trilayer nickelates, providing new constraints on the ordering wave vector of the density-wave instability and the mechanism related to superconductivity.

Using Ni L3-edge resonant inelastic X-ray scattering (RIXS), this study compares low-energy spin excitations in the octahedral Ruddlesden-Popper (RP) phase Nd9Ni8O25 (non-superconducting) and its reduced planar phase Nd9Ni8O18 (exhibiting superconducting correlations at approximately 5 K). The results show that the octahedral phase exhibits a spin-density wave (SDW) ground state with an ordering wave vector of (1/4,1/4), where the low-energy spectrum is dominated by weakly dispersive paramagnons along the (0,π) and (π,π) directions; in contrast, the planar phase displays an elastic peak at (1/3,0) with dispersionless magnetic excitations at an energy of about 65 meV. Polarization-resolved RIXS further confirms the distinct nature of magnetic excitations in the two phases. These findings systematically reveal fundamental differences in the ground states and spin excitations between the two structural families, providing critical insights into the mechanism of nickelate superconductivity.

This study employs high-pressure synthesis and hydrostatic high-pressure transport techniques to systematically modulate the bandwidth and band filling in the bilayer nickelate La₃Ni₂O₇ family, aiming to investigate their effects on superconductivity and non-superconducting state properties. By partially substituting La with smaller Nd (which increases NiO₆ octahedral tilting and reduces bandwidth), the pressure required for the superconducting phase is significantly elevated; conversely, co-introducing Sr for hole doping reverses this trend, lowering the onset pressure of superconductivity. In the non-superconducting state, up to three characteristic resistance anomalies are observed, evolving with pressure, likely corresponding to charge density wave and spin density wave orders that compete with superconductivity. A comprehensive comparison of phase diagrams across samples with different compositions indicates that independent control of bandwidth and filling is key to unraveling the mechanism of unconventional superconductivity and its competing orders in this system.

The recent observation of high-𝑇𝑐 superconductivity in the bilayer nickelate La3⁢Ni2⁢O7 under pressure has garnered significant interest. While researches have predominantly focused on the role of electron-electron interactions in the superconducting mechanism, the impact of electron-phonon coupling (EPC) has remained elusive and unexplored. In this Letter, we perform first-principles calculations to study the phonon spectrum and electron-phonon coupling within La3⁢Ni2⁢O7 under pressure and explore the interplay between EPC and electronic interactions on the superconductivity by employing functional renormalization group (FRG) approach. Our calculations reveal that EPC alone is insufficient to trigger superconductivity in La3⁢Ni2⁢O7 under pressure. We identify unique out-of-plane and in-plane breathing phonon modes which selectively couple with the Ni 𝑑𝑧2 and 𝑑𝑥2−𝑦2 orbitals, showcasing an orbital-selective EPC. Within the bilayer two-orbital model, it is revealed that solely electronic interactions foster 𝑠±-wave pairing characterized by notable frustration in the band space, leading to a relatively low transition temperature. Remarkably, we find that the out-of-plane EPC can act in concert with electronic interactions to promote the interlayer pairing in the 𝑑𝑧2 orbital, partially releasing the pairing frustration and thus elevating 𝑇𝑐. In contrast, the inclusion of in-plane EPC only marginally affects the superconductivity, distinct from the cuprates. Potential experimental implications in La3⁢Ni2⁢O7 are also discussed.

The discovery of pressurized superconductivity at 80 K in La3Ni2O7 officially brings nickelates into the family of high-temperature superconductors, which gives rise to not only new insights but also mysteries in the strongly correlated superconductivity. More recently, the sibling compound La4Ni3O10 was also shown to be superconducting below about 25 K under pressure, further boosting the popularity of nickelates in the Ruddlesden-Popper phase. In this study, combining high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structures of La4Ni3O10 at ambient pressure. We reveal a high resemblance of La4Ni3O10 with La3Ni2O7 in the orbital-dependent fermiology and electronic structure, suggesting a similar electronic correlation between the two compounds. The temperature-dependent measurements imply an orbital-dependent energy gap related to the density-wave transition in La4Ni3O10. By comparing the theoretical pressure-dependent electronic structure, clues about the superconducting high-pressure phase can be deduced from the ambient measurements, providing crucial information for deciphering the unconventional superconductivity in nickelates.

This study employed scanning superconducting quantum interference microscopy to perform local magnetic susceptibility and magnetic flux imaging on infinite-layer nickelate superconductor Nd₀.₈Sr₀.₂NiO₂ thin films, aiming to elucidate the correlation between the zero-temperature superfluid density and the superconducting transition temperature. Owing to micron-scale inhomogeneities in the samples, spatial statistical analysis revealed that when the local (T_c) exceeds 8 K, (T_c) exhibits a linear relationship with (\rho_{s0}); conversely, when (T_c) is below 8 K, the dependence becomes sublinear (approximately (T_c \propto \rho_{s0}^{1/2})). This overall behavior closely resembles observations in overdoped cuprate superconductors, suggesting a potentially intimate intrinsic connection between the superconducting mechanisms of infinite-layer nickelate and cuprate superconductors.

Based on the bilayer two-orbital Hubbard model, this study systematically investigates the effects of electronic correlations on the Fermi surface topology and superconducting pairing symmetry in La₃Ni₂O₇ using the time-dependent variational principle cluster perturbation theory (TDVP-CPT) and large-scale density matrix renormalization group (DMRG) methods. TDVP-CPT calculations on clusters containing up to 16 physical sites reveal that electronic correlations drive significant orbital-selective low-energy spectral reconstruction: the spectral weight of the d_{z²} orbital is progressively depleted, the γ band sinks below the Fermi level, while the α and β bands exhibit a pseudogap, ultimately leading to the formation of a Fermi arc dominated by the d_{x²-y²} orbital in the strong-coupling regime. DMRG calculations further demonstrate that the dominant superconducting pairing correlations evolve consistently with this Fermi surface reconstruction, transitioning from interlayer spin singlet pairing mediated primarily by the d_{z²} orbital in the weak-coupling regime to pairing dominated by the d_{x²-y²} orbital in the strong-coupling regime, while maintaining s±-wave symmetry throughout. The study indicates that the disappearance of the γ Fermi surface does not suppress superconductivity but instead signifies a correlation-driven change in the pairing channel, with key intermediate mechanisms including interlayer antiferromagnetic fluctuations, Hund coupling, and interorbital hybridization.

To resolve the unsettled superconducting pairing symmetry in pressurized La₃Ni₂O₇, this study employs a four-orbital Wannier Hamiltonian and incorporates the self-energy from single-site two-orbital dynamical mean-field theory (DMFT) into the random phase approximation (RPA), constructing self-energy-renormalized particle–hole bubbles to replace the bare bubbles while retaining the same local Slater-Kanamori interaction vertices. Conventional RPA calculations reveal that the dominant pairing belongs to the B₂g dxy channel, but once the DMFT self-energy is included, the pairing hierarchy is reversed: the sign-changing A₁g s± state becomes dominant, the B₁g dx²-y² channel takes the second place, and the original B₂g instability is strongly suppressed. Pocket-resolved decomposition and orbital-resolved susceptibility analyses show that this reversal originates from the selective renormalization of the d3z²-r² orbital, which filters out γ-pocket scattering processes that favor dxy pairing while preserving distributed inter-pocket scattering conducive to s±. Further employing the dual Bethe-Salpeter equation with local DMFT vertices to compute the static spin susceptibility yields a broad finite-momentum magnetic response that is weak near the Γ point, reinforcing the spin-fluctuation background for the s± state at the two-particle level. These results demonstrate that strong correlation effects in La₃Ni₂O₇ are not minor corrections; properly treating correlation-renormalized quasiparticles is essential for accurately predicting the superconducting pairing symmetry.

This paper reports inverse superconducting transitions realized beyond the liquid-helium cooling limit in Eu-based infinite-layer nickelates (EuxNd1‑xNiO₂ and EuxPr1‑xNiO₂). Through magnetic-field tuning, the zero-resistance superconducting state is observed to be confined between a lower critical temperature (Tc‑inv ≈ 2.6–5.4 K) and a higher normal Tc in both overdoped and underdoped regions; raising the temperature or increasing the current density can drive the system from a resistive state into superconductivity, which then vanishes again at higher temperatures and currents. Systematic temperature-dependent transport measurements reveal that this inverse superconducting transition in the Kelvin range arises from the temperature-driven alternating dominance of a compensating effective magnetic field associated with Eu²⁺ 4f⁷ moments and the upper critical field, supported by a temperature-induced re-entrant superconductivity phenomenon where superconductivity reappears at around 300 mK under an applied magnetic field. This work establishes a high-temperature superconductor system with magnetically reconstructed interactions as a platform for exploring quantum phenomena that reverse the paradigm of thermal decoherence, and opens application avenues for the inverse design of quantum phase-transition devices.

This study presents a detailed structural analysis of epitaxial La₃Ni₂O₇₋δ thin films using scanning transmission electron microscopy combined with electron energy loss spectroscopy. The films were prepared on SrLaAlO₄ substrates via pulsed laser deposition and exhibited significantly distinct superconducting properties after different ozone annealing treatments. It was found that the stabilization of the superconducting phase is closely related to oxygen stoichiometry uniformity, epitaxial strain, and specific stacking structural motifs such as bilayers and polytypes. By correlating the rich morphology of stacking polytypes with transport behavior, a theoretical framework for understanding metastable superconducting phases in bilayer nickelate thin films was established. The results reveal the critical roles of oxygen content, lattice strain, and structural ordering in achieving ambient-pressure superconductivity, providing a clear pathway for designing new nickel-based superconducting materials.