Paper

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.

Through high-throughput first-principles simulations, this study systematically compares the thermodynamic stability of delafossite (D1), ordered rock salt variant (D2), and infinite-layer (IL) oxides at ABO₂ stoichiometry, constructing phase diagrams encompassing 2,346 elemental combinations. The results demonstrate that for nickelates, palladates, and platinate, the delafossite structure exhibits stability comparable to or even superior to the infinite-layer phase, with competition between these two phases and the perovskite phase. Electronic structure analysis reveals that delafossite compounds feature an inverted cation order, with the Fermi surface dominated by d_{z^2} orbital contributions, distinctly different from the d_{x^2-y^2} characteristics of the infinite-layer phase. Among all candidate systems, the La-Ni combination is the thermodynamically optimal choice for stabilizing the infinite-layer structure. Furthermore, hole doping via Ca, Sr, and Ba systematically enhances the relative stability of the infinite-layer phase across the three transition metal families. These findings elucidate the fundamental challenges in synthesizing substrate-free bulk infinite-layer oxides and provide guidance for the experimental exploration of novel superconducting compounds.

Using the unrestricted Hartree-Fock method based on a multiorbital Hubbard-Hund model, we investigate the density-wave phase diagram of the low-pressure bilayer nickelate La₃Ni₂O₇. Our calculations reveal that in the orthorhombic phase, the electron system first develops a double-stripe spin-density-wave order with wave vector Q_Y = (0, π) at about 150 K; subsequently, at about 130 K, the pure double-stripe spin state becomes unstable against a commensurate charge density wave, resulting in a spin-modulated double-stripe ordered state where the magnetic moments and charge densities on the in-plane Ni1 and Ni2 sites are modulated, forming low-spin sites. This charge order parameter is an order of magnitude smaller than the magnetic order parameter and induces additional band gaps and folded Fermi surfaces in the electronic structure. The study establishes the hierarchical relationship between spin-density-wave and charge-density-wave orders in La₃Ni₂O₇, provides important clues for understanding the connection between the ambient-pressure ordered phases and the high-pressure superconducting phase, and proposes suggestions for further experimental verification.

In the ambient-pressure normal state of the trilayer Ruddlesden-Popper nickelate La₄Ni₃O₁₀, researchers observed a phonon thermal Hall effect enhanced by density-wave order. The material undergoes a density-wave transition at about 140 K, below which the thermal Hall response sharply increases; the thermal Hall angle rises from 1.5‰ at 160 K to 6‰ near 100 K, peaks at ~7‰ at 70 K, and two distinct plateaus appear in the thermal Hall resistivity. The longitudinal thermal conductivity shows almost no magnetic field dependence and has a negligible electronic contribution, confirming that phonons dominate both longitudinal and transverse thermal transport. The characteristic energy extracted from thermal Hall data is about 4.1 meV, which closely matches the magnon–phonon dispersion crossing energy of 3.2 meV, indicating that magnon–phonon hybridization induced by spin-density-wave order is the core mechanism enhancing the thermal Hall effect. This work reveals the significant modulation of phonon transport by spin–lattice coupling in nickelates and points out that such dynamic coupling may participate in suppressing antiferromagnetic order and promoting superconductivity under high pressure via softening of optical phonons, providing a new perspective for understanding the intertwining of charge, spin, and lattice degrees of freedom in unconventional superconductors.