Paper

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 investigates the pairing symmetry of the five-layer bulk superconductor La$_6$Ni$_5$O$_{12}$ by employing an effective five-layer Hubbard model constructed within density functional theory and variational Monte Carlo methods. Calculations indicate that the spin-singlet $d_{x^2-y^2}$-wave superconducting state is energetically dominant, whereas extended $s$-wave pairing is strongly suppressed. Based on the optimized state, the tunneling density of states is predicted to exhibit a V-shape, the superfluid density shows a linear low-temperature depletion, the Knight shift decreases, and the spin-lattice relaxation rate follows $1/T_1T\propto T^2$, characteristics that collectively point to a clean nodal superconductor. Furthermore, this framework reveals that infinite-layer LaNiO$_2$ possesses a robust $d$-wave tendency, while three-layer La$_4$Ni$_3$O$_8$ lies near the boundary between $d$-wave superconductivity and a non-superconducting state at nominal filling. These results establish La$_6$Ni$_5$O$_{12}$ as a platform for probing pairing symmetry in nickelates and suggest a prevalent $d_{x^2-y^2}$-wave pairing trend within the reduced Ruddlesden-Popper nickelate family.

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.

Using high-quality La₃Ni₂O₇ single crystals, the researchers established a high-precision pressure–temperature phase diagram by accurately measuring in-plane and out-of-plane resistivity. They resolved two density-wave-related anomalies with distinct pressure dependences: the low-temperature anomaly T₁ is initially suppressed and then sharply enhanced after a structural transition at about 10 GPa, while the high-temperature spin-density-wave anomaly T₂ increases monotonically. The pressure-induced structural transition not only raises the resistivity in both directions but also significantly enhances the low-temperature resistivity anisotropy, indicating that the density-wave order profoundly affects charge dynamics. Once pressure completely suppresses the density-wave phase, zero-resistance superconductivity emerges at the phase boundary with an onset temperature up to 68 K, and the normal-state resistivity exhibits a linear temperature dependence over a wide range from just above the superconducting transition to 300 K, with the scattering rate falling within the Planckian limit. These results reveal that pressure reconstructs anisotropic charge transport by tuning the density-wave order, thereby generating a strongly scattering strange-metal state and superconductivity, and establish that density-wave correlations and Planckian dissipation are defining characteristics of La₃Ni₂O₇.

Using a two-orbital bilayer model, this study systematically calculates the electronic Raman response in different Raman channels via both multiorbital and band-sum methods to distinguish the controversial pairing symmetry in the bilayer nickelate superconductor La₃Ni₂O₇. By comparing the Raman susceptibilities obtained from the multiorbital approach and the band-sum approximation, it is found that the Raman response can effectively differentiate various pairing symmetries and identify the Fermi-pocket-dependent gap sizes in fully gapped and nodal superconducting states. Specifically, nodal dₓ²⁻ᵧ²/dₓᵧ-wave pairing exhibits robust power-law behavior at low energies, distinctly different from fully gapped pairing; for s±-wave pairing, detailed gap anisotropy on the β pocket can be determined. The study also emphasizes the crucial role of multiorbital effects in shaping the Raman spectra, and points out that electronic Raman scattering, as a symmetry-resolving probe, provides a powerful means to determine the superconducting gap structure of unconventional superconductors, offering significant experimental implications for understanding the superconducting mechanism of bilayer nickelates.