Paper

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.

Using density functional theory, we calculate the electronic structure of the bilayer infinite-layer nickelate La₃Ni₂O₅F, revealing its ideal two-dimensional character and the coexistence of two distinct quasiparticle behaviors. After treating the oxygen/fluorine disorder with the virtual crystal approximation, band structure calculations show that the conventional Ni dpσ band forms a hole-like Fermi surface, whereas an E* band originating from interstitial density gives rise to a cylindrical electron Fermi surface, resulting in a self-doping of 0.18 electrons. This interstitial density is distributed between the La layers that lack apical oxygen, and as the Ni dₓz/dyz bands approach the M point with parallel linear dispersion, they couple with it to form a nearly non-analytic Dirac point, exhibiting an exotic interstitial-orbital band coupling effect. Concurrently, the d_z² band undergoes a symmetry-driven splitting of approximately 1 eV through interaction with the interstitial density. This dual electron–hole character is expected to govern normal-state transport and far-infrared properties, may influence the superconducting state of nickelates, and offers a fresh perspective for understanding the physics of infinite-layer nickelates.

This study investigates the dimensionality of the superconducting state in infinite-layer nickel oxides by mapping the vortex phase diagram of superconducting Pr0.8Sr0.2NiO2 thin films from multiple perspectives. Experimental results reveal that low-disorder films exhibit a quasi-two-dimensional vortex liquid-to-glass transition, while increasing disorder drives the system into a pure two-dimensional state. This finding indicates that pure two-dimensionality is not an intrinsic property but an extrinsic phenomenon caused by the decoupling of NiO2 layers due to enhanced disorder. The work establishes disorder as a key tuning parameter for superconductivity in infinite-layer nickel oxides and identifies that disorder primarily resides within the NiO2 layers, offering two fundamental insights for understanding this class of materials.

Disorder-Induced Suppression of Superconductivity in Infinite-Layer Nickelates

This study employs resonant inelastic X-ray scattering (RIXS) to directly compare the electronic and magnetic excitation properties of trilayer nickelate La₄Ni₃O₁₀ and bilayer La₃Ni₂O₇. The results show that La₄Ni₃O₁₀ exhibits more itinerant behavior, evidenced by broader Ni d-d orbital excitations and a stronger fluorescence background, indicating weaker electronic correlations than in the bilayer system. Despite the weaker correlations, clear collective spin excitations are observed, including dispersive acoustic and optical magnon branches as well as incommensurate spin density waves (SDW). Using linear spin-wave theory analysis, the interlayer superexchange interaction Jz is extracted to be approximately 22 meV, significantly smaller than that in La₃Ni₂O₇. The weaker electron correlations and reduced interlayer magnetic exchange together account for the substantially lower superconducting transition temperature of the trilayer compound (about 30 K) compared to the bilayer (about 80 K). This study establishes interlayer magnetic coupling and electronic correlations as key parameters for superconductivity in layered nickelates, providing important constraints for understanding the superconducting pairing mechanism in this emerging family.

Using muon spin rotation/relaxation (μSR), this study systematically measured a series of infinite-layer nickelate (La,Sr)NiO₂ samples spanning from the parent compound through the superconducting dome to the overdoped regime (Sr doping 0% ≤ x ≤ 25%) to probe the magnetic ground state and temperature-dependent static and dynamic magnetism. The results show that, regardless of doping level, local magnetic moments undergo spin freezing at temperatures on the order of tens of kelvin and enter a glassy state, and no anomaly is observed near the superconducting onset, indicating that the magnetism is intrinsic and essentially independent of hole concentration. With increasing hole doping, the glassy state shows only a weak tendency toward destabilization. These observations suggest that magnetism and superconductivity in nickelates are largely decoupled, and that their indirect interactions need to be understood within a multiorbital framework.