Paper

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 Ni (L_3)-edge resonant inelastic X-ray scattering (RIXS), this work systematically investigates the evolution of electronic and spin excitations with carrier doping in coherently compressively strained La(_{3-x})Sr(_x)Ni(_2)O(_7)/SrLaAlO(_4) thin films, covering the superconducting ((x = 0, 0.09, 0.21)) and overdoped non-superconducting ((x = 0.38)) regimes. In the superconducting films, dispersive spin excitations persist along the ([H,H]) and ([H,0]) directions, with the dispersion remaining almost doping-independent and exhibiting minimal damping, while the spectral weight only moderately decreases, indicating robust bistripe spin correlations. However, in the non-superconducting film at (x = 0.38), the magnetic response becomes strongly broadened and weakened, accompanied by significantly enhanced damping and a spectral weight reduction of approximately 50%, signaling the collapse of coherent bistripe spin excitations. The simultaneous disappearance of magnetic coherence with superconductivity directly establishes the link between doping-controlled magnetism and superconductivity in layered nickelate thin films.

Through muon spin rotation/relaxation and resistivity measurements combined with oxygen isotope substitution, the pressure and isotope effects on density wave transitions in the trilayer Ruddlesden-Popper nickelate La₄Ni₃O₁₀ were systematically investigated. Under ambient pressure, two incommensurate spin density wave (SDW) transitions were observed at 132 K and 80–90 K; the magnetic structure reveals that the outer two Ni layers exhibit an antiferromagnetically coupled SDW order, while the inner layer has a smaller magnetic moment, and a c-axis component of the magnetic moment emerges below T*. The abrupt onset of the internal field at T_SDW indicates that the SDW transition resembles a first-order phase change and is closely intertwined with the charge density wave (CDW) occurring at the same temperature. Under applied pressure, T_SDW, T*, and T_CDW are uniformly suppressed at a rate of approximately -13 K/GPa, differing from the behavior in bilayer La₃Ni₂O₇ where pressure increases the separation between SDW and CDW. Substitution of ¹⁶O with ¹⁸O raises T_CDW; in the region where CDW and SDW are intertwined, T_SDW also exhibits a significant isotope effect similar in magnitude to the shift in T_CDW, whereas no isotope effect is observed for the SDW at T* where it evolves independently. These results reveal the strongly intertwined nature of SDW and CDW in La₄Ni₃O₁₀ and suggest that pressure-induced suppression of the CDW order may be a key mechanism for high-pressure superconductivity in Ruddlesden-Popper nickelates.

Using first-principles density functional theory calculations, we systematically investigate the electron-doping effects of tetravalent element substitution in double-layer Ruddlesden-Popper nickelate La₃Ni₂O₇ thin films. Unlike cuprates, cerium (Ce) doping is found to be inefficient in introducing electron carriers into low-energy bands, whereas zirconium (Zr), hafnium (Hf), and thorium (Th) serve as effective electron dopants. These elemental substitutions significantly enhance the interlayer hopping integral t⊥ between Ni-dz² orbitals, potentially strengthening the interlayer superexchange coupling J⊥ and thereby potentially increasing the superconducting transition temperature Tc. Using the constrained random phase approximation to evaluate interaction parameters, we find that electron doping increases the occupancy of low-energy orbitals (including Ni-dx²-y² and dz² along with their hybridized oxygen orbitals) and alters the electron filling ratio between in-plane and interlayer orbitals. Structural analysis reveals that differences in dopant ionic radii cause variations in Ni–O bond lengths, with Zr and Hf inducing lattice contraction and Th exhibiting the strongest doping effect. These results indicate that Zr, Hf, and Th are promising candidates for achieving electron doping in La₃Ni₂O₇, offering a new avenue to clarify the ongoing debate over the electron pairing mechanism in this system.

By combining density functional theory and dynamical mean-field theory, the evolution of the electronic structure, magnetism, and correlation effects in the infinite-layer nickelate LaNiO₂ under electron and hole doping is investigated. The results reveal that, due to the presence of rare-earth 5d states, the self-doping effect of the Ni-d_{x²-y²} band exhibits significant asymmetry: hole doping strongly suppresses self-doping, whereas electron doping, while enlarging the rare-earth 5d electron pocket, does not effectively hole-dope the Ni-d_{x²-y²} band. This difference directly impacts the magnetic response—hole doping rapidly suppresses antiferromagnetic order, while electron doping maintains the antiferromagnetic state as the ground state. Despite these disparities, the electronic correlations in both doping regimes are dominated by the Ni-d_{x²-y²} orbital, suggesting that a single-band description may be applicable in both electron- and hole-doped regions.

High-temperature superconductivity was discovered in the pressurized nickelate La3Ni2O7 which has a unique bilayer structure and mixed valence state of nickel. The properties at ambient pressure contain crucial information of the fundamental interactions and bosons mediating superconducting pairing. Here, using X-ray absorption spectroscopy and resonant inelastic X-ray scattering, we identified that Ni 3 $${d}_{{x}^{2}-{y}^{2}}$$, Ni 3 $${d}_{{z}^{2}}$$, and ligand oxygen 2p orbitals dominate the low-energy physics with a small charge-transfer energy. Well-defined optical-like magnetic excitations soften into quasi-static spin-density-wave ordering, evidencing the strong electronic correlation and rich magnetic properties. Based on an effective Heisenberg spin model, we extract a much stronger inter-layer effective magnetic superexchange than the intra-layer ones and propose two viable magnetic structures. Our findings emphasize that the Ni 3 $${d}_{{z}^{2}}$$orbital bonding within the bilayer induces novel electronic and magnetic excitations, setting the stage for further exploration of La3Ni2O7 superconductor.