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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.

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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.

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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.

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