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This study systematically analyzes the electronic correlation behavior of La₃Ni₂O₇ within the superconducting pressure range using first-principles simulations combined with constrained random phase approximation and ab initio molecular dynamics. The results show that, accompanying the structural phase transition, the effective on-site repulsion of Ni e_g orbitals is significantly enhanced, attributed to the dynamic balance between orbital localization and competing screening channels, particularly the spacer-layer La bands. This enhancement region aligns remarkably well with the experimentally observed right-triangular superconducting dome, reaching a peak correlation strength at 18 GPa that corresponds to the highest superconducting critical temperature. Finite-temperature simulations further clarify the boundaries of the structural phase diagram, while calculations on Ac₃Ni₂O₇ confirm the critical role of A-site cations in the pressure-driven evolution of electronic correlations. These findings directly reveal how structural changes drive unconventional superconductivity by modulating the strength of electronic correlations.

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4 linked papers

This study systematically investigated the evolution of PrNiOₓ thin films at various intermediate stages of topological reduction (x = 2–3) using soft X-ray absorption spectroscopy. By comparing Ni L-edge experimental spectra with single-cluster and double-cluster ligand-field model calculations, it was found that none of the samples exhibited a pure d⁹ electronic configuration. Quantitative analysis based on the charge sum rule revealed that even in the most reduced films, the average number of Ni 3d holes remained 1.35, while superconducting samples displayed higher hole counts, challenging previous assumptions regarding the hole doping limit. Concurrent changes in the O K-edge absorption spectra during reduction indicated the presence of O 2p holes even in the most reduced films. Collectively, these results suggest that a complex hole doping mechanism arises from the interplay between self-doping effects and oxygen non-stoichiometry.

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Research indicates that the two-step superconducting transition observed in La₂PrNi₂O₇₋δ thin films originates from their granular superconducting nature, where two superconducting phases with distinct critical temperatures coexist and couple through a Josephson junction network. For films grown via pulsed laser deposition and subsequently ozone-annealed, transport measurements reveal a pronounced secondary low-temperature transition even when the residual resistance is minimal near 30 K, resulting in a zero-resistance temperature of only about 10 K. The hysteresis in magnetoresistance and the sensitive response to weak magnetic fields align with the effective field model of granular superconductors, ruling out the possibility of a spin-glass phase. Structural characterization identifies oxygen inhomogeneity and local structural disorder, such as monolayer phase intercalation, as the primary causes of the observed phase separation. These findings elucidate the microscopic mechanisms underlying the complex superconducting behavior in bilayer nickelate films and underscore that improving oxygen uniformity is crucial for achieving bulk superconductivity with higher zero-resistance temperatures, thereby providing a foundation for subsequent spectroscopic studies.

We report systematic magnetotransport measurements on high-crystallinity overdoped infinite-layer nickelate La₁₋ₓSrₓNiO₂ thin films (x = 0.20–0.24), revealing two prominent normal-state features in pulsed magnetic fields up to 62 T: the magnetoresistance violates Kohler’s rule and exhibits H-linear behavior at high H/T limits, while the normal-state resistivity consistently follows a T² dependence below 30 K. These results demonstrate the coexistence of H-linear magnetoresistance and T² resistivity in this model unconventional superconductor, providing new insights into the transport properties of the normal ground state that hosts superconductivity in infinite-layer nickelates.

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