Ni-O hybridization-driven electronic reconstruction across the superconducting dome in an infinite-layer nickelate

Summary

This study systematically characterizes the evolution of unoccupied states in the infinite-layer nickel oxide La_1-xCa_xNiO₂ as a function of doping and temperature using O K-edge and Ni L-edge X-ray absorption spectroscopy. Superconductivity emerges in the doping range of x = 0.18 to 0.27. Near x ≈ 0.20–0.23, a redistribution of low-energy spectral weight occurs: Ni 3d-dominated states decrease while O 2p hybridized states increase, indicating an orbital-selective crossover in Ni–O covalency. This crossover coincides precisely with the sign reversal of the Hall coefficient and precedes the suppression of the superconducting critical temperature at higher doping levels. B_y directly linking transport anomalies and the superconducting dome to measurable Ni–O orbital reorganization, these results represent a critical step toward establishing a unified orbital-resolved phase diagram for infinite-layer nickelates and offer a practical route for designing superconductivity through hybridization engineering.

Materials

• La_1-xCa_xNiO₂
• LaNiO₂
• La₀.77Ca₀.23NiO₂
• La₀.65Ca₀.35NiO₂
• La₀.82Ca₀.18NiO₂

Methods

• X-ray absorption spectroscopy (XAS)
• X-ray linear dichroism (XLD)
• PLD
• Topotactic reduction

Keywords

• ni o hybridization
• orbital selective crossover
• superconducting dome
• hall coefficient sign reversal
• electronic reconstruction
• covalency
• hole doping

Highlights

• Mapping of doping- and temperature-dependent unoccupied states in La_1-xCa_xNiO₂ using O K-edge and Ni L-edge XAS.
• Identification of an additional O K-edge pre-edge feature 'b' that emerges in the superconducting regime and weakens in the overdoped regime.
• Discovery that the crossover in Ni–O covalency near x≈0.20-0.23 coincides with Hall coefficient sign reversal and precedes T_c reduction.
• Observation of weak X-ray linear dichroism in La-based films, contrasting with stronger dichroism in Nd-based analogues.
• Providing a unified orbital-resolved phase diagram for infinite-layer nickelates linking transport anomalies to orbital redistribution.
• Demonstration that hybridization control may serve as a practical route to engineer superconductivity.

Conclusions

• O₂p–Ni₃d hybridization remains robust across the doping series of infinite-layer La_1-xCa_xNiO₂.
• In the superconducting regime, an additional spectral feature appears in the O K-edge pre-edge region.
• Across the superconducting window (0.18 ≤ x ≤ 0.27), low-energy spectral weight redistributes: Ni₃d-dominated contributions decrease while O₂p-hybridized states increase, most rapidly near x ≈ 0.20-0.23.
• This crossover coincides with a sign change in the Hall coefficient, indicating an orbital-selective reorganization beyond simple carrier modulation.
• At higher doping (x > 0.23), the continued growth of O-hybridized character tracks the suppression of T_c, linking the downturn of the superconducting dome to a hybridization-driven electronic evolution.
• Temperature- and doping-dependent Ni L₃-edge measurements reveal weak XLD in La-based films, implying smaller anisotropy between in-plane and out-of-plane unoccupied 3d states compared to Nd-based analogues.
• These findings place Ni–O covalency at the centre of electronic-structure changes across the superconducting dome in La-based infinite-layer nickelates.
• The results refine the phase diagram by connecting transport anomalies to orbital redistribution and suggest hybridization control as a practical route for tuning superconducting behavior.

Main claims

• Superconductivity in La_1-xCa_xNiO₂ occurs for 0.18 ≤ x ≤ 0.27.
  • Evidence: Abstract: 'Superconductivity occurs for 0.18 ≤ x ≤ 0.27.',Full text: T_c data from Ref. [36] and superconducting dome characterization.
• Near x ≈ 0.20-0.23, low-energy spectral weight redistributes: Ni₃d-dominated states decrease while O₂p-hybridized states increase.
  • Evidence: Abstract: 'Near x≈0.20-0.23, low-energy spectral weight redistributes: Ni₃d-dominated states decrease while O₂p-hybridized states increase.',O K-edge and Ni L-edge XAS spectra show doping-dependent intensity variations.
• This orbital-selective crossover coincides with sign reversal of the Hall coefficient and precedes the reduction of T_c at higher doping.
  • Evidence: Abstract: 'coincides with a sign reversal of the Hall coefficient and precedes the reduction of the superconducting critical temperature at higher doping.',Full text: comparison of XAS trends with transport data from Ref. [36].
• Results provide a key step toward a unified orbital-resolved phase diagram for infinite-layer nickelates and a practical route to engineer superconductivity via hybridization control.
  • Evidence: Abstract: 'key step toward a unified, orbital-resolved phase diagram… and a practical route to engineer superconductivity via hybridization control.',Full text: concluding remarks linking orbital reorganization to superconducting dome.

Workflow

• sample_preparation — Thin films of La_1-xCa_xNiO₂ were successfully synthesized across the doping range.
  • Materials: La_1-xCa_xNiO₃ perovskite precursor films; SrTiO₃ (STO) substrate (uncapped)
  • Methods: Pulsed laser deposition (PLD); Soft-chemistry topotactic reduction
  • Observations: Conversion from perovskite to infinite-layer phase
• xas_measurement — XAS reveals doping- and temperature-dependent evolution of unoccupied states and Ni-O hybridization.
  • Materials: La_1-xCa_xNiO₂ thin films
  • Methods: O K-edge X-ray absorption spectroscopy (XAS); Ni L-edge XAS; Total electron yield (TEY) mode; Polarization-dependent X-ray linear dichroism (XLD)
  • Observations: Pre-peak feature 'a' at ≈530.7 eV in O K-edge; Emergence of feature 'b' near x=0.18, strongest at x=0.23; Ni L₃-edge intensity peaks near x=0.18; Weak XLD in La-based films
• data_analysis — Low-energy spectral weight transfers from Ni₃d-dominated to O₂p-hybridized states with doping, coinciding with Hall sign change.
  • Materials: XAS spectra; Transport data (Hall coefficient, T_c) from Ref. [36]
  • Methods: Integrated spectral weight analysis (Region I of O K-edge); Peak intensity ratio analysis (Ni L-edge); Temperature differential ΔIT calculation
  • Observations: Ni L-edge intensity peaks at x=0.18, then decreases; O K-edge integrated area (Region I) peaks at x=0.23; ΔIT increases with doping up to x=0.23, then decreases; Sign reversal of Hall coefficient near x≈0.20-0.23
• interpretation — Ni-O hybridization-driven electronic reconstruction links transport anomalies to the superconducting dome in infinite-layer nickelates.
  • Materials: XAS results; Transport phase diagram; Literature on nickelates and cuprates
  • Methods: Comparison of XAS and transport trends; Orbital-selective crossover analysis
  • Observations: Orbital reorganization most rapid near x≈0.20-0.23; T_c suppressed at higher doping as O-hybridized character grows; Ni-O covalency central to electronic evolution