Spin and orbital excitations in undoped infinite layers: a comparison between superconducting PrNiO₂ and insulating CaCuO₂

Summary

This study systematically compares the spin and orbital excitation properties of undoped superconducting infinite-layer nickelate PrNiO₂ and insulating cuprate CaCuO₂ using momentum-resolved and polarization-resolved resonant inelastic X-ray scattering (RIXS) measurements. The results show that the in-plane magnetic exchange integral of PrNiO₂ (approximately 46 meV) is significantly smaller than that of CaCuO₂ (approximately 82 meV), while the out-of-plane exchange integrals are similar (approximately 6–7 meV), indicating that both materials support three-dimensional antiferromagnetic order with comparable three-dimensionality of spin-spin correlations. The orbital excitations (intra-3d transitions) are well described by a single-ion model, but the Ni-dxy peak energy is notably lower than that of Cu-dxy, with opposite dispersion directions—nickelate exhibits orbital excitation propagation driven by nearest-neighbor orbital superexchange coupling, whereas cuprate is dominated by next-nearest-neighbor coupling. Despite a significant difference in charge-transfer energy (larger in the nickelate), the spin and orbital excitation characteristics are generally highly similar, with key distinctions only in the energy and dispersion of the Ni-dxy peak, attributed to differing orbital superexchange coupling mechanisms. This work reveals the core commonalities in magnetism and orbital dynamics between infinite-layer nickelates and cuprates, while also indicating smaller spin fluctuation energies and stronger localization of doped charges on metal sites in the nickelates.

Materials

• PrNiO₂
• CaCuO₂

Methods

• Resonant Inelastic X-ray Scattering (RIXS)
• Polarization-resolved RIXS
• Linear Spin Wave (LSW) theory
• Single-ion cross-section calculations
• Spin-orbital model

Keywords

• three dimensional antiferromagnetic order
• orbital superexchange coupling
• charge transfer energy
• self doping
• orbiton propagation

Highlights

• Momentum- and polarization-resolved RIXS measurements on nominally undoped, superconducting PrNiO₂ are compared with the reference infinite layer cuprate CaCuO₂.
• The Ni-dxy peak lies at significantly lower energy and shows an opposite dispersion to that of Cu-dxy, attributed to different orbital superexchange couplings.
• The in-plane exchange integrals are approximately half in PrNiO₂ (46 meV) compared to CaCuO₂ (82 meV), while out-of-plane values are comparable (6-7 meV).
• In PrNiO₂, the magnon peak broadening is nearly constant (≈30 meV) over the explored momentum range, less than half the broadening in superconducting Bi₂₂₀₁ at lowest doping.

Conclusions

• In PrNiO₂, the in-plane magnetic exchange integrals are smaller than in CaCuO₂, whereas the out-of-plane values are similar, indicating that both materials support a three-dimensional antiferromagnetic order.
• The orbital dispersion in the infinite-layer nickelate primarily involves nearest neighbor orbital superexchange interaction, which in cuprates is strongly hampered by coupling to magnons.
• Infinite-layer nickelates are unconventional superconductors closely related to cuprates, but lacking some of the ingredients that enhance T_c in the latter.
• Self-doping has a much milder impact on spin order than chemical doping, endowing infinite-layer nickelates with a non-disruptive way to achieve superconductivity which is absent in copper oxides.

Main claims

• In-plane magnetic exchange integral in PrNiO₂ (≈46 meV) is about half that in CaCuO₂ (≈82 meV), but out-of-plane exchange is similar (≈6-7 meV), indicating comparable three-dimensional antiferromagnetic correlations.
  • Evidence: LSW fits to RIXS spin wave dispersions give J₁ values; out-of-plane exchange Jc is comparable
• Orbital excitations show distinct differences: Ni-dxy peak is lower in energy (1.29 eV) than Cu-dxy (1.65 eV) and exhibits opposite dispersion direction.
  • Evidence: RIXS maps show dispersion of dxy peak; PNO dispersion is maximal at Γ and decreases; CCO is minimal at Γ and increases
• The opposite orbiton dispersion is due to dominant nearest-neighbor orbital superexchange in PNO versus next-nearest-neighbor in CCO, originating from different covalency and charge-transfer energy.
  • Evidence: Three-band model calculations give NN orbital exchange dominant in PNO; NNN dominant in CCO; NN orbiton hopping in PNO enabled by Hund's exchange

Workflow

• sample_preparation — High-quality infinite-layer thin films of both systems.
  • Materials: PrNiO₂ thin films; CaCuO₂ thin films
  • Methods: pulsed laser deposition; topotactic reduction for PrNiO₂
  • Observations: PrNiO₂ is superconducting (T_c ≈10 K); CaCuO₂ is insulating
• rixs_measurements — Comparative spin and orbital excitation spectra.
  • Materials: Ni-L₃ edge for PrNiO₂; Cu-L₃ edge for CaCuO₂
  • Methods: momentum-resolved RIXS with polarization analysis; ERIXS spectrometer at ESRF ID₃₂
  • Observations: spin excitations up to ≈230 meV (PNO) and ≈320 meV (CCO); orbital excitations at 1-3 eV
• spin_wave_analysis — In-plane exchange smaller in nickelate; out-of-plane similar.
  • Materials: RIXS dispersion data
  • Methods: linear spin wave theory with SpinW
  • Observations: J₁ ≈46 meV (PNO) vs 82 meV (CCO); Jc ≈6 meV (PNO) vs 7 meV (CCO)
• orbital_excitation_analysis — Orbiton propagation driven by nearest-neighbor orbital superexchange in PNO vs next-nearest-neighbor in CCO.
  • Materials: RIXS orbital excitation data
  • Methods: single-ion cross-section calculations; orbiton dispersion fitting
  • Observations: dxy peak at 1.29 eV (PNO) vs 1.65 eV (CCO); opposite dispersion direction