Spin Fluctuations in the Rare-Earth Doped Bilayer Nickelates

Summary

The spin fluctuations in rare-earth Pr and Nd doped bilayer nickelates La₂LnNi₂O_7-δ (Ln = La, Pr, Nd) were investigated under ambient pressure using inelastic neutron scattering. In the undoped La₃Ni₂O_7-δ, a flat spin fluctuation mode at 45 meV was observed; upon doping, this mode splits into two modes at 43 and 48 meV, with an additional weak mode appearing at approximately 60 meV. Notably, the spin fluctuation intensity in La₂NdNi₂O_7-δ is significantly higher than that in La₃Ni₂O_7-δ and La₂PrNi₂O_7-δ. These results are consistent with a description based on the stripe-type antiferromagnetic Heisenberg model, indicating that rare-earth doping enhances the interlayer magnetic coupling, with the interlayer exchange coupling SJ⊥ increasing from about 60 meV to 69–73 meV, while the intralayer coupling remains weak (≤3.5 meV). This enhancement may account for the increase in superconducting transition temperature from 80 K to near 100 K following rare-earth doping. This work reveals the regulatory role of rare-earth doping on spin dynamics and superconducting pairing in bilayer nickelates.

Materials

• La₂LnNi₂O₇-δ (Ln=La, Pr, Nd) powder

Methods

• Inelastic neutron scattering (INS)
• Linear spin-wave theory
• Heisenberg model

Keywords

• spin fluctuations
• interlayer magnetic coupling
• flat spin mode
• rare earth doping
• enhanced tc

Highlights

• Our results are consistent with an enhanced interlayer coupling within the stripe-type Heisenberg model framework.
• The stronger spin excitations in La₂NdNi₂O₇-δ probably support higher T_c under pressure than the parent compound.

Conclusions

• Rare-earth doping enhances the interlayer magnetic coupling, with interlayer exchange coupling J⊥ increasing from about 60 meV to 69–73 meV.
• The intralayer coupling remains weak (≤3.5 meV).
• This enhancement may account for the increase in superconducting transition temperature from 80 K to near 100 K following rare-earth doping.

Main claims

• In La₂NdNi₂O₇-δ, the flat spin fluctuation mode at 45 meV splits into two modes at 43 and 48 meV with an additional weak mode at ≈60 meV, and spin fluctuation intensity is significantly higher than in undoped and Pr-doped samples.
  • Evidence: Abstract: 'this mode splits into two modes at 43 and 48 meV, with an additional weak mode appearing at approximately 60 meV. Notably, the spin fluctuation intensity in La₂NdNi₂O₇-δ is significantly higher'
• Rare-earth doping enhances interlayer magnetic coupling S⊥J⊥ from ≈60 meV to 69-73 meV, which may account for the increase in T_c from 80 K to near 100 K.
  • Evidence: Abstract: 'the interlayer exchange coupling S⊥J⊥ increasing from about 60 meV to 69–73 meV… This enhancement may account for the increase in superconducting transition temperature'

Workflow

• Sample preparation
  • Materials: La₂LnNi₂O₇-δ (Ln=La, Pr, Nd) powder samples
  • Methods: Sol-gel method
  • Observations: Phase purity confirmed, magnetic susceptibility and heat capacity measurements
• Inelastic neutron scattering — Spin fluctuations in Nd-doped sample are stronger than in pure and Pr-doped
  • Materials: Powder samples
  • Methods: Time-of-flight INS on MERLIN and PANTHER spectrometers
  • Observations: CEF excitations at low energies; Flat spin fluctuation mode around 45 meV splits into two modes in doped samples; Additional mode at ≈60 meV
• Theoretical fitting — Enhanced interlayer coupling may account for increased T_c (up to ≈100 K) in rare-earth doped samples
  • Materials: Experimental dispersion
  • Methods: Stripe-type Heisenberg model; Linear spin wave theory (SpinW)
  • Observations: Interlayer coupling S⊥J⊥ increases from ≈60 meV to 69-73 meV upon doping; Intralayer coupling remains weak (≤3.5 meV)