Electron Doping of La₃Ni₂O₇ Thin Films: Candidate Metal Dopants and Their Potential Impact on Superconductivity

Summary

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 T_c. 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.

Materials

• La₃Ni₂O₇
• Ce-doped La₃Ni₂O₇
• Zr-doped La₃Ni₂O₇
• Hf-doped La₃Ni₂O₇
• Th-doped La₃Ni₂O₇

Methods

• DFT
• Constrained random-phase approximation (cRPA)

Keywords

• electron doping
• interlayer hopping t⊥
• superexchange coupling j⊥
• pairing mechanism
• hund's coupling

Highlights

• Unlike in cuprates, Ce doping fails to introduce electron carriers into low-energy bands in La₃Ni₂O₇.
• Electron doping alters the electron filling ratio between in-plane and interlayer orbitals, which may affect pairing symmetry.
• First systematic DFT study of electron doping in La₃Ni₂O₇ thin films, identifying promising dopants.

Conclusions

• Cerium (Ce) doping is inefficient for introducing electron carriers into low-energy bands of La₃Ni₂O₇.
• Zirconium (Zr), hafnium (Hf), and thorium (Th) act as effective electron dopants in La₃Ni₂O₇ thin films.
• Electron doping significantly increases the interlayer hopping t⊥ between dz2 orbitals, potentially enhancing superexchange coupling J⊥ and possibly T_c.
• These candidate dopants provide a route to clarify the debate on pairing mechanisms in bilayer nickelates.

Main claims

• Cerium doping does not effectively introduce electron carriers into La₃Ni₂O₇, unlike in cuprates.
  • Evidence: low-energy bands of La₃Ni₂O₇:Ce almost coincide with pristine La₃Ni₂O₇,Ni-O bond lengths remain unchanged upon Ce doping,Ce retains largelyCe₃+ valence state, not Ce₄+
• Zirconium, hafnium, and thorium are effective electron dopants for La₃Ni₂O₇, enhancing interlayer hopping and potentially increasing T_c.
  • Evidence: Ni-dz2 and dx2-y2 bands shift downward in energy upon doping,interlayer hopping integral t⊥ increases significantly (ratio t⊥/t‖ from 1.7 to 2.5),Bader charge analysis shows increased charge supply to the system,Hund's coupling increases relative to Hubbard U, potentially boosting superconductivity

Workflow

• model_setup — Thin film models with various dopants and substrates are constructed for DFT calculations.
  • Materials: one-unit-cell thick La₃Ni₂O₇ film on substrates (LaAlO₃, SrTiO₃, NdGaO₃)
  • Methods: density functional theory (DFT) with VASP; projector augmented wave (PAW) pseudopotentials; PBE exchange-correlation functional; DFT+U with U=3.5 eV for Ni-3d orbitals
  • Observations: structures relaxed to convergence
• electronic_structure_calculation — Zr, Hf, Th are effective electron dopants; Ce is not.
  • Materials: La₃Ni₂O₇ thin films with dopants
  • Methods: DFT band structure calculations; density of states (DOS) analysis
  • Observations: Ce doping shows no significant change in low-energy bands; Zr, Hf, Th doping shifts Ni-dz2 and dx2-y2 bands downward in energy; orbital occupancy increases for both in-plane and out-of-plane orbitals
• structural_analysis — Structural changes correlate with dopant ionic radii and affect electron doping efficiency.
  • Materials: relaxed atomic structures
  • Methods: analysis of Ni-O bond lengths and La-O bonds
  • Observations: Ce doping leaves Ni-O bonds nearly unchanged; Zr/Hf doping causes lattice contraction due to smaller ionic radii; Th doping also contracts lattice but has stronger electron doping effect
• charge_and_orbital_analysis — Electron doping modifies charge distribution and orbital occupancy, with Th showing strongest effect.
  • Materials: DFT charge densities
  • Methods: Bader charge analysis; integrated DOS for orbital occupancy
  • Observations: Th supplies additional charge to the system; Sr removes charge; electron doping increases occupancy of low-energy orbitals; relative ratio of carriers in in-plane vs out-of-plane orbitals varies with dopant
• tight_binding_and_interaction_parameters — Electron doping enhances interlayer hopping and Hund's coupling, potentially raising T_c.
  • Materials: DFT band structures
  • Methods: Wannier90 tight-binding fitting; constrained random-phase approximation (cRPA) for Hubbard U and Hund's coupling
  • Observations: interlayer hopping t⊥ increases from ≈1.7 to ≈2.5 relative to in-plane hopping; Hund's coupling increases relative to average Hubbard U