Jahn-Teller distortion on strained La₃Ni₂O₇ thin films

Summary

This study systematically analyzed the electronic structure of strained La₃Ni₂O₇ thin films using density functional theory calculations, revealing that biaxial compressive strain primarily elongates the outer apical Ni–O bonds while leaving the inner apical Ni–O bonds nearly unchanged, thereby significantly enhancing the Jahn–Teller splitting energy Δ_JT, yet the interlayer d_z² orbital hopping parameter t_⊥^z exhibits only a weak variation. Given that superconductivity emerges only when the in-plane lattice constant falls below a critical value, these results identify strain-enhanced Δ_JT as a key microscopic tuning parameter. The calculated Fermi surface topology and Hall response agree well with angle-resolved photoemission spectroscopy (ARPES) and Hall measurements on LaAlO₃ and SrLaAlO₄ substrates, confirming that Jahn–Teller distortion plays a central role in optimizing superconductivity in bilayer nickelates.

Materials

• La₃Ni₂O₇

Methods

• density functional theory (DFT) calculations
• tight-binding model
• Wannier90
• DFT
• Wannier function analysis
• SCAN meta-GGA
• Crystal Orbital Hamilton Population (ICOHP)

Keywords

• jahn teller distortion
• interlayer hopping
• fermi surface topology
• hall response
• jahn teller splitting
• biaxial compressive strain
• apical ni o bonds
• fermi surface reconstruction
• hall coefficient
• lifshitz transition

Highlights

• Biaxial compressive strain primarily elongates the outer apical Ni-O bonds while leaving the inner apical Ni-O bonds nearly unchanged.
• The Jahn-Teller splitting energy Δ_JT is strongly enhanced under strain, while the interlayer d_z2 orbital hopping parameter t_⊥^z varies only weakly.
• Identifies Jahn-Teller distortion as the leading electronic effect of epitaxial strain in bilayer nickelate thin films.
• Provides a microscopic link between lattice strain and superconductivity, suggesting that ΔJT controls the critical in-plane lattice constant for superconductivity.

Conclusions

• Strain-enhanced Jahn-Teller splitting is a key microscopic tuning parameter for superconductivity in La₃Ni₂O₇.
• The calculated Fermi surfaces and Hall response agree with ARPES and Hall measurements on LAO and SLAO substrates.
• Biaxial compressive strain primarily elongates the outer apical Ni-O bond while leaving the inner apical bond nearly unchanged, leading to a strong enhancement of Jahn-Teller splitting ΔJT.
• The interlayer dz2 hopping t⊥z changes only weakly under strain; strain-enhanced ΔJT is the key microscopic tuning parameter for superconductivity.
• Calculated Fermi surfaces and Hall response for LAO and SLAO substrates agree with ARPES and Hall measurements.

Main claims

• Biaxial compressive strain selectively enhances Jahn-Teller splitting energy Δ_JT while leaving interlayer hopping nearly unchanged
  • Evidence: DFT calculations show outer apical Ni-O bond elongates, inner bond unchanged, Δ_JT increases strongly with decreasing a,Interlayer hopping t_⊥^z shows only weak variation
• Jahn-Teller distortion plays a central role in optimizing superconductivity in strained La₃Ni₂O₇ thin films
  • Evidence: Calculated Fermi surface topology and Hall response agree with ARPES and Hall measurements,Superconductivity appears only below a critical in-plane lattice constant, correlated with increased Δ_JT
• Biaxial compressive strain primarily elongates the outer apical Ni-O bond while leaving the inner apical bond nearly unchanged, leading to strong enhancement of Jahn-Teller splitting ΔJT but weak change in interlayer hopping t⊥ᶻ.
  • Evidence: DFT calculations show d_outer increases from 2.05 to 2.25 Å while d_inner remains ≈1.90 Å; ΔJT increases linearly with decreasing a; t⊥ᶻ changes only slightly
• Strain-enhanced Jahn-Teller splitting is the relevant microscopic tuning parameter for superconductivity in strained films.
  • Evidence: Superconductivity appears only below critical in-plane lattice constant; ΔJT changes dramatically with strain while other parameters change little
• Calculated Fermi surfaces and Hall responses for LAO and SLAO substrates agree with ARPES and Hall measurements.
  • Evidence: For SLAO, two pockets (α and β); for LAO, additional γ pocket; Hall coefficient negative and 4-5 times smaller for LAO, matching experiment

Workflow

• Density functional theory calculations — Biaxial compressive strain primarily elongates outer apical Ni-O bond
  • Materials: La₃Ni₂O₇ thin film crystal structure
  • Methods: VASP; meta-GGA SCAN functional
  • Observations: Outer apical Ni-O bond (d_ot) elongates; Inner apical Ni-O bond (d_om) nearly unchanged
• Tight-binding model extraction — Strain-enhanced Δ_JT is the dominant electronic effect
  • Materials: DFT band structure
  • Methods: Wannier90; Fitting to 4-band tight-binding model
  • Observations: Jahn-Teller splitting Δ_JT strongly enhanced; Interlayer hopping t_⊥^z weakly varies
• Fermi surface and Hall coefficient calculation — JT distortion controls Fermi surface and Hall response
  • Materials: Tight-binding model
  • Methods: Calculate Fermi surface topology; Calculate Hall coefficient with uniform scattering rate
  • Observations: Fermi surface agrees with ARPES; Hall coefficient magnitude for LAO is 4-5 times smaller than SLAO, consistent with experiment
• Interpretation — JT distortion is a key tuning parameter for superconductivity in bilayer nickelates
  • Methods: Comparison with bulk hydrostatic pressure case
  • Observations: Bulk pressure compresses both bonds equally, unlike epitaxial strain which selectively enhances Δ_JT
• dft_calculations — Strain-induced asymmetric structural response.
  • Materials: La₃Ni₂O₇ thin film structure
  • Methods: density functional theory with SCAN meta-GGA functional; Wannier downfolding using Wannier90
  • Observations: outer apical Ni-O bond elongates under compressive strain; inner apical bond nearly unchanged
• tight_binding_model_extraction — Strain mainly enhances Jahn-Teller splitting, not interlayer hopping.
  • Materials: DFT band structure
  • Methods: Wannier90 to obtain TB parameters
  • Observations: Jahn-Teller splitting ΔJT strongly increases with decreasing a; interlayer hopping t⊥ᶻ changes weakly
• fermi_surface_and_hall_response_calculation — Jahn-Teller splitting controls Fermi surface topology and Hall response.
  • Materials: TB model parameters for SLAO and LAO substrates
  • Methods: Boltzmann transport equation with constant scattering rate
  • Observations: γ pocket present for LAO but absent for SLAO; Hall coefficient negative and more negative for SLAO