Jahn-Teller distortion on strained La₃Ni₂O₇ thin films

摘要

该研究通过密度泛函理论计算系统分析了应变状态下La₃Ni₂O₇薄膜的电子结构，发现双轴压缩应变主要拉伸外层顶角Ni-O键，而内层顶角Ni-O键几乎不变，进而显著增强Jahn-Teller分裂能Δ_JT，但层间d_z²轨道跳跃参数t_⊥^z变化微弱。由于超导仅出现在面内晶格常数低于临界值时，该结果将应变增强的Δ_JT识别为关键微观调控参数。计算的费米面结构和霍尔响应与LaAlO₃和SrLaAlO₄衬底上的ARPES及霍尔测量结果吻合，证实Jahn-Teller畸变在优化双层镍酸盐超导中起核心作用。

材料

• La₃Ni₂O₇

方法

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

关键词

• 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

亮点

• 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.

结论

• 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.

主要论断

• Biaxial compressive strain selectively enhances Jahn-Teller splitting energy Δ_JT while leaving interlayer hopping nearly unchanged
  • 证据: 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
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: For SLAO, two pockets (α and β); for LAO, additional γ pocket; Hall coefficient negative and 4-5 times smaller for LAO, matching experiment

研究流程

• Density functional theory calculations — Biaxial compressive strain primarily elongates outer apical Ni-O bond
  • 材料: La₃Ni₂O₇ thin film crystal structure
  • 方法: VASP; meta-GGA SCAN functional
  • 观察: 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
  • 材料: DFT band structure
  • 方法: Wannier90; Fitting to 4-band tight-binding model
  • 观察: 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
  • 材料: Tight-binding model
  • 方法: Calculate Fermi surface topology; Calculate Hall coefficient with uniform scattering rate
  • 观察: 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
  • 方法: Comparison with bulk hydrostatic pressure case
  • 观察: Bulk pressure compresses both bonds equally, unlike epitaxial strain which selectively enhances Δ_JT
• dft_calculations — Strain-induced asymmetric structural response.
  • 材料: La₃Ni₂O₇ thin film structure
  • 方法: density functional theory with SCAN meta-GGA functional; Wannier downfolding using Wannier90
  • 观察: 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.
  • 材料: DFT band structure
  • 方法: Wannier90 to obtain TB parameters
  • 观察: 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.
  • 材料: TB model parameters for SLAO and LAO substrates
  • 方法: Boltzmann transport equation with constant scattering rate
  • 观察: γ pocket present for LAO but absent for SLAO; Hall coefficient negative and more negative for SLAO