Persistent structural distortions and absent superconductivity in trilayer nickelate thin films

摘要

该研究通过原子级精确合成、电输运测量、皮米级电子显微镜和同步辐射X射线衍射，系统探究了三层镍酸盐La₄Ni₃O₁₀薄膜的应变效应。尽管压缩外延应变能有效抑制母体密度波序并提升晶体对称性（如消除面外八面体旋转），但即使在最大压缩应变（-2.8%）下也未观测到超导电性。关键结构表征发现，压缩应变无法完全消除薄膜中特有的面内八面体旋转，这种旋转在每一三层单元的内层与外层中呈现层间不等效性，且持续存在。同步辐射X射线衍射显示面内旋转幅度随压缩应变单调减小，但未完全消失。相比之下，双层体系La₃Ni₂O₇中压缩应变能完全抑制所有八面体旋转从而诱导超导。该结果揭示了三层与双层体系的关键差异，表明单纯依靠外延应变工程无法实现三层镍酸盐的环境压力超导，需探索其他调控手段。

材料

• La₄Ni₃O₁₀
• La₃Ni₂O₇ (for comparison)

方法

• Reactive oxidative molecular-beam epitaxy (MBE)
• X-ray diffraction (XRD)
• Resistivity and Hall transport measurements
• Annular dark-field scanning transmission electron microscopy (ADF-STEM)
• Multislice electron ptychography (MEP)
• Synchrotron X-ray diffraction (crystal truncation rods and high dynamic range reciprocal space mapping)
• First-principles density functional theory (DFT) calculations

关键词

• octahedral rotations
• density wave suppression
• layer inequivalence
• compressive strain
• absence of superconductivity
• structural distortion

亮点

• We identify a previously unobserved structure in RP nickelates, the I₄/mmm space group, demonstrating the capability of epitaxial strain to induce unique structural variants.
• Compressive strain suppresses out-of-plane rotations but leaves in-plane rotations persistent, a structural evolution pathway distinct from that under hydrostatic pressure.

结论

• Compressive epitaxial strain suppresses the parent density wave in La₄Ni₃O₁₀ thin films but does not induce superconductivity.
• A structural distortion unique to strained n=3 thin films is identified: persistent, layer-inequivalent octahedral rotations around the c-axis.
• These results highlight key differences between the n=3 and n=2 systems, suggesting that ambient-pressure superconductivity in the n=3 may require new methods beyond epitaxial strain engineering.

主要论断

• Compressive epitaxial strain suppresses the parent density wave in La₄Ni₃O₁₀ thin films but does not stabilize superconductivity.
  • 证据: Transport measurements show no superconducting transition even at -2.8% compressive strain,Density wave transition is absent for compressive strains on LAO and SLAO
• Compressive strain completely suppresses out-of-plane octahedral rotations in La₄Ni₃O₁₀ thin films, raising crystal symmetry.
  • 证据: ADF-STEM Fourier transforms show disappearance of half-order peaks under compressive strain
• In-plane octahedral rotations persist in the inner perovskite layers even under large compressive strain, unlike in bilayer La₃Ni₂O₇.
  • 证据: Multislice electron ptychography shows persistent oxygen incoherence in inner layers for all strain states,Synchrotron XRD shows finite (1.5 0.5) peak even on SLAO
• The structural evolution under compressive strain differs from that under hydrostatic pressure; in thin films, out-of-plane rotations are eliminated first, while in bulk, in-plane rotations are eliminated first.
  • 证据: Thin film structural phases under compressive strain: a0 a0 c- with in-plane rotations,Bulk pressure phase diagram: a- a- c0 then a0 a0 c0

研究流程

• Thin film synthesis — High qualityLa₄Ni₃O₁₀ thin films with four distinct epitaxial strain states were successfully synthesized.
  • 材料: La; Ni; distilled ozone; pseudo-cubic substrates: SrLaAlO₄ (SLAO), LaAlO₃ (LAO), NdGaO₃ (NGO), SrTiO₃ (STO)
  • 方法: reactive oxygen molecular beam epitaxy (MBE) with dynamic layer-by-layer shuttering
  • 观察: High quality thin films with coherent strain; clear superlattice XRD peaks; Kiessig fringes; limited intergrowths
• Electrical transport measurements — Compressive epitaxial strain suppresses the parent density wave order in La₄Ni₃O₁₀ thin films, but does not induce superconductivity.
  • 材料: La₄Ni₃O₁₀ thin films; Pt or Pd contacts
  • 方法: Resistivity vs temperature measurements; Hall effect measurements in PPMS with AC current; e-beam deposition of contacts; wirebonding
  • 观察: For tensile strain (NGO, STO): density wave transition observed at ≈134 K and ≈152 K respectively; For compressive strain (LAO, SLAO): no density wave transition observed; No superconducting transition for any film
• Out-of-plane octahedral rotation analysis via ADF-STEM — Out-of-plane octahedral rotations are completely suppressed under compressive epitaxial strain, raising crystal symmetry from orthorhombic to tetragonal.
  • 材料: cross-sectional lamellas of La₄Ni₃O₁₀ thin films
  • 方法: Annular dark-field scanning transmission electron microscopy (ADF-STEM); Fourier transform analysis of images
  • 观察: Tensile strain films (STO, NGO): clear half-order peaks indicating out-of-plane rotations; Compressive strain films (LAO, SLAO): no half-order peaks, indicating suppression of out-of-plane rotations
• In-plane octahedral rotation analysis via multislice electron ptychography — In-plane octahedral rotations persist in the inner perovskite layers under all compressive strain states, even on SLAO with -2.8% strain.
  • 材料: cross-sectional lamellas of La₄Ni₃O₁₀ thin films
  • 方法: Multislice electron ptychography (4D-STEM); maximum likelihood iterative phase retrieval; EMPAD-G₂ detector
  • 观察: Inner perovskite layers show blurred oxygen columns due to in-plane rotations; Outer layers appear sharp; This blurring observed for all strain states including SLAO
• Synchrotron X-ray diffraction for in-plane rotation quantification — In-plane octahedral rotation magnitude decreases monotonically with compressive strain but does not vanish completely.
  • 材料: La₄Ni₃O₁₀ thin films on substrates
  • 方法: Crystal truncation rod (CTR) measurements at (1.5 0.5 L); high dynamic-range reciprocal space mapping (HDRM)
  • 观察: CTR peak intensity at (1.5 0.5 L) decreases with compressive strain; Barely detectable on SLAO; HDRM confirms systematic suppression of in-plane rotation signal
• First-principles calculations — First-principles calculations corroborate experimental findings on structural evolution and density wave suppression.
  • 材料: La₄Ni₃O₁₀ crystal structure models
  • 方法: Density functional theory (DFT) using GGA+U; QUANTUM ESPRESSO implementation
  • 观察: Calculations reproduce suppression of out-of-plane rotations under compressive strain; Persistence of in-plane rotations confirmed; Density wave instability suppression predicted