Structural modifications in strain-engineered bilayer nickelate thin films

摘要

The discovery of high-temperature superconductivity in bulk La₃Ni₂O₇ under high hydrostatic pressure1−4 and biaxial compression in epitaxial thin films5−8 has ignited significant interest in understanding the interplay between atomic and electronic structure in these compounds. Subtle changes in the nickel-oxygen bonding environment are thought to be key drivers for stabilizing superconductivity, but specific details of which bonds and which modifications are most relevant remains so far unresolved. While direct, atomic-scale structural characterization under hydrostatic pressure is beyond current experimental capabilities, static stabilization of strained La₃Ni₂O₇ films provides a platform well-suited to investigation with new picometer-resolution electron microscopy methods. Here, we use multislice electron ptychography (MEP)₉,10 to directly measure the atomic-scale structural evolution of La₃Ni₂O₇ thin films across a wide range of biaxial strains tuned via substrate choice. B_y resolving both the cation and oxygen sublattices, we study the strain-dependent evolution of atomic bonds, providing the opportunity to isolate and disentangle the effects of specific structural motifs for stabilizing superconductivity. We identify the lifting of crystalline symmetry through modification of the nickel-oxygen octahedral distortions under compressive strain as a key structural ingredient for superconductivity and identify in-plane lattice compression as a common attribute between bulk and thin film superconductivity. Building upon the detailed structures obtained by MEP, we introduce a theoretical framework to disentangle coupled structural distortions in corner-sharing octahedra11, which suggest that both known superconducting geometries of La₃Ni₂O₇ (hydrostatic pressure and compressive strain) suppress local t2g orbital mixing in the low-energy Ni bands by raising the octahedral symmetry.

材料

• La₃Ni₂O₇

方法

• multislice electron ptychography (MEP)

关键词

• strain engineering
• octahedral distortions
• superconductivity
• t2g orbital mixing

亮点

• Multislice electron ptychography directly measures atomic-scale structural evolution with picometer resolution, resolving both cation and oxygen sublattices.

结论

• Lifting of crystalline symmetry through modification of the nickel-oxygen octahedral distortions under compressive strain is a key structural ingredient for superconductivity.
• In-plane lattice compression is a common attribute between bulk and thin film superconductivity.

主要论断

• Crystalline symmetry lifting via octahedral distortion modification under compressive strain is a key structural ingredient for superconductivity in La₃Ni₂O₇ thin films.
  • 证据: Abstract: 'We identify the lifting of crystalline symmetry through modification of the nickel-oxygen octahedral distortions under compressive strain as a key structural ingredient for superconductivity',Full text: 'the Ni-planar O angles are aligned unidirectionally in compressive films but alternate in tensile films… Additional half-order peaks visible in tensile strained films and lack thereof in compressive films indicate relatively lower and higher crystalline symmetries respectively.'
• In-plane lattice compression is a common attribute between bulk (high-pressure) and thin-film superconductivity in La₃Ni₂O₇.
  • 证据: Abstract: 'identify in-plane lattice compression as a common attribute between bulk and thin film superconductivity',Full text: 'The IP spacing of the superconducting film on SLAO closely matches that of bulk samples at the symmetry raising structural transition and onset of superconductivity.'

研究流程

• thin_film_synthesis_and_transport — Compressive strain (SLAO) induces superconductivity; tensile strain (STO) gives insulating behavior.
  • 材料: La₃Ni₂O₇; Substrates: SrLaAlO₄ (SLAO), LaAlO₃ (LAO), NdGaO₃ (NGO), SrTiO₃ (STO)
  • 方法: Pulsed laser deposition (SLAO film); Molecular beam epitaxy (LAO, NGO, STO films); Post-growth O₂ annealing; Resistivity measurement
  • 观察: SLAO film: superconducting onset ≈42K; LAO: metallic; NGO: metallic but less; STO: insulating
• structural_characterization_by_xrd_and_mep — Lifting of crystalline symmetry via octahedral distortion modification under compressive strain is key for superconductivity; in-plane compression is common with bulk.
  • 材料: Thin film cross-sections; FIB lamellae
  • 方法: X-ray diffraction (Nelson-Riley fitting); Annular dark-field STEM; Multislice electron ptychography (MEP) with EMPAD-G₂ detector; Bayesian optimization of reconstruction parameters
  • 观察: Compressive films: higher symmetry (no half-order peaks in FT), unidirectional Ni-planar O angles; Tensile films: alternating Ni-planar O angles, half-order peaks present; In-plane lattice compression common to superconducting films and bulk high-pressure phases; Out-of-plane Ni-O bond length increases under compressive strain
• theoretical_octahedral_decomposition — Both superconducting geometries suppress local t2g orbital mixing by raising octahedral symmetry.
  • 材料: Structural models from MEP and bulk literature
  • 方法: DFT (VASP, GGA-PBE); Four-octahedra model decomposition into L, G, R distortions; Band structure projection onto t2g manifold
  • 观察: Superconducting geometries (compressive film, high-pressure bulk) show suppressed t2g hybridization in low-energy Ni bands; Suppression mainly due to R distortions (octahedral rotations); L distortions mainly shift band energies