Pressure and strain tuning of the alternating bilayer-trilayer Ruddlesden-Popper nickelate: crystal and electronic structure

摘要

通过第一性原理计算，本工作研究了混合双层-三层Ruddlesden-Popper镍酸盐La₇Ni₅O₁₇在静水压和双轴压缩应变下的晶体与电子结构。通过分析高对称P₄/mmm结构中动力学不稳定声子模的不可约表示，作者确定了具有八面体倾斜的动力学稳定低对称C₂/c结构。施加压力和压缩应变均会抑制八面体倾斜，使结构四方化，这一行为与传统Ruddlesden-Popper相类似。在电子结构方面，静水压和应变下的总体特征相似，但区别在于三层块中d_z²成键带的位置：在30 GPa压力下该带穿过费米能级，而任何强度的压缩应变均使其维持在费米能级以下。这种应变诱导的电子结构变化与传统双层镍酸盐中观察到的行为一致，为理解这类材料中压力与应变对超导电性的不同影响提供了关键线索。

材料

• La₇Ni₅O₁₇

方法

• First-principles calculations (DFT)
• Phonon calculations (ALAMODE)
• Group theory analysis

关键词

• octahedral tilts
• tetragonalization
• dz2 bonding band
• bilayer trilayer hybrid
• fermi surface pocket

亮点

• First systematic study of pressure and strain effects on the hybrid bilayer-trilayer RP nickelate La₇Ni₅O₁₇.
• Identification of a dynamically stable C₂/c structure with octahedral tilts.
• The dz2 pocket emerges under pressure but is absent under compressive strain, providing a test for the role of this pocket in superconductivity.

结论

• Both pressure and compressive strain suppress octahedral tilts, tetragonalizing the structure.
• The dz2 bonding band from the trilayer block crosses the Fermi level at 30 GPa but remains below for compressive strain.
• The electronic structure under strain mimics that at 15 GPa more than at 30 GPa.
• The extra corner pocket from the trilayer block is fragile and can be shifted by on-site Coulomb repulsion.

主要论断

• Under hydrostatic pressure, the structure becomes tetragonal with octahedral tilts suppressed above 25 GPa.
  • 证据: Abstract,Full text: Under pressure, the structure becomes tetragonal, with the tilts of oxygen octahedra being completely suppressed at ≈25 GPa.
• The dz2 bonding band from the trilayer block crosses the Fermi level at 30 GPa, but remains below it under compressive strain.
  • 证据: Abstract,Full text: This band crosses the Fermi level at a pressure of 30 GPa, but it remains below it for any level of compressive strain.
• The electronic structure under strain is similar to that at 15 GPa, not 30 GPa, due to differences in lattice parameters.
  • 证据: Full text,Section III C: The electronic structure at this strain mimics better that obtained at 15 GPa.

研究流程

• structural_optimization
  • 材料: La₇Ni₅O₁₇
  • 方法: DFT with Quantum ESPRESSO
  • 观察: C₂/c structure with octahedral tilts at ambient; tetragonalization under pressure and strain
• phonon_analysis
  • 方法: frozen-phonon method (ALAMODE); group theory analysis
  • 观察: dynamically unstable modes at R point; mode hardening with pressure
• electronic_structure_calculation
  • 方法: DFT band structure; Fermi surface analysis
  • 观察: dz2 bonding band from trilayer crosses Fermi level at 30 GPa but not under strain
• comparison
  • 方法: hopping parameter extraction
  • 观察: hoppings similar to isolated bilayer and trilayer