Orbital dimerization-induced first-order structural phase transition: a case study in La₃Ni₂O₇

摘要

该研究以新发现的镍基超导体La₃Ni₂O₇为例，揭示了其一级结构相变背后的“轨道二聚化”机制。作者指出，标准密度泛函理论（DFT）及考虑原子内关联的LDA+DMFT方法均无法重现该相变的突然性，因为这些方法忽略了关键的原子间关联。通过构建包含低能活性轨道的多体有效哈密顿量并进行精确对角化，他们发现当Ni-O-Ni键角趋于平直时，层间Ni轨道之间会产生强烈的反铁磁超交换作用，形成自旋单态键合，从而引起总能量急剧下降。这种能量降低在特定构型处产生新的局域能量极小值，与倾斜构型共存，完美解释了实验中观测到的一级相变特征，以及高低压结构的共存现象。该机制不仅改变了晶格键合性质，还导致低能电子结构定性变化（如超导电性的出现）。该工作建立的DFT加多体修正计算框架具有普适性，适用于含开壳层d/f电子的离子材料，为理解此类体系的结构与电子性质关联提供了关键微观机理。

材料

• La₃Ni₂O₇

方法

• DFT
• LDA+U
• LDA+DMFT
• Exact diagonalization
• Wannier functions
• First-principles calculations

关键词

• orbital dimerization
• first order structural phase transition
• spin singlet bond
• inter atomic correlation
• ni o ni bond angle
• spin fractionalization

亮点

• This is the first identification of a general orbital dimerization mechanism for pressure-induced first-order structural phase transitions.
• The developed many-body framework improves DFT total energy by including inter-atomic correlations.
• The mechanism explains the coexistence of tilted and straight octahedra structures over a wide pressure range.

结论

• The first-order structural phase transition in La₃Ni₂O₇ is reproduced by incorporating inter-atomic correlations via a full many-body treatment of low-energy active orbitals.
• The mechanism involves orbital dimerization through spin-singlet bond formation, leading to abrupt energy reduction.
• State-of-the-art DFT and LDA+DMFT methods fail to capture the first-order nature because they omit inter-atomic correlations.
• The orbital dimerization qualitatively alters the low-energy electronic structure, effectively fractionalizing Ni₂+ ionic spin to 1/2 and producing cuprate-like dynamics.

主要论断

• Standard LDA and LDA+DMFT fail to reproduce the first-order nature of the structural transition; inter-atomic correlation is essential.
  • 证据: Abstract,Full text: LDA and LDA+DMFT display a smooth behavior… completely misses the first-order nature.
• A full many-body treatment reveals an abrupt energy reduction due to inter-atomic orbital dimerization through spin-singlet bonding.
  • 证据: Abstract,Full text: The energy improvement near 180° is associated with the emergence of strong inter-layer superexchange and corresponding spin-singlet correlation.
• The orbital dimerization mechanism qualitatively alters low-energy electronic structure, fractionalizing Ni₂+ spin to 1/2.
  • 证据: Abstract,Full text: Ni₂+ ionic spins are effectively fractionalized from 1 into 1/2, leaving only half-filled effective orbital active.

研究流程

• DFT_calculation
  • 材料: La₃Ni₂O₇
  • 方法: LDA; LDA+DMFT; LDA+U
  • 观察: smooth energy variation with tilt angle in LDA and DMFT
• many_body_model_construction
  • 方法: exact diagonalization of effective Hamiltonian for active orbital subspace
• energy_analysis
  • 方法: total energy calculation with many-body correction; double counting removal
  • 观察: double-minimum structure in total energy; strong energy reduction near 180° bond angle
• interpretation — First-order structural transition is due to inter-atomic orbital dimerization via spin-singlet bond formation.
  • 方法: spin-singlet bond analysis
  • 观察: strong interlayer superexchange at straightened octahedra