Theoretical study on ambient pressure superconductivity in La₃Ni₂O₇ thin films: structural analysis, model construction, and robustness of s±-wave pairing

摘要

该研究通过理论方法分析了La₃Ni₂O₇薄膜在常压下的超导电性。研究首先基于第一性原理结构优化构建模型哈密顿量，固定面内晶格常数至实验所用衬底（LSAT、LAO和SLAO），并额外采用基于实验测定晶体结构的模型。运用考虑完全动量和频率依赖的格林函数与配对相互作用的涨落交换近似（FLEX）求解线性化Eliashberg方程。结果表明，电子结构（包括γ-费米口袋的存在与否）取决于所采用的晶体结构及能带计算中是否包含+U修正，但s±-波配对对称性始终保持鲁棒。这种鲁棒性主要源于配对由有限能量自旋涨落介导，该机制对费米面拓扑细节不敏感，并在轨道表示中产生近乎与动量无关的层间d3z²−r²配对间隙函数。另一方面，薄膜的超导转变温度（约40 K）约为加压块材（约80 K）的一半，这一现象在FLEX框架内仅能通过采用基于实验测定晶体结构得到的小层间跃迁参数|t⊥|模型来理解，但也不排除其他因素的可能性。

材料

• La₃Ni₂O₇

方法

• DFT
• FLEX
• Wannier function
• linearized Eliashberg equation
• PBEsol+U

关键词

• s± wave pairing
• spin fluctuations
• gamma pocket
• bilayer hubbard model
• tc reduction

亮点

• Ambient pressure superconductivity in La₃Ni₂O₇ thin films is achieved
• The interlayer Ni-O-Ni bond angle deviation from 180° has small impact on superconductivity within FLEX
• The gap function is nearly momentum-independent for interlayer d3z2-r2 pairing
• FLEX results agree with cDMFT, DCA, and VMC non-perturbative approaches
• The origin of the large discrepancy between theoretical and experimental crystal structures remains an open question

结论

• s±-wave pairing symmetry remains robust regardless of details in band structure
• The robustness is due to finite energy spin fluctuations that are insensitive to Fermi surface topology
• The reduction of T_c in thin films to about 40 K from about 80 K in bulk can be explained by the small interlayer hopping derived from the experimentally determined crystal structure

主要论断

• s±-wave pairing symmetry is robust in La₃Ni₂O₇ thin films under ambient pressure, regardless of the presence or absence of the γ-pocket and details of the crystal structure.
  • 证据: FLEX calculations for three different crystal structures (theoretical I₄/mmm, theoretical I₄/m, experimental I₄/m) all yield s±-wave pairing.,Gap function in orbital representation is nearly momentum-independent for interlayer d3z2-r2 pairing.,Finite-energy spin fluctuations, insensitive to Fermi surface topology, mediate the pairing (see Fig. 9 and discussion in Sec. III.3.3).
• The reduced T_c (≈40 K) in thin films compared to pressurized bulk (≈80 K) can be understood within FLEX only if the experimentally determined crystal structure (small |t⊥| ≈ 0.4 eV) is adopted.
  • 证据: Eigenvalue of linearized Eliashberg equation (λ) is significantly smaller for experimental structure than for theoretical structures (Fig. 8(a)).,λ decreases monotonically with reducing interlayer hopping |t⊥| (Fig. 8(c)).,Theoretical structures give λ comparable to pressurized bulk, failing to explain reduced T_c.
• The pairing mechanism is dominated by finite-energy (>0.6 eV) spin fluctuations, not by Fermi surface nesting.
  • 证据: Dynamical spin susceptibility shows that low-energy spin fluctuations are momentum-dependent and enhanced by better nesting, but high-energy fluctuations are nearly momentum-independent (Fig. 9).,λ is not enhanced by the appearance of the γ-pocket (better nesting) but decreases when |t⊥| is small (where nesting is worse).,Gap function in orbital representation is nearly momentum-independent, consistent with pairing mediated by finite-energy fluctuations.

研究流程

• First-principles structural optimization — Structural parameters obtained as function of in-plane lattice constant; bond angle approaches 180° for smaller lattice constant
  • 材料: La₃Ni₂O₇ thin films; substrates (LSAT, LAO, SLAO)
  • 方法: DFT structural optimization with PBEsol functional; projector augmented wave (PAW) method; VASP code
  • 观察: Determined lattice constants and Ni-O-Ni bond angles as function of substrate
• Tight-binding model construction — Interlayer hopping integral |t⊥| differs significantly between theoretically determined and experimentally determined structures
  • 材料: DFT-optimized structures; experimentally determined structure (Ref. [24])
  • 方法: Extraction of Wannier functions (WANNIER₉₀); Two-orbital bilayer Hubbard model with Ni-3dx2-y2 and 3d3z2-r2 orbitals
  • 观察: Hopping parameters and level offset extracted; interlayer hopping |t⊥| ≈ 0.6 eV for theoretical structures, ≈0.4 eV for experimental structure
• Fluctuation exchange approximation (FLEX) calculation — s±-wave pairing symmetry robust across different crystal structures and Fermi surface details
  • 材料: Two-orbital bilayer Hubbard models for three crystal structures (theoretical I₄/mmm, theoretical I₄/m, experimental I₄/m)
  • 方法: FLEX approximation; self-consistent calculation of self-energy and spin-fluctuation-mediated pairing interaction; full momentum and frequency dependence of Green's function
  • 观察: Eigenvalue of linearized Eliashberg equation obtained; gap function in orbital and band representation
• Analysis of superconductivity and pairing mechanism — Finite-energy spin fluctuations mediate robust s±-wave pairing; absence of γ-pocket does not suppress superconductivity
  • 材料: FLEX results for all models
  • 方法: Solution of linearized Eliashberg equation; analytical continuation of dynamical spin susceptibility via Padé approximation; analysis of gap function and pairing glue
  • 观察: Gap function in orbital representation nearly momentum-independent for interlayer pairing; spin susceptibility shows finite-energy spin fluctuations as pairing glue