Theoretical study of superconductivity in freestanding infinite-layer nickelate membranes under pressure: mitigation of excess correlation enhances T_c

摘要

该研究基于第一性原理计算构建了七轨道有效模型，采用涨落交换（FLEX）近似对自由站立无限层镍酸盐Nd₀.₈₅Sr₀.₁₅NiO₂膜在压力下的超导电性进行了理论分析。结果表明，随着压力增大，超导转变温度T_c单调上升，与近期实验结果一致。这种增强归因于Ni原子极低价态导致的过强电子关联得到缓解，使得有效电子相互作用参数U显著降低，从而减少了准粒子阻尼并增强了自旋涨落介导的d波配对。此外，声子计算确认晶体结构在90 GPa内保持动态稳定。研究通过对比不同U值的模型，指出仅采用较大的U值（约5.1 eV）才能复现实验趋势，而较小U值会导致T_c过早饱和甚至出现穹顶形状，从而支持了无限层镍酸盐中过强关联抑制超导、压力缓解关联进而提升T_c的机制。

材料

• Nd₀.85Sr₀.15NiO₂

方法

• First-principles calculations
• Fluctuation exchange (FLEX) approximation
• Phonon calculations
• Seven-orbital effective model

关键词

• superconductivity
• electron correlations
• d wave pairing
• spin fluctuation
• self doping

亮点

• Seven-orbital effective model constructed from first-principles calculations.
• Pressure induces a decrease in the effective on-site Coulomb interaction U, leading to enhanced T_c.
• Phonon calculations confirm dynamical stability of the crystal structure up to 90 GPa.
• Comparison with experimental data supports the mechanism of correlation mitigation enhancing superconductivity.

结论

• Superconducting transition temperature T_c increases monotonically with pressure in freestanding infinite-layer nickelate membranes.
• Enhancement of superconductivity is attributed to mitigation of excessively strong electron correlations due to low valence of Ni.
• A model with sufficiently large U (≈5.1 eV) is more plausible for infinite-layer nickelates than one with smaller values.

主要论断

• The seven-orbital model explains the monotonic increase in superconducting transition temperature T_c under pressure observed in experiment.
  • 证据: FLEX calculation shows eigenvalue lambda (proxy for T_c) increases monotonically with pressure (Fig. 4(b)).,This trend is consistent with experimental T_c increase reported in Ref. [36].
• The enhancement of T_c is attributed to mitigation of excessively strong electron correlations under pressure.
  • 证据: Intraorbital interaction U decreases from ≈5.1 eV under pressure (Fig. 4(a)).,Hypothetical models confirm that reducing U (while keeping other parameters fixed) increases lambda.,Smaller U values (4.0, 3.5 eV) give non-monotonic T_c, inconsistent with experiment, supporting that only a large ambient U can explain the data.
• The crystal structure remains dynamically stable up to 90 GPa.
  • 证据: Phonon dispersion calculations show no imaginary modes up to 90 GPa (Fig. 1(c)).

研究流程

• Structural optimization and phonon calculation — Crystal structure remains dynamically stable up to 90 GPa, validating subsequent superconductivity analysis.
  • 材料: Nd₀.85Sr₀.15NiO₂ freestanding membrane
  • 方法: First-principles DFT (Quantum ESPRESSO); Phonon calculations (phonopy)
  • 观察: Lattice parameters decrease under pressure; No imaginary phonon modes up to 90 GPa
• Band structure and model construction — The seven-orbital model captures essential pressure-induced electronic structure changes.
  • 材料: Nd₀.85Sr₀.15NiO₂ under pressure
  • 方法: Wannierization (wannier90); Construction of seven-orbital effective model
  • 观察: Increase in overall band width; Enhanced three-dimensionality; Self-doping effect: Ni-d occupancy decreases, Fermi pocket enlarges
• Calculation of interaction parameters — Pressure mitigates excessively strong electronic correlations by reducing U.
  • 材料: Nd₀.85Sr₀.15NiO₂ seven-orbital model
  • 方法: Constrained RPA (RESPACK)
  • 观察: Intraorbital interaction U_d decreases from ≈5.1 eV at ambient to lower values under pressure; Hund's coupling J remains nearly constant; Ratio U/J decreases significantly
• Superconductivity analysis via FLEX — Monotonic enhancement of T_c under pressure is reproduced only with realistic U≈5.1 eV, supporting the mechanism that pressure alleviates excessive correlations to boost superconductivity.
  • 材料: Seven-orbital model with various U values
  • 方法: Fluctuation exchange (FLEX) approximation; Eliashberg equation (d-wave)
  • 观察: Eigenvalue lambda monotonically increases with pressure for U=5.1 eV; Hypothetical models with only U or J changed confirm U reduction drives lambda increase; Smaller U (4.0, 3.5 eV) leads to premature saturation or dome-shaped T_c