Pauli-limited upper critical field and anisotropic depairing effect of La₂.82Sr₀.18Ni₂O₇ superconducting thin film

摘要

本研究采用外延生长的La₂.82Sr₀.18Ni₂O₇薄膜（超导转变温度约31.6 K），通过高场输运测量（最高58 T）系统表征了上临界场及其各向异性。近转变温度附近，超导呈现厚度限制的二维特征；冷却后，面外相干长度减小至小于薄膜厚度（6 nm），表明体系向本征三维体超导转变。基于Ginzburg-Landau模型分析，测得零温下面内和面外上临界场分别为82 T和45 T，各向异性比γ≈1.34，与块材Ruddlesden-Popper镍酸盐相当。低温下面内方向上临界场因自旋顺磁对破裂效应被强烈压制，接近Pauli极限（58 T），而面外方向几乎不受影响。这种各向异性Pauli限制解释了上临界场各向异性降低的现象，并支持薄膜中超导本质上是三维体超导的结论。结果凸显了自旋顺磁效应在决定该类镍酸盐高场超导相图中的关键作用。

材料

• La₂.82Sr₀.18Ni₂O₇

方法

• magnetotransport measurements
• high-field transport up to 58T
• X-ray diffraction
• Ginzburg-Landau (GL) model fitting
• Werthamer-Helfand-Hohenberg (WHH) theory
• two-band model fitting
• angular-dependent magnetoresistance

关键词

• upper critical field
• pauli limit
• anisotropic depairing
• spin paramagnetic pair breaking
• coherence length
• dimensional crossover
• maki parameter
• multiband superconductivity

亮点

• Near T_c, superconductivity exhibits thickness-limited two-dimensional characteristics; upon cooling, a crossover to intrinsic three-dimensional bulk superconductivity occurs.
• The WHH fit yields a large Maki parameter (αM=21) and small spin-orbit scattering (λso=0.3), consistent with strong spin-paramagnetic pair-breaking.
• The two-band model for Hc₂(c) indicates that intraband coupling dominates the stabilization of superconductivity.

结论

• High-field transport measurements reveal large upper critical fields with a small anisotropy ratio γ≈1.34, comparable to bulk Ruddlesden-Popper nickelates.
• At low temperatures, the in-plane upper critical field is strongly suppressed by spin-paramagnetic pair breaking and approaches the Pauli limit (58 T), while Hc₂(c) remains largely unaffected.
• This anisotropic Pauli limitation accounts for the reduced upper critical field anisotropy and supports the conclusion that superconductivity in these films is fundamentally three-dimensional bulk like.

主要论断

• The in-plane upper critical field is strongly suppressed by spin-paramagnetic pair breaking and approaches the Pauli limit (58 T).
  • 证据: Abstract,Full text: At low temperatures, the in-plane upper critical field Hc₂^ab is strongly suppressed by spin-paramagnetic pair breaking and approaches the Pauli limit (Hc₂^Pauli=58 T).
• The out-of-plane upper critical field remains largely unaffected by spin-paramagnetic effects.
  • 证据: Abstract,Full text: Hc₂^c remains largely unaffected.
• The superconductivity in the film is three-dimensional bulk like at low temperatures.
  • 证据: Abstract,Full text: Supports the conclusion that superconductivity in these films is fundamentally three-dimensional bulk like.

研究流程

• sample_preparation
  • 材料: La₂.82Sr₀.18Ni₂O₇; SrLaAlO₄ substrate
  • 方法: reactive molecular-beam epitaxy; ozone annealing
• transport_measurement
  • 方法: electrical transport measurements; pulsed magnetic field up to 58T
• data_analysis
  • 方法: Ginzburg-Landau model; WHH model; two-band model
  • 观察: upper critical field anisotropy γ≈1.34; in-plane Hc₂ approaches Pauli limit 58T
• interpretation — Superconductivity in thin films is fundamentally three-dimensional bulk like, with anisotropic Pauli limitation.