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

Summary

This study employs epitaxial La₂.82Sr₀.18Ni₂O₇ thin films (with a superconducting transition temperature of approximately 31.6 K) to systematically characterize the upper critical field and its anisotropy via high-field transport measurements up to 58 T. Near the transition temperature, superconductivity exhibits thickness-limited two-dimensional characteristics; upon cooling, the out-of-plane coherence length decreases to below the film thickness (6 nm), indicating a transition to intrinsic three-dimensional bulk superconductivity. Based on the Ginzburg-Landau model, the zero-temperature in-plane and out-of-plane upper critical fields are determined to be 82 T and 45 T, respectively, yielding an anisotropy ratio γ≈1.34, comparable to that of bulk Ruddlesden-Popper nickelates. At low temperatures, the in-plane upper critical field is strongly suppressed by the spin paramagnetic pair-breaking effect, approaching the Pauli limit (58 T), while the out-of-plane direction remains largely unaffected. This anisotropic Pauli limiting explains the reduced anisotropy of the upper critical field and supports the conclusion that superconductivity in the films is inherently three-dimensional bulk superconductivity. The results highlight the critical role of spin paramagnetic effects in determining the high-field superconducting phase diagram of these nickelates.

Materials

• La₂.82Sr₀.18Ni₂O₇

Methods

• 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

Keywords

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

Highlights

• 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.

Conclusions

• 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.

Main claims

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

Workflow

• sample_preparation
  • Materials: La₂.82Sr₀.18Ni₂O₇; SrLaAlO₄ substrate
  • Methods: reactive molecular-beam epitaxy; ozone annealing
• transport_measurement
  • Methods: electrical transport measurements; pulsed magnetic field up to 58T
• data_analysis
  • Methods: Ginzburg-Landau model; WHH model; two-band model
  • Observations: 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.