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

TL;DR

This study explores the upper critical magnetic fields and anisotropic depairing effects in La2.82Sr0.18Ni2O7 superconducting thin films, revealing a crossover from two-dimensional to three-dimensional superconductivity and the significant influence of spin-paramagnetic effects.

Contribution

It provides the first detailed analysis of the anisotropic upper critical fields and spin-paramagnetic effects in La2.82Sr0.18Ni2O7 thin films, demonstrating their impact on superconductivity.

Findings

Large upper critical fields with small anisotropy ratio (~1.34).

Superconductivity transitions from 2D to 3D with decreasing temperature.

Hc2(ab) approaches the Pauli limit, Hc2(Pauli)=58 T, due to spin-paramagnetic pair breaking.

Abstract

We investigate the upper critical field and superconducting anisotropy of epitaxial La2.82Sr0.18Ni2O7 thin films, which show a sharp superconducting transition at Tc=31.6 K. Near Tc, superconductivity exhibits thickness-limited two-dimensional characteristics. Upon cooling, the out-of-plane coherence length decreases below the sample thickness of 6 nm, corresponding to a 3-unit-cell film, indicating a crossover to intrinsic three-dimensional bulk superconductivity. High-field transport measurements reveal large upper critical fields with a small anisotropy ratio gama~1.34, comparable to bulk Ruddlesden-Popper nickelates. At low temperatures, the in-plane (ab) upper critical field Hc2(ab) is strongly suppressed by spin-paramagnetic pair breaking and approaches the Pauli limit (Hc2(Pauli)=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. Our results highlight the essential role of spin-paramagnetic effects in shaping the high-field superconducting phase diagram of Ruddlesden-Popper nickelates.