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

Abstract

We investigate the upper critical field and superconducting anisotropy of epitaxial La_2.82Sr_0.18Ni₂O₇ thin films, which show a sharp superconducting transition at $T_c=31.6K$. Near $T_c$, superconductivity exhibits thickness-limited two-dimensional characteristics. Upon cooling, the out-of-plane coherence length ${\xi }_c$ 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 ${\gamma =H}_{c2}^{\mathrm{ab}}$/$H_{c2}^c$ $\approx 1.34$, comparable to bulk Ruddlesden-Popper nickelates. At low temperatures, the in-plane (ab) upper critical field $H_{c2}^{\mathrm{ab}}$ is strongly suppressed by spin-paramagnetic pair breaking and approaches the Pauli limit ($H_{c_2}^{Pauli}=58T$), while $H_{c2}^c$ 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.