The recent observation of superconductivity in thin-film infinite-layer nickelates^1,2,3 offers a different angle from which to investigate superconductivity in layered oxides⁴. A wide range of candidate models have been proposed^5,6,7,8,9,10, which emphasize single- or multi-orbital electronic structure, Kondo or Hund’s coupling and analogies to cuprates. Further experimental characterization of the superconducting state is needed to develop a full understanding of the nickelates. Here we use magnetotransport measurements to probe the superconducting anisotropy in Nd_0.775Sr_0.225NiO₂. We find that the upper critical field is surprisingly isotropic at low temperatures despite the layered crystal structure. In a magnetic field, the superconductivity is strongly Pauli-limited, such that the paramagnetic effect dominates over orbital de-pairing. Underlying this isotropic response is a substantial anisotropy in the superconducting coherence length, which is at least four times longer in-plane than out-of-plane. A prominent low-temperature upturn in the upper critical field indicates the presence of an unconventional ground state.

最近对薄膜无限层镍酸盐^{1,2,3中超导性的观察为研究层状氧化物4}中的超导性提供了不同的角度。已经提出了广泛的候选模型^5,6,7,8,9,10，它们强调单轨道或多轨道电子结构、Kondo 或 Hund 的耦合以及与铜酸盐的类比。需要对超导状态进行进一步的实验表征，以全面了解镍酸盐。在这里，我们使用磁输运测量来探测 Nd _0.775 Sr _0.225 NiO _{2中的超导各向异性}. 我们发现，尽管存在层状晶体结构，但上临界场在低温下却令人惊讶地各向同性。在磁场中，超导性受到强烈的泡利限制，因此顺磁效应在轨道去配对中占主导地位。这种各向同性响应的基础是超导相干长度的显着各向异性，其平面内至少是平面外的四倍。上临界场中显着的低温上升表明存在非常规基态。