Isotropic Pauli-Limited Superconductivity in the Infinite Layer Nickelate Nd${}_{0.775}$Sr${}_{0.225}$NiO${}_2$

Abstract

The recent observation of superconductivity in thin film infinite-layer nickelates${}^{1-3}$ offers a different angle to investigate superconductivity in layered oxides${}^4$. A wide range of candidate models have been proposed${}^{5-10}$, emphasizing single- or multi-orbital electronic structure, Kondo or Hund's coupling, and analogies to cuprates. Clearly, 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${}_{0.775}$Sr${}_{0.225}$NiO${}_2$. 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.

9 pages, 4 figures, 1 supplementary info