Isotropic Pauli-Limited Superconductivity in the Infinite Layer Nickelate Nd$_{0.775}$Sr$_{0.225}$NiO$_{2}$

TL;DR

This study reveals that superconductivity in Nd$_{0.775}$Sr$_{0.225}$NiO$_{2}$ is isotropic and Pauli-limited, with evidence of unconventional ground state properties, challenging expectations based on layered crystal structure.

Contribution

The paper provides the first detailed magnetotransport measurements showing isotropic superconductivity and strong Pauli-limiting effects in infinite-layer nickelates.

Findings

Superconductivity is surprisingly isotropic at low temperatures.

Superconductivity is strongly Pauli-limited, with paramagnetic effects dominating.

An unconventional ground state is suggested by the low-temperature upturn in the critical field.

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.