Large upper critical fields and dimensionality crossover of superconductivity in infinite-layer nickelate La${}_{0.8}$Sr${}_{0.2}$NiO${}_2$

The recently emerging superconductivity in infinite-layer nickelates, with isostructure and isoelectron of cuprates, provides a new platform to explore the pairing mechanism of high-temperature superconductors. In this work, we studied the upper critical field ($H_{\mathrm{c}2}$) of a high-quality La${}_{0.8}$Sr${}_{0.2}$NiO${}_2$ thin film with superconducting transition temperature, $T_{\mathrm{c}}$ = 18.8 K, using high magnetic field up to 56 T. A very large $H_{\mathrm{c}2}$, $\sim$ 40 T for $H$ $\text{Arrowvert}$ $c$ and $\sim$ 52 T for $H$ $\text{Arrowvert}$ $ab$, was confirmed, which suggests that infinite-layer nickelates also have great application potential. The anisotropy of $H_{\mathrm{c}2}$ monotonically decreases from $\sim$ 10 near $T_{\mathrm{c}}$ to $\sim$ 1.5 at 2 K. Angle dependence of $H_{\mathrm{c}2}$ confirms the crossover of superconductivity from two-dimensional (2D) to three-dimensional (3D) as the temperature decreases. We discussed that the interstitial orbital effect causes the weakening of anisotropy. The observed abnormal upturning of $H_{\mathrm{c}2}$ at low temperatures is found to be a universal behavior independent of film quality and rare earth elements. Therefore, it should not be the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state due to the fact that it is in the dirty limit and insensitive to disorder.