Evolution of the superfluid density in infinite-layer nickelates

Nickelate superconductors provide a valuable new platform for the study of unconventional superconductivity that is complementary to the cuprates. One of the central puzzles about high-temperature superconductors is what factors determine the scale of their superconducting transition temperature ($T_\mathrm{c}$). To address this question for infinite-layer nickelates, we present a systematic mutual inductance study of the superfluid density across the doping-dependent superconducting dome of $\mathrm{Nd}_{1-x}\mathrm{Sr}_x\mathrm{NiO}_2$. We observe a weak superfluid stiffness that exhibits an approximately square-root correlation with $T_\mathrm{c}$. We also find a strong interplay between Nd magnetism and the superconducting phase, manifested as a substantial low-temperature suppression of superfluid density. These observations highlight the importance of superconducting phase fluctuations in limiting $T_\mathrm{c}$ and unexpectedly strong coupling between the Nd 4$f$ moments and the superfluid.