The recently discovered infinite-layer nickelate superconductor offers a new platform for exploring high-temperature superconductivity. In this work, we employ time-resolved ultrafast optical spectroscopy to investigate the excited carrier dynamics in Eu-doped (${\mathrm{Sm}}_{0.69}{\mathrm{Ca}}_{0.05}{\mathrm{Eu}}_{0.26}$)${\mathrm{NiO}}_2$ thin film, which exhibits superconductivity with $T_{c0}=16\text{K}$. The formation of superconducting gap was identified with estimating the gap size of about 2.5 meV, which can be eliminated with a small pump fluence of $0.32\mu \mathrm{J}/{\mathrm{cm}}^2$. The relaxation trajectories of quasiparticles exhibit a high similarity to those of the Ruddlesden-Popper nickelates, supporting the commonality in their electronic structures and superconductivity. More interestingly, from the temperature dependence of fast component, we estimate an electron-phonon coupling (EPC) strength of $\approx 0.42$, which would predict a transition temperature of only 3.5 K. This suggests that EPC alone is insufficient to explain superconductivity in the $R{\mathrm{NiO}}_2$ system, and there must be other interactions that help hold Cooper pairs together.

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• Electron-phonon coupling
• Superconductivity
• Unconventional superconductors
• Ultrafast femtosecond pump probe