Revealing the similarity to Ruddlesden-Popper nickelates and electron-phonon coupling in the infinite-layer nickelate superconductor (Sm_0.69Ca_0.05Eu_0.26)NiO₂ by pump-probe spectra

The recently discovered infinite-layer nickelate superconductor offers a new platform for exploring hightemperature superconductivity. In this work, we employ time-resolved ultrafast optical spectroscopy to investigate the excited carrier dynamics in Eu-doped (Sm_0.69⁢Ca_0.05⁢Eu_0.26)NiO₂ thin film, which exhibits superconductivity with 𝑇_𝑐⁢0 = 16K. 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 𝜇⁢J/cm². 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 ≈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⁢NiO₂ system, and there must be other interactions that help hold Cooper pairs together. © 2025 American Physical Society.