Distinct ultrafast dynamics of bilayer and trilayer nickelate superconductors regarding the density-wave-like transitions.

In addition to the pressurized high-temperature superconductivity, bilayer and trilayer nickelate superconductors LaNiO (n = 2 and 3) exhibit many intriguing properties at ambient pressure, such as orbital-dependent electronic correlation, non-Fermi liquid behavior, and density-wave transitions. Here, using ultrafast reflectivity measurement, we observe a drastic difference between the ultrafast dynamics of the bilayer and trilayer nickelates at ambient pressure. We observe a coherent phonon mode in LaNiO involving the collective vibration of La, Ni, and O atoms, which is absent in LaNiO. Temperature-dependent relaxation time diverges near the density-wave transition temperature of LaNiO, while it is inversely proportional to the temperature in LaNiO above ∼150 K, suggesting a non-Fermi liquid behavior of LaNiO. Moreover, we estimate the electron-phonon coupling constants to be 0.05-0.07 and 0.12-0.16 for LaNiO and LaNiO, respectively, suggesting a relatively minor role of electron-phonon coupling in the electronic properties of LaNiO at ambient pressure. The relevant microscopic interaction and dynamic information are essential for further studying the interplay between superconductivity and density-wave transitions in nickelate superconductors.