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 La_n+1Ni_nO_3n+1 (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 La₄Ni₃O₁₀ involving the collective vibration of La, Ni, and O atoms, which is absent in La₃Ni₂O₇. Temperature-dependent relaxation time diverges near the density-wave transition temperature of La₄Ni₃O₁₀, while it is inversely proportional to the temperature in La₃Ni₂O₇ above ∼150 K, suggesting a non-Fermi liquid behavior of La₃Ni₂O₇. Moreover, we estimate the electron–phonon coupling constants to be 0.05–0.07 and 0.12–0.16 for La₃Ni₂O₇ and La₄Ni₃O₁₀, respectively, suggesting a relatively minor role of electron–phonon coupling in the electronic properties of La_n+1Ni_nO_3n+1 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.