Multiband Metallic Ground State in Multilayered Nickelates La_3Ni_2O_7 and La_4Ni_3O_10 Probed by ^139La-NMR at Ambient Pressure

We report a ^139La-NMR study of polycrystalline samples of multi(n)-layered nickelates, La_3Ni_2O_7-δ (n=2) and La_4Ni_3O_10-δ (n=3), at ambient pressure. Measurements of the nuclear magnetic resonance (NMR) spectra and nuclear spin relaxation rate (1/T_1) indicate the emergence of a density wave order with a gap below T^*∼150 K for La_3Ni_2O_7-δ and ∼130 K for La_4Ni_3O_10-δ. The finite value of 1/T_1 below T^* indicates metallic ground states with the remaining density of states at the Fermi level (E_ F) under the density wave order. These features are attributed to multiple d electron bands with different characteristics. Above T^*, the gradual decrease in 1/T_1T upon cooling implies the presence of a band with flat dispersion near E_ F. From our microscopic probes, we point out that these nickelates (n=2 and 3) possess similar electronic states despite the difference in the formal valence of the Ni-d electron states, which provides a basis for understanding the novel high-T_ c superconductivity under high pressures.