Multiband Metallic Ground State in Multilayered Nickelates La$_3$Ni$_2$O$_7$ and La$_4$Ni$_3$O$_{10}$ Probed by $^{139}$La-NMR at Ambient Pressure

TL;DR

This study uses $^{139}$La-NMR to reveal that multilayered nickelates La$_3$Ni$_2$O$_{7- ext{delta}}$ and La$_4$Ni$_3$O$_{10- ext{delta}}$ exhibit density wave order and metallic ground states at ambient pressure, with implications for high-temperature superconductivity.

Contribution

First microscopic NMR investigation showing density wave order and metallic states in multilayered nickelates at ambient conditions, linking their electronic structure to potential superconductivity.

Findings

Density wave order with a gap below ~150 K and ~130 K for the two compounds.

Metallic ground states with residual density of states at the Fermi level.

Presence of multiple $d$ electron bands with different characteristics.

Abstract

We report a $^{139}$La-NMR study of polycrystalline samples of multi($n$)-layered nickelates, La$_3$Ni$_2$O$_{7-\delta}$ ($n=2$) and La$_4$Ni$_3$O$_{10-\delta}$ ($n=3$), 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 $T^*\sim150$ K for La$_3$Ni$_2$O$_{7-\delta}$ and $\sim130$ K for La$_4$Ni$_3$O$_{10-\delta}$. The finite value of $1/T_1$ below $T^*$ indicates metallic ground states with the remaining density of states at the Fermi level ($E_{\rm F}$) 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 $E_{\rm F}$. 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-$T_{\rm c}$ superconductivity under high pressures.