Evolution of Electronic Correlations in the Ruddlesden-Popper Nickelates

Abstract: We report on optical studies of the Ruddlesden-Popper nickelates La${n+1}$Ni${n}$O${3n+1}$ with $n = 2$ (La${3}$Ni${2}$O${7}$), $n = 3$ (La${4}$Ni${3}$O${10}$) and $n = \infty$ (LaNiO${3}$). As the number of the NiO${6}$ octahedra layers $n$ grows, the ratio of the kinetic energy determined from the experimental optical conductivity and that from band theory $K{\text{exp}}/K_{\text{band}}$ increases, suggesting a reduction of electronic correlations. While the strong electronic correlations in the bilayer La${3}$Ni${2}$O${7}$ place it on the verge of the Mott insulating phase, the trilayer La${4}$Ni${3}$O${10}$ and infinite-layer LaNiO${3}$ exhibit moderate electronic correlations, falling into the regime of correlated metals. The evolution of the electronic correlations in La${n+1}$Ni${n}$O${3n+1}$ is likely to be dominated by the Ni-$d_{z^2}$ orbital. Our results provide important information for understanding the superconductivity in Ruddlesden-Popper nickelates.