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.
Categories: cond-mat.supr-con cond-mat.str-el