Evolution of electronic correlations in the Ruddlesden-Popper nickelates

We report on optical studies of the Ruddlesden-Popper nickelates Lan+1NinO3n+1, with n = 2 (La3Ni2O7), n = 3 (La4Ni3O10), and n = infinity (LaNiO3). As the number of the NiO6 octahedra layers n grows, the ratio of the kinetic energy determined from the experimental optical conductivity and that from band theory Kexp/Kband increases, suggesting a reduction of electronic correlations. While the bilayer La3Ni2O7 lies on the verge of the Mott insulating phase, the trilayer La4Ni3O10 and infinite-layer LaNiO3 fall into the regime of correlated metals. The evolution of the electronic correlations in Lan+1NinO3n+1 is likely to be dominated by the Ni dz2 orbital. Our results provide important information for understanding the superconductivity in Ruddlesden-Popper nickelates.