Two dimensional Mott physics in the rare earth nickelates

The strong electron correlations inherent in the rare-earth nickelate system (RNiO$_{3}$) lead to a metal-insulator transition, the temperature of which can be tuned by changing the rare-earth ion, R. Bulk LaNiO$_{3}$ is metallic at all temperatures, and NdNiO$_{3}$ undergoes a metal-insulator transition at 150 K. However, reducing the thickness of both LaNiO$_{3}$ and NdNiO$_{3}$ strongly affects the transport behavior, where LaNiO$_{3}$ undergoes a thickness-driven metal-insulator transition below $\sim$4 unit cells. Here, we identify the physics of this transition and demonstrate two-dimensional metallic behavior in thin films. We show that by direct chemical doping of LaNiO$_{3}$ thin films we can restore metallic behavior and tune the conductivity. We apply the same technique to thin films of NdNiO$_{3}$ and control the metal-insulator transition temperature. Finally, combining artificial confinement and doping, we observe metallicity in nickelate layers as thin as two unit cells. The effects of both structural and charge-carrier modifications on the transport properties of the thin films will be discussed.