Optical conductivity and magnetic properties of ultrathin epitaxial LaNiO$_{3}$ under compressive and tesile strain.*

The perovskite oxides RNiO$_{3,}$ (R = rare earth) exhibit a charge transfer metal-insulator (MI) transition to an antiferromagnetic state as a function of temperature and rare earth ionic radius. Bulk, stochiometric LaNiO$_{3}$ is the only member of the series to exhibit metallic conductivity at all temperatures (4-300 K). Motivated by the expectation that reduced dimensionality and strain will alter the nickelate series MI transition, we have grown ultrathin stoichiometric epitaxial films of LaNiO$_{3}$ on LaAlO$_{3 }$and (LaSr) (Al,Ta)O$_{3}$ substrates by rf magnetron sputtering. We present magnetic susceptibility and optical conductivity measurements up to 3 eV, and from 2 to 300 K on films as thin as 3 nm. Optical conductivity is measured by near normal incidence reflectivity and indicates strong thickness and strain dependent deviations from the Drude model. Low frequency reflectivity data extrapolates to measured DC conductivity which indicates a thickness and strain dependent MI transition. We compare our results to bulk measurements and to models of the ground state and transport properties of the nickelates.