M. K. Stewart¹, J. Liu², R. K. Smith¹, B. C. Chapler¹, C. Yee³, K. Haule³, J. Chakhalian², and D. N. Basov^1
1Department of Physics, University of California-San Diego
²Department of Physics, University of Arkansas
³Department of Physics, Rutgers University

We present an optical study of LaNiO₃ in the range between 10 meV and 6 eV. Thin films of varying thickness were epitaxially grown by pulsed laser deposition on LaAlO₃ and SrTiO₃ substrates. The samples were investigated using near-normal incidence reflectance and variable angle spectroscopic ellipsometry at temperatures ranging from 20 K to 298 K. Several interband transitions are evident in the optical conductivity above 1 eV, in good agreement with band structure calculations. No Drude peak is observed however, in stark contrast with theoretical works predicting a finite density of states at the Fermi energy. This experimental finding of a vanishing Drude spectral weight, compared to a finite electron kinetic energy obtained from band structure calculations, is indicative of strong electronic correlations in LaNiO₃. Optical conductivity of LaNiO₃/LaAlO₃ superlattices reveals small changes in the band structure of LaNiO₃ induced by heterostructuring.