Metal-insulator transition in manganites: Changes in optical conductivity up to 22 eV

Abstract

The electronic response of doped manganites at the transition from the paramagnetic insulating to the ferromagnetic metallic state in ${\text{La}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$ for $x=0.3$ and 0.2 was investigated by dc conductivity, ellipsometry, and vacuum ultraviolet reflectance for energies between 0 and 22 eV. A stabilized Kramers-Kronig transformation yields the optical conductivity and reveals changes in the optical spectral weight up to 22 eV at the metal-to-insulator transition. In the observed energy range, the spectral weight is conserved within 0.3%. The redistribution of spectral weight in this surprisingly broad energy range has important ramifications for the effective low-energy physics. We discuss the importance of the charge-transfer, Coulomb on-site, Jahn-Teller, and long-range Coulomb screening effects to the electronic structure.