Optical conductivity of perovskite manganites

Abstract

We calculate the optical conductivity $\ensuremath{\sigma}(\ensuremath{\omega})$ for doped rare-earth manganites based on the recently proposed microscopic ``two fluid'' $\ensuremath{\ell}\ensuremath{-}b$ model. We study the temperature dependence of $\ensuremath{\sigma}(\ensuremath{\omega})$ for ${\text{La}}_{0.825}{\text{Sr}}_{0.175}{\text{MnO}}_{3}$, which has a metallic ground state. At low temperatures, the calculated $\ensuremath{\sigma}(\ensuremath{\omega})$ shows a ``two-peak'' structure consisting of a far-infrared coherent Drude peak and a broad mid-infrared ``polaron'' peak, as observed in experiments. Upon heating, the Drude peak rapidly loses spectral weight, and $\ensuremath{\sigma}(\ensuremath{\omega})$ crosses over to having just a single broad mid-infrared peak. The temperature dependence of the mid-infrared peak and the spectral weight transfer between the two peaks are also in agreement with experimental findings. We also study the doping dependence of $\ensuremath{\sigma}(\ensuremath{\omega})$ for the same compound. The integrated spectral weight under the Drude peak increases rapidly as the doping level is increased from an underdoped, insulating state ($x=0.1$) to a highly doped, metallic state ($x=0.3$), again in agreement with trends seen experimentally.