Incoherent-to-coherent crossover of optical spectra inLa0.825Sr0.175MnO3:Temperature-dependent reflectivity spectra measured on cleaved surfaces

Abstract

Optical reflectivity spectra were measured on cleaved surfaces of ${\mathrm{La}}_{0.825}{\mathrm{Sr}}_{0.175}{\mathrm{MnO}}_{3}$ single crystals ${(T}_{\mathrm{C}}=283$ K) over a temperature range 10--295 K. The optical conductivity $\ensuremath{\sigma}(\ensuremath{\omega})$ shows incoherent-to-coherent crossover with decreasing temperature. The minimum metallic conductivity ${\ensuremath{\sigma}}_{\mathrm{min}}$ of this compound was determined by the dc resistivity $\ensuremath{\rho}(T)$ measurements of Al-substituted crystals $({\mathrm{La}}_{0.825}{\mathrm{Sr}}_{0.175})({\mathrm{Mn}}_{1\ensuremath{-}z}{\mathrm{Al}}_{z}){\mathrm{O}}_{3}$ and was found to be 2000--3000 ${\ensuremath{\Omega}}^{\ensuremath{-}1}{\mathrm{cm}}^{\ensuremath{-}1}.$ This indicates that the dc conductivity of ${\mathrm{La}}_{0.825}{\mathrm{Sr}}_{0.175}{\mathrm{MnO}}_{3}$ is smaller than ${\ensuremath{\sigma}}_{\mathrm{min}}$ over a wide temperature range below ${T}_{\mathrm{C}}$ even though $\ensuremath{\rho}(T)$ is metallic $(d\ensuremath{\rho}/dTg0).$ The present results suggest that there are two types of the ferromagnetic-metallic phase below ${T}_{\mathrm{C}}$---a ``high-temperature incoherent'' metallic (HIM) and a ``low-temperature coherent'' metallic phase. ``Colossal magnetoresistance'' is a characteristic of the HIM phase.