Incoherent-to-coherent crossover of optical spectra in ${\mathrm{La}}_{0.825}{\mathrm{Sr}}_{0.175}{\mathrm{MnO}}_3:$ Temperature-dependent reflectivity spectra measured on cleaved surfaces

Optical reflectivity spectra were measured on cleaved surfaces of ${\mathrm{La}}_{0.825}{\mathrm{Sr}}_{0.175}{\mathrm{MnO}}_3$ single crystals ${(T}_{\mathrm{C}}=283\mathrm{}$ K) over a temperature range 10–295 K. The optical conductivity $\sigma \left(\omega \right)$ shows incoherent-to-coherent crossover with decreasing temperature. The minimum metallic conductivity ${\sigma }_{\min }$ of this compound was determined by the dc resistivity $\rho \mathrm{}\left(T\right)\mathrm{}$ measurements of Al-substituted crystals $\left({\mathrm{La}}_{0.825}{\mathrm{Sr}}_{0.175}\right)\left({\mathrm{Mn}}_{1-z}{\mathrm{Al}}_z\right){\mathrm{O}}_3$ and was found to be 2000–3000 ${\Omega }^{-1}{\mathrm{cm}}^{-1}.$ This indicates that the dc conductivity of ${\mathrm{La}}_{0.825}{\mathrm{Sr}}_{0.175}{\mathrm{MnO}}_3$ is smaller than ${\sigma }_{\min }$ over a wide temperature range below $T_{\mathrm{C}}$ even though $\rho \mathrm{}\left(T\right)\mathrm{}$ is metallic $(d\rho /dT>\mathrm{}0).\mathrm{}$ 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.