Incoherent-to-coherent crossover of optical spectra in La0.825Sr0.175MnO3:Temperature-dependent reflectivity spectra measured on cleaved surfaces

Optical reflectivity spectra were measured on cleaved surfaces of La0.825Sr 0.175MnO3 single crystals (T-C = 283 K) over a temperature range 10-295 K. The optical conductivity sigma(omega) shows incoherent-to-coherent crossove r with decreasing temperature. The minimum metallic conductivity sigma(min) of this compound was deter mined by the de resistivity p(T) measurements o f Al-substituted crystals (La0.825Sr0.175)(Mn1-zAlz)O-3 and was found to be 2000-3000 Omega(-1)cm(-1). This indicates that the de conductivity of La0. 825Sr0.175MnO3 is smaller than sigma(min) over a wide temperature range bel ow T-C even though p(T) is metallic (dp/dT>O). The present results suggest that there are two types of the ferromagnetic-metallic phase below Tc-a "hi gh-temperature incoherent" metallic (HIM) and a "low-temperature coherent" metallic phase. "Colossal mag netoresistance'' is a characteristic of the H IM phase, [S0163-1829(99)01242-4].