Abstract
Optical reflectivity spectra were measured on cleaved surfaces of single crystals K) over a temperature range 10–295 K. The optical conductivity shows incoherent-to-coherent crossover with decreasing temperature. The minimum metallic conductivity of this compound was determined by the dc resistivity measurements of Al-substituted crystals and was found to be 2000–3000 This indicates that the dc conductivity of is smaller than over a wide temperature range below even though is metallic The present results suggest that there are two types of the ferromagnetic-metallic phase below —a “high-temperature incoherent” metallic (HIM) and a “low-temperature coherent” metallic phase. “Colossal magnetoresistance” is a characteristic of the HIM phase.
- Received 13 May 1999
DOI:https://doi.org/10.1103/PhysRevB.60.13011
©1999 American Physical Society