Annealing effects on the optical conductivity of single crystal (formula presented) (formula presented)

Optical reflectance spectra were measured in the temperature range 70–295 K, and in the energy range of 0.006–6 eV for single crystals of (formula presented) before and after annealing. The conductivity spectrum of the unannealed (formula presented) in its low-temperature metallic phase features a Drude-like peak, the spectral weight of which is dramatically increased by annealing the sample. Annealing also repairs a negative anomaly in the conductivity, which is thought to be associated with a surface layer damaged by polishing. A secondary ion mass spectrometry measurement shows, however, that the surface valence becomes depth dependent upon annealing. On the basis that the skin depth is far greater than the extent of annealing damages in the low-energy spectral region, analysis of the temperature dependence of the effective number of carriers (formula presented) below 0.5 eV is presented. © 2001 The American Physical Society.