Doping dependence of the optical properties

We analyze the doping dependence of the optical conductivity of the (Formula presented) (BKBO) system in the full doping range from (Formula presented) to (Formula presented). We have used the results of our reflectivity and ellipsometric measurements at (Formula presented), as well as the results obtained by other experimental groups at different doping levels, to analyze the doping dependence of the spectral weight distribution. At the insulator-metal transition boundary we observe the appearance of a Drude-like free-carrier peak in the optical conductivity. In the metallic phase, we have performed measurements in the normal as well as the superconducting state. The data show that BKBO becomes a better metal with increasing (Formula presented) in the metallic regime due to a decrease in the scattering rate of the metallic charge carriers, while the density of the mobile (Drude) charge carriers does not significantly change with doping. The London penetration depth (Formula presented) is also found to be almost doping (and (Formula presented)) independent while the superconducting gap scales with (Formula presented) as (Formula presented).