Doping dependence of the optical properties of Ba 1 − x K x Bi O 3

We analyze the doping dependence of the optical conductivity of the ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}\mathrm{Bi}{\mathrm{O}}_{3}$ (BKBO) system in the full doping range from $x=0$ to $x=0.46$. We have used the results of our reflectivity and ellipsometric measurements at $x=0.31, 0.38, 0.4, 0.46$, 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 $x$ 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 ${\ensuremath{\lambda}}_{L}$ is also found to be almost doping (and ${T}_{c}$) independent while the superconducting gap scales with ${T}_{c}$ as $\frac{2{\ensuremath{\Delta}}_{s}}{{k}_{b}{T}_{c}}=4.2\ifmmode\pm\else\textpm\fi{}0.3$.