Optical spectroscopy of K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$ superconductors

We measured the temperature dependent optical reflectivity $R(\omega$) and extracted the complex optical conductivity $\sigma(\omega)$ of the K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$ superconductors. At room temperature, both $R(\omega$) and $\sigma_{1}(\omega)$ have semiconducting behavior. Upon cooling, at temperatures depending on the S-concentration, the low frequency reflectivity increases in absolute value and shows a sharp upturn, consistent with metallic behavior. The phonon spectrum of $\sigma_{1}(\omega)$, which is very different from similar Fe-based superconductors and also doping dependent will be explained in terms of the changes in occupancy of the Fe-sites with doping. An anomalous feature in optical reflectivity is observed in the superconducting state and its possible origin and association with the condensation of free carriers will be discussed.