Effective medium approximation and the complex optical properties of the   inhomogeneous superconductor K_{0.8}Fe_{2-y}Se_2
  The in-plane optical properties of the inhomogeneous iron-chalcogenide superconductor K_{0.8}Fe_{2-y}Se_2 with a critical temperature Tc = 31 K have been modeled in the normal state using the Bruggeman effective medium approximation for metallic inclusions in an insulating matrix. The volume fraction for the inclusions is estimated to be ~ 10%; however, they appear to be highly distorted, suggesting a filamentary network of conducting regions joined through weak links. The value for the Drude plasma frequency in the inclusions is much larger than the volume average, which when considered with the reasonably low values for the scattering rate, suggests that the transport in the grains is always metallic. Estimates for the dc conductivity and the superfluid density in the grains places the inclusions on the universal scaling line close to the other homogeneous iron-based superconductors. 
C. C. Homes, Z. J. Xu, J. S. Wen, and G. D. Gu
cond-mat.supr-con