Effective medium approximation and the complex optical properties of the inhomogeneous superconductor K0.8Fe2− ySe2

The in-plane optical properties of the inhomogeneous iron-chalcogenide superconductor K0.8Fe2−ySe2 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 plasma frequency ωp,D in the inclusions is much larger than the volume average, which when considered with the reasonably low values for the scattering rate 1/τD , suggests that the transport in the grains is always metallic. Estimates for the dc conductivity σdc and the superfluid density ρs0 in the grains place the inclusions on the universal scaling line ρs0/8 4.4σdcTc close to other homogeneous iron-based superconductors.