Effective medium approximation and the complex optical properties of the inhomogeneous superconductor K_{0.8}Fe_{2-y}Se_2

TL;DR

This paper models the optical properties of inhomogeneous K_{0.8}Fe_{2-y}Se_2 superconductor using effective medium theory, revealing metallic inclusions within an insulating matrix and their impact on superconducting behavior.

Contribution

It introduces a modeling approach for the inhomogeneous superconductor using the Bruggeman effective medium approximation, highlighting the metallic nature of inclusions and their influence on optical properties.

Findings

Inclusions occupy about 10% volume and form a filamentary network.

Transport within grains remains metallic with high plasma frequency.

Inclusions align with universal scaling for iron-based superconductors.

Abstract

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.