Optical conductivity of superconducting K0.8Fe2−ySe2single crystals: Evidence for a Josephson-coupled phase

Abstract

The optical properties of the iron-chalcogenide superconductor K{0.8}Fe{2-y}Se2, with a critical temperature Tc = 31 K, have been measured over a wide frequency range in the a-b planes above and below Tc. The conductivity is incoherent at room temperature, but becomes coherent (Drude-like) at T ~ Tc; however, R_\Box ~ 64 k-Ohm, well above the threshold for the superconductor-insulator transition at R_\Box = h/4e^2 ~ 6.9 k-Ohm. Below Tc, the superfluid density \rho_{s0} ~ 48 x 10^3 cm^{-2} places this material on the scaling line \rho_{s0}/8 ~ 4.4 \sigma_{dc} Tc, but in a region associated with Josephson coupling, suggesting this material is inhomogeneous and constitutes a Josephson phase.