Hint of a condensate in K$_{\mathbf{0.8}}$Fe$_{\mathbf{2-y}}$Se$_{\mathbf{2}}$

The optical properties of the iron-chalcogenide superconductor K$_{0.8}$Fe$_{2-y}$Se$_2$ with a critical temperature $T_c = 31$~K have been measured over a wide frequency range in the {\em a-b} planes above and below $T_c$. The conductivity is incoherent at room temperature, but becomes coherent (Drude-like) with $\omega_{p,D}\simeq 430 \pm 20$~cm$^{-1}$ and $1/\tau_D \simeq 70\pm 5$~cm$^{-1}$ at $T\simeq T_c$; however, $\omega_{p,D}$ is an order of magnitude smaller than what is observed in other iron-based superconductors. The highly anisotropic nature of these materials suggests that the transport is best described by a sheet resistance $R_\Box = \rho_{dc}/d \simeq 64$~k$\Omega$ (per sheet), well above the threshold for the superconductor-insulator transition at $R_\Box = h/4e^2 \simeq 6.9$~k$\Omega$. Below $T_c$, $\omega_{p,S} \simeq 220\pm 20$~cm$^{-1}$ resulting in a superfluid density $\rho_{s0} \equiv \omega_{p,S}^2 \simeq 48 \times 10^3$~cm$^{-2}$, placing this material on the scaling line $\rho_{s0}/8 \simeq 4.4\, \sigma_{dc}\, T_c$ observed for the cuprates, but in a region associated with Josephson coupling, suggesting this material is inhomogeneous and constitutes a Josephson phase.\footnote{C. C. Homes {\em et al.}, arXiv:1110.5529}