The optical properties of the iron-chalcogenide superconductor K0.8Fe2−ySe2 with a critical temperature Tc = 31 K have been measured over a wide frequency range in the ab planes above and below Tc. The conductivity is incoherent at room temperature but becomes coherent (Drude-like) at T Tc; however, R 320 k , well above the threshold for the superconductor-insulator transition atR = h/4e2 6.9 k...
