Optical conductivity of superconducting Rb2Fe4Se5 single crystals

We report the complex dielectric function of high-quality nearly-stoichiometric Rb2Fe4Se5 single crystals with T-c = 32 K determined by wide-band spectroscopic ellipsometry and time-domain transmission spectroscopy in the spectral range of 1 meV <= h omega <= 6.5 eV at temperatures of 4 K <= T <= 300 K. This compound simultaneously displays a superconducting and a semiconducting optical response. It reveals a direct band gap of approximate to 0.45 eV determined by a set of spin-controlled interband transitions. Below 100 K, in the lowest terahertz spectral range, we observe a clear metallic response characterized by the negative dielectric permittivity epsilon(1) and bare (unscreened)omega(pl) approximate to 100 meV. At the superconducting transition, this metallic response exhibits a signature of a superconducting gap below 8 meV. Our findings suggest a coexistence of superconductivity and magnetism in this compound as two separate phases.