Optical conductivity of superconducting Rb2Fe4Se5
  We report the complex dielectric function of high-quality nearly-stoichiometric Rb2Fe4Se5 (RFS) single crystals with Tc=32 K determined by wide-band spectroscopic ellipsometry and time-domain transmission spectroscopy in the spectral range 1 meV<=$\hbar$\omega<=6.5 eV at temperatures 4 K<=T<=300 K. This compound simultaneously displays a superconducting and a semiconducting optical response. It reveals a direct band-gap of 0.45 eV determined by a set of spin-controlled interband transitions. Below 100 K we observe in the lowest THz spectral range a clear metallic response characterized by the negative dielectric permittivity \epsilon 1 and bare (unscreened) \omega pl=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. 
A. Charnukha, J. Deisenhofer, D. Pr\"opper, M. Schmidt, Z. Wang, Y.   Goncharov, A. N. Yaresko, V. Tsurkan, B. Keimer, A. Lo...
cond-mat.supr-con cond-mat.str-el