Optical conductivity of superconducting $\mathbfRb_\mathbf2\mathbfFe_\mathbf4\mathbfSe_\mathbf5$ single crystals
We report the complex dielectric function of high-quality nearly-stoichiometric Rb2Fe4Se5 single crystals with Tc=32 K determined by wide-band spectroscopic ellipsometry and time-domain transmission spectroscopy in the spectral range of 1 meV≤ℏω≤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 ≈ 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 ɛ1 and bare (unscreened) ωpl≈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.