Electronic correlations and unusual superconducting response in the optical properties of the iron chalcogenide FeTe_0.55 Se_0.45

The in-plane complex optical properties of the iron-chalcogenide superconductor FeTe_0.55 Se_0.45 have been determined above and below the critical temperature T_c =14 K. At room temperature the conductivity is described by a weakly interacting Fermi liquid; however, below 100 K the scattering rate develops a frequency dependence in the terahertz region, signaling the increasingly correlated nature of this material. We estimate the dc conductivity σ_dc (T≳ T_c) ≃3500±400 Ω^-1 cm^-1 and the superfluid density ρ_s0 ≃9±1× 10⁶ cm ^-2, which places this material close to the scaling line ρ_s0 /8≃8.1 σ_dc T_c for a BCS dirty-limit superconductor. Below T_c the optical conductivity reveals two gap features at Δ1,2 ≃2.5 and 5.1 meV.