Optical properties of the iron-chalcogenide superconductor FeTe0.55Se0.45

The complex optical properties of the iron-chalcogenide superconductor FeTe0.55Se0.45 with Tc=14 K have been examined over a wide frequency range for light polarized in the Fe–Te(Se) planes above and below Tc. At room temperature the optical response may be described by a weakly interacting Fermi liquid; however, just above Tc this picture breaks down and the scattering rate takes on a linear frequency dependence. Below Tc there is evidence for two gap features in the optical conductivity at Δ1≃2.5meV and Δ2≃5.1meV. Less than 20% of the free carriers collapse into the condensate for T⪡TcT⪡Tc, and this material is observed to fall on the universal scaling line for a BCS dirty-limit superconductor in the weak-coupling limit.