Electronic correlations and unusual superconducting response in the optical properties of the iron chalcogenide FeTe0.55Se0.45

The in-plane complex optical properties of the iron-chalcogenide superconductor FeTe0.55Se0.45 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 sigma(dc)(T >= T-c) similar or equal to 3500 +/- 400 Omega(-1) cm(-1) and the superfluid density rho(s0) similar or equal to 9 +/- 1 x 10(6) cm(-2), which places this material close to the scaling line rho(s0)/8 similar or equal to 8.1 sigma T-dc(c) for a BCS dirty-limit superconductor. Below T-c the optical conductivity reveals two gap features at Delta(1,2)similar or equal to 2.5 and 5.1 meV.