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 FeTe{sub 0.55}Se{sub 0.45} have been determined above and below the critical temperature T{sub 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}{sub dc}(T {ge} T{sub c}) = 3500 {+-} 400 {Omega}{sup -1} cm{sup -1} and the superfluid density {rho}{sub s0} = 9 {+-} 1 x 10{sup 6} cm{sup -2}, which places this material close to the scaling line {rho}{sub s0}/8 = 8.1{sigma}{sub dc}T{sub c} for a BCS dirty-limit superconductor. Below T{sub c} the optical conductivity reveals two gap features at {Delta}{sub 1,2} = 2.5 and 5.1 meV.