Speaker: Christopher Homes, Brookhaven National Lab
Abstract: The complex conductivity of the iron-arsenic superconductor BaFe1.85Co0.15As2 (Tc ≅ 25 K) and the iron-chalcogenide superconductor FeTe0.55Se0.45 (Tc ≅ 14 K) have been determined over a wide frequency range for light polarized in the planes at a variety of temperatures above and below Tc. The optical properties of the superconducting materials are compared with their metallic parent compounds, BaFe2As2 and Fe1.03Te. Despite being multiband systems, the low-frequency conductivity in the electron-doped iron-arsenic material is dominated by a single (electron) pocket, which can be described by a simple Drude model. Not surprisingly, the electron-boson coupling constant λ ≅ 1, indicating that this material is not strongly correlated. Below Tc there is a clear signature of the superconductivity in the optical conductivity and there is evidence that more than one energy scale is involved in the superconductivity. However, at low temperature (T << Tc) less than 50% of the free carriers have collapsed into the condensate indicating that this material is not in the clean limit . In contrast, the low-frequency conductivity of the iron-chalcogenide compounds is non-Drude at low temperature, with λ ≅ 5 - 7 in the superconducting material, indicating a much greater degree of electronic correlation. Below Tc there is once again evidence for two energy scales in the superconductivity - this may be interpreted as either the isotropic gapping of the electron and hole pockets, or the anisotropic gapping of the electron pocket (s+/- model) . Both the iron-arsenic and the iron-chalcogenide materials fall close to the universal scaling line for a BCS dirty-limit superconductor in the weak-coupling limit, as do the majority of the high-temperature copper-oxide superconductors.
 J. J. Tu et al., Phys. Rev. B 82, 174509 (2010).
 C. C. Homes et al., Phys. Rev. B 81, 180508(R) (2010); J. Phys. Chem. Solids (in press).