Optical properties of the iron arsenic superconductor BaFe1.85Co0.15As2

The transport and complex optical properties of the electron-doped iron-arsenic superconductor BaFe1.85Co0.15As2 with T-c=25 K have been examined in the Fe-As planes above and below T-c. A Bloch-Gruneisen analysis of the resistivity yields a weak electron-phonon coupling constant lambda(ph)similar or equal to 0.2. The low-frequency optical response in the normal state appears to be dominated by the electron pocket and may be described by a weakly interacting Fermi liquid with a Drude plasma frequency of omega(p,D) similar or equal to 7840 cm(-1) (similar or equal to 0.972eV) and scattering rate 1/tau(D) similar or equal to 126 cm(-1) (similar or equal to 15 meV) just above T-c. The frequency-dependent scattering rate 1/tau(omega) has kinks at similar or equal to 12 and 55 meV that appear to be related to bosonic excitations. Below T-c the majority of the superconducting plasma frequency originates from the electron pocket and is estimated to be omega(p,S) similar or equal to 5200 cm(-1) (lambda(0) similar or equal to 3000 angstrom) for T << T-c, indicating that less than half the free carriers in the normal state have collapsed into the condensate, suggesting that this material is not in the clean limit. Supporting this finding is the observation that this material falls close to the universal scaling line for a Bardeen, Cooper, and Schrieffer dirty-limit superconductor in the weak-coupling limit. There are two energy scales for the superconductivity in the optical conductivity and photoinduced reflectivity at Delta(1)(0) similar or equal to 3.1 +/- 0.2 meV and Delta(2)(0) similar or equal to 7.4 +/- 0.3 meV. This corresponds to either the gapping of the electron and hole pockets, respectively, or an anisotropic s-wave gap on the electron pocket; both views are consistent with the s(+/-) model.