  The transport and complex optical properties of the electron-doped
iron-arsenic superconductor BaFe1.85Co0.15As2 with Tc = 25 K have been examined
in the Fe-As planes above and below Tc. A Bloch-Gruneisen analysis of the
resistivity yields a weak electron-phonon coupling constant lambda_ph ~ 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 ~ 7840 cm-1 (~ 0.972 eV) and
scattering rate 1/tau_D ~ 125 cm-1 (~ 15 meV) just above Tc. The
frequency-dependent scattering rate 1/tau(omega) has kinks at ~ 12 and 55 meV
that appear to be related to bosonic excitations. Below Tc the majority of the
superconducting plasma frequency originates from the electron pocket and is
estimated to be omega_p,S ~ 5200 cm-1 (lambda0 ~ 3000 Angstroms) for T << Tc,
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 BCS dirty-limit superconductor
in the weak-coupling limit. There are two energy scales for the
superconductivity in the optical conductivity and photo-induced reflectivity at
Delta1 ~ 3.1 +/- 0.2 meV and Delta2 ~ 7.4 +/- 0.3 meV. This corresponds to
either the gaping 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.
