Two-band Bardeen-Cooper-Schrieffer superconducting state of the iron pnictide compound Ba(Fe 0 . 9 Co 0 . 1 ) 2 As 2

The optical conductivity and permittivity spectra of Ba(Fe${}_{0.9}$Co${}_{0.1}$)${}_{2}$As${}_{2}$ films (${T}_{c}=20 \text{K}$) are analyzed together with the literature data on photoemission and specific heat. In the superconducting state at all temperatures up to ${T}_{c}$ the temperature dependence of the magnetic field penetration depth is well described within the generalized two-band BCS model with intraband and interband pairing interactions considered. It is shown that the small superconducting energy gap $2{\ensuremath{\Delta}}^{(1)}=3.7\text{meV}$ develops in the electron subsystem while the larger gap $2{\ensuremath{\Delta}}^{(2)}=7.9\text{meV}$ opens in the hole subsystem. The normal state parameters, such as plasma frequency and scattering rate, of electron and hole conduction bands are determined.