Infrared reflectivity measurements on several 122 iron pnictides reveal the existence of two electronic subsystems. The one gapped due to the spin-density-wave transition in the parent materials, such as EuFe 2 As 2 , is responsible for superconductivity in the doped compounds, such as Ba͑Fe 0.92 Co 0.08 ͒ 2 As 2 and Ba͑Fe 0.95 Ni 0.05 ͒ 2 As 2 . Analyzing the dc resistivity and scattering rate of this contribution, a hidden T 2 dependence is found in the normal state. The second subsystem gives rise to incoherent background, present in all 122 compounds, which is basically temperature independent but affected by the superconducting transition.

The temperature-dependent optical reflectivity and complex transmissivity of an epitaxially grown Ba͑Fe 0.9 Co 0.1 ͒ 2 As 2 thin film were measured over a wide frequency range ͑4 -35 000 cm −1 ͒. The opening of the superconducting gap 2⌬ 0 = ͑3.7Ϯ 0.3͒ meV is directly observed by vanishing optical conductivity at 30 cm −1 for T Ͻ T c = 20 K. While in this range the measured temperature-and frequency-dependent electrodynamic properties agree well with the BCS predictions of a nodeless order parameter, unexpectedly a strong quasiparticle absorption shows up below 1.5 meV. The spectral weight of the condensate 1.94ϫ 10 7 cm −2 corresponds to a penetration depth = 3600 Å.

The temperature dependence of the ab-plane optical reflectivity of Ba(Fe0.92Co0.08)2As2 and Ba(Fe0.95Ni0.05)2As2 single crystals is measured in a wide spectral range. Upon entering the superconducting regime, the reflectivity in both compounds increases considerably at low frequency, leading to a clear gap in the optical conductivity below 100 cm −1 . From the analysis of the complex conductivity spectra we obtain the penetration depth λ(T ) = (3500 ± 350)Å for Ba(Fe0.92Co0.08)2As2 and (3000 ± 300)Å for Ba(Fe0.95Ni0.05)2As2. The calculated superfluid density ρs of both compounds nicely fits Homes' scaling relation ρs = (125 ± 25)σ dc Tc. . Here we present a comprehensive optical study on optimal electron-doped Ba(Fe 0.92 Co 0.08 ) 2 As 2 (T c =25 K) and Ba(Fe 0.95 Ni 0.05 ) 2 As 2 (T c =20 K) single crystals. The superconducting gap is clearly observed in reflectivity spectra for T < T c . The spectral weight analysis on optical conductivity provides information on the penetration depth and the superfluid density.Single crystals of Ba(Fe 0.92 Co 0.08 ) 2 As 2 and Ba(Fe 0.95 Ni 0.05 ) 2 As 2 were synthesized using selfflux method [3]. The platelets with a typical size of 2 mm × 2 mm × 0.1 mm have naturally flat and shiny surfaces. The resistivity and susceptibility evidence sharp superconducting transitions at 25 K and 20 K, indicating that samples are doped uniformly [4]. The temperature dependence of optical reflectivity was measured in a wide frequency range from 20 to 37 000 cm −1 using a coherentsource spectrometer in the THz range, infrared Fourier transform spectrometers (Bruker IFS 66v/s and IFS 113v) and a Woollam spectroscopic ellipsometer extending up to the ultraviolet. The lowfrequency extrapolation was done according to the dc conductivity measured on the same crystals by standard four-probe method. The optical conductivity was calculated from the reflectivity spectra using Kramers-Kronig analysis. Fig. 1 shows the temperature dependent reflectivity and conductivity of Ba(Fe 1−x M x ) 2 As 2 . The R(ω) spectra show a good metallic behavior above T c as the reflectivity goes towards unity at low frequencies and increases by cooling. Upon entering the superconducting regime, the reflectivity starts to increase rapidly with a change in curvature below 70 cm −1 and 60 cm −1 , respectively. This yields

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