Superconducting energy gap and c-axis plasma frequency of (Nd, Sm)FeAsO0.82F0.18 superconductors from infrared ellipsometry

We present far-infrared ellipsometric measurements of polycrystalline samples of the pnictide superconductor RFeAsO0.82F0.18 (R = Nd and Sm). We find evidence that the electronic properties are strongly anisotropic such that the optical spectra are dominated by the weakly conducting c-axis response similar to the cuprate high-temperature superconductors. We deduce an upper limit of the c-axis superconducting plasma frequency of omega(SC)(pl,c) <= 260 cm(-1) corresponding to a lower limit of the c-axis magnetic penetration of lambda(c) >= 6 mu m and lambda(c)/lambda(ab) >= 30 as compared to lambda(ab) = 185 nm from muon spin rotation [A. Drew et al., arXiv:0805.1042 [Phys. Rev. Lett. (to be published)]]. We also observe a gaplike suppression of the conductivity in the superconducting state with a shoulderlike feature at omega(SC)* approximate to 300 cm(-1) and spectral shape which is consistent with an unconventional order parameter with 2 Delta approximate to omega(SC)* approximate to 37 meV.