Superconducting Energy Gap andc-Axis Plasma Frequency of(Nd,Sm)FeAsO0.82F0.18Superconductors from Infrared Ellipsometry

Abstract

We present far-infrared ellipsometric measurements of polycrystalline samples of the pnictide superconductor $R{\mathrm{FeAsO}}_{0.82}{\mathrm{F}}_{0.18}$ ($R=\mathrm{Nd}$ 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 ${\omega }_{\mathrm{pl},c}^{\mathrm{SC}}\le 260{\mathrm{cm}}^{-1}$ corresponding to a lower limit of the $c$-axis magnetic penetration depth of ${\lambda }_c\ge 6\mu \mathrm{m}$ and ${\lambda }_c/{\lambda }_{ab}\ge 30$ as compared to ${\lambda }_{ab}=185\mathrm{nm}$ 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 ${\omega }_{\mathrm{SC}}^{*}\approx 300{\mathrm{cm}}^{-1}$ and spectral shape which is consistent with an unconventional order parameter with $2\Delta \approx {\omega }_{\mathrm{SC}}^{*}\approx 37\mathrm{meV}$.