Superconducting energy gap and c-axis plasma frequency of (Nd,Sm)O0.82F0.18FeAs superconductors from infrared ellipsometry

Abstract

We present ellipsometric measurements of the far-infrared dielectric response of polycrystalline samples of the new pnictide superconductor RO0.82F0.18FeAs (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 as in the cuprate high-temperature superconductors (HTSC). Accordingly, we obtain an upper limit of the c-axis superconducting plasma frequency of \(\omega_{{\rm pl},c}^{\rm SC}\leq 260\cm\) which corresponds to a lower limit of the c-axis magnetic penetration depth of \(\lambda_c\geq6\mum\) and an anisotropy of \(\lambda_c/\lambda_{ab}\geq 30\) as compared to \(\lambda_{ab}=185\) nm from muon spin rotation [A. Drew {\it et al.}, arXiv:0805.1042]. Also in analogy to the cuprate HTSC, our spectra exhibit the signatures of a gap-like suppression of the conductivity in the superconducting state with a large gap magnitude of \(2\Delta\approx300\cm\) (37 meV) and a ratio of \(2\Delta/k_{\rm B}\tc\approx8\) that is suggestive of strong coupling.